[29 Chev]

This is awesome participation @29 Chev thank you for taking the time. I hope you don't mind if I point folks here when appropriate.

Thank you for the kind words. It is my hope that others will learn how to take a bit of time and diagnose an electrical problem so that it can be found and repaired. I have read so many posts where someone has a problem and they have replaced component A and B and the problem still exists and they want to know what component they should replace next - to me that type of repairing is like going to the county fair, plunking hard earned money down on the counter, spinning a game show style wheel and seeing where the wheel stops. Eventually you may find the faulty component but it will usually cost a lot more in time and money than what doing basic diagnostics and replacing the faulty part will in the long run - just my opinion. Not saying that diagnosing an electrical system problem is easy as it is usually harder than writing an article like this about it - not all test point locations are easy to locate and then to actually get to on a vehicle can also be a challenge.

Perhaps if the forum allows it and other people find these types of posts helpful they can be pinned or moved to a better location so they are easier for others to find - just a thought.

A 1966 Chevrolet Impala electrical system looks rather simple in relation to todays modern multi module Impalas but such is progress. If I compare the wiring of a 1929 Chevrolet passenger car to that of the 1966 Impala I could make the same comparison as a 1929 Chevrolet only had one actual fuse and very few electrical circuits. Keyed ignition circuit for the primary winding to the ignition coil (which is rather clever since it has an antitheft device built in), parking, head, tail, stop and dome lamps, brake light switch and a mechanical operated foot pedal to engage the starter motor. That was about it as far as wiring circuits other than the two battery cables, generator and cutout wiring.

 

[29 Chev]

Let us fast forward from the year 1966 to 1989 which I believe was the first year that fuel injection was used on a Chevrolet Caprice 305E engine - it was a OBD1 system and used a TBI system. Attached is an image of the under the hood wiring diagram for that vehicle. At first glance it looks quite challenging compared to the 1966 version but I have marked the positive portions of the circuit with red that are "hot at all times" and except for the main battery cable all the circuit wires are protected by a number of fusible links - 4 by my count. There are now two positive wire circuits that go to the alternator - one to the B+ post and one to the connector. There is also a connection at a junction box that goes to a pin on the ECM module. In this scenario we will also have a no start / no crank situation and when the key is turned to the run or start position no idiot lamps illuminate on the dash but the headlights and tail lights work and when the key is turned to the start position the headlights remain the same brightness as they are when the key is in the off position.
If interested stay tuned and we will attempt to create a plan to find out why the engine will not crank over when the key is turned.

 

[29 Chev]

We need to formulate a plan but since the wiring diagram and number of components have increased compared to the 1966 Impala situation we will have to look at a few more wiring diagram pages to understand better how things receive their power since there are four fusible link wires shown in the under the hood wiring page. I have attached two other pages of the wiring diagram that may be of interest to us. One is of the ECM since it has a wire going to it that comes from the battery junction box – it is marked “JB” on the ECM page and I have high lighted it in red to where it connects to an inline fuse. The other page is of the ignition switch, fuse panel and other areas that may be of interest since at least two of the fusible link circuits connect to them under the dash area.

 

[29 Chev]

As stated in this case the 89 Caprice with a 305E engine and automatic transmission has the same no start no crank problem but in doing some preliminary checks we have learned a few things that are different to what we encountered with the 1966 Impala scenario. If we turn the headlight switch on the headlights come on nice and bright and if we turn the key to the start position with them on the headlights do not dim or go out. This would indicate that the battery appears to be connected under the hood and has enough life in it to light the headlights and when the key is turned to the crank position the starter is not making any difference to the headlight brightness like it would if there was a discharged battery or bad connection at the battery. We have also observed that the dash idiot lights (charge indicator and oil pressure) do not illuminate and the radio does not work when the key is in the on (run) position and when we turn the key to the start position the brake warning light and the temperature indicator light don’t come on and the starter does not make any noise like it might with a discharged battery. It is like the key switch is not working and if we move the key switch to the accessory position the radio still does not work. After studying the wiring diagram pages for a few minutes we can identify the various circuits that are required to make the starter crank the engine, make the radio work and cause the charge indicator and oil pressure lights to illuminate. Since we know the battery can power the headlights we can assume it should have enough power to also illuminate the idiot lights, power the radio when the key is in the run position and when the key is moved to the crank position the starter should at least attempt to crank the engine if everything was working correctly. We can now create a plan to begin to diagnose this problem concentrating on the circuit or circuits that provide power to the ignition switch and also to the starter solenoid. We must also keep in mind that this could be a mechanical problem where the rod that moves inside the steering column when the key is turned has become dislodged from the ignition switch or something has broken. Since it is easier to test for power at a couple of locations in the circuits than to remove the panel under the dash to get to the steering column in my opinion I will opt to grab an incandescent test light to do a couple of tests as the first steps of my plan. The nice thing about a plan is that you can make it using the initial data you have gathered and then alter it if the tests results don’t find the problem. Keeping a plan as simple as possible will usually get tests results fairly quickly and then you can move on to either modifying the plan or fixing the problem if you discover the problem.

We know the battery has power since the headlights work so we can assume that both the ground and positive connections at the battery are good – if they were not and one was open or had high resistance then the headlights would not work. Let us examine closely how the headlights and the ignition switch get there power on the positive portion of the circuit. I have attached the two portions of the wiring diagram pages that will help us with that. If we look at the under the hood portion we can see that point (A) is where point (C) gets its power via the main positive battery cable which terminates at the large stud on the starter solenoid. We can also see that power goes to a pin on the ECM via battery junction block at point (F) via a smaller wire coming from point (A) – this portion of the circuit only feeds power to the ECM so it should not create the problem we are experiencing so we will move on and see where power goes once it leaves point (C). We can see we have four fusible links that connect to point C to feed various circuits. Looking at the diagram the first one feeds the A/C blower relay (at a splice in the wiring harness) and also the taillights and light switch – we know this must work because the lights work. The second one feeds a wire on the alternator plug – this portion of the circuit should not affect the problem we are experiencing. The third one feeds the B+ post on the alternator – this portion of the circuit should not cause the problem we are experiencing. The fourth one feeds the ignition switch – this one is a concern as it may be causing our problem so we need to test it at a convenient spot (if possible) to make sure it is not open and is capable of supplying current to power the ignition switch.

Now let us move on the second portion of the wiring diagram we need to concentrate on so we can find a convenient spot to test the circuit. If we look at point (Z) we can see this is where the fusible link wire terminates and connects to the ignition switch connector at point (X). We can see there is a splice connection in the harness at point (W) that feeds two wires going to the ignition switch and also a third wire that connects to one side of the stop / hazard fuse at point (V) and also to the power accessory Circuit Breaker. Since the fuse panel will be relatively easy to access compared with the ignition switch connector we will test at the stop / hazard fuse with our test light to make sure there is power at point (V) of the circuit. We will connect the clip of our test light to a good ground point and then touch the probe to both terminals of the stop / hazard fuse (one at a time) and see if the test lamp lights. We are touching both terminals (one at a time) to make sure we are connecting to the hot side of the fuse in case the fuse is blown. We do this test with the test lamp and discover it does not illuminate indicating there is no power at this point in the circuit this may be out problem. We will do one more test by touching the probe of the test lamp to one of the terminal of the tail lamp fuse to make sure the light does actually work and we have a good ground connection. We do this test and the light illuminates so we can now be certain we have found a problem with the circuit that feeds the ignition switch. At this point we know that the circuit is open or has excessive resistance somewhere between point (V) at the fuse box and point (C) at the starter and we may suspect that the fusible link is open but more testing will be needed to determine whether it is the terminal at the end of the fusible link, the fusible link itself or a spot in the wire from the fusible ling to point (V) that is the actual problem. We can however determine if that is the only problem that is creating our problem. If we have a fused jumper wire that is about 15 feet long and made from 14 gauge wire and has a 30 amp fuse we can connect one end to the positive battery terminal and the other end to the stop / hazard fuse terminal at point V. We can temporarily remove the stop / hazard fuse and push a male spade terminal gently into the fuse panel terminal and clip onto it with the test lead and see if the radio and idiot lights work when the key is turned to the on position – you may have to move the male spade to the other terminal of the fuse box if you did not select the hot one initially. If the radio and idiot lights work you can then make sure the vehicle is in park and that nothing under the hood (wires, tools, etc.) will get caught and see if the engine cranks and starts. If it does you know you have located the problem area of the circuit and that once you determine and repair the open portion of the circuit the vehicle will be fixed – if not you can figure out a new modified plan and continue testing. The nice thing is with the fused jumper wire in place you can now start and move the vehicle so it is easier to work on as you will probably have to get underneath the vehicle to remove the fusible link wire so it can be tested for an open or excessive resistance. If you find the fusible link actually opened up you now have a new question you can ask yourself – why did it? The answer may or may not be found – old age, something in the circuit created an overload situation, corrosion got into the fusible link wire. etc. If you repair the circuit and don’t find any obvious reason as to why the circuit failed you can suspect that a component that is being powered by the circuit may be failing and at some point in the future the problem may reoccur but for now you have diagnosed and repaired a no start no crank problem.

It is my hope that by following along the reader can see that even though wiring diagrams can become complicated compared to older vehicles it is still possible to do a few checks and tests, observe the results and by breaking the wiring diagrams down to areas of interest a problem can be diagnosed and found using relatively simple tests and a logical approach.

 

[29 Chev]

A brief post with some information before we move on to another no start - no crank scenario - first I will mention test lights. I usually recommend the use of a 12 volt incandescent test light rather than an LED style test light when testing electrical circuits - there are two reasons for this. An incandescent test light usually requires a minimum of 250 milliamps (or more) of current to flow through the bulb filament and make the bulb actually light whereas an LED style test lamp will not draw nearly as much current to make it light so an LED test light will not load a circuit nearly as well as an incandescent test light will if the circuit has a resistance problem. Sometimes if a circuits integrity is questionable I will use a seal beam lamp such as a H6054 (which will draw a little over 2 amps on low beam or about 5 amps on high beam) as a load on a circuit and measure the voltage present where I would expect to see battery voltage with the seal beam connected and not connected and observe for any difference in the voltage readings. There should be 0.01 or less volts difference in a circuit if there is no excessive resistance between when the seal beam is connected or disconnected to a circuit as a load - any higher of a voltage difference in the two readings would usually indicate there is a point in the circuit that is suffering a voltage drop due to excessive resistance. The other thing to remember about LED lamps is that they usually require a minimum voltage difference of 0.5 to 0.7 volts between the two connection points before the LED inside will actually turn on and any current will actually flow. If the voltage difference in a circuit is less than the thresh hold voltage required for the LED to turn on the LED test lamp may actually act like and open switch in a circuit. An incandescent test light will allow current to flow as soon as there is any voltage difference in the connection points in a circuit so it acts like a piece of resistance wire in a circuit - sometimes this difference can affect test results depending on the circuit you are testing and the voltage difference between the points you are testing across.
Second I will mention that service information and wiring diagrams are usually created by human beings and while for the most part they are usually correct and accurate sometimes they may not be identical to the vehicle you are working on. Another thing to remember is that the vehicle you are working on may not be identical to the way it was when it left the factory. Collisions, fire damage, water damage (flooding), an owner who decided to make two vehicles out of one by combining parts are just a few of the ways that vehicles can become modified from the way they were originally designed and produced. Another factor is that most auto manufacturers will sometimes change things during a production run that may or may not be catalogued and documented depending on what the change was. I will also mention "fleet vehicles" that may have been produced to serve as police cars ,taxi cabs or destined for sale in another part of the world that may not have been sold for their intended purpose and ended up being equipped with "unique" options just to keep people on their toes.

 

[29 Chev]

Let us now fast forward to a 2010 Chevrolet Impala with a 3.5L engine and see what we can do when we encounter a no start / no crank on a vehicle like this. We have now entered the world of what can be considered modern vehicles – progress, a desire for fewer emissions, more anti theft measures, safety and a motoring public that thinks it needs to be connected 24/7 with the latest and greatest gizmos and gadgets have made a vehicle and how it functions electronically much more complicated. Instead of having just a single control module utilizing one computer and one processor to perform calculations based on inputs from a handful of sensors we now have a vehicle equipped with many modules and each is equipped with its own computer and processor and these much more advanced modules communicate with each other many times per second when the vehicle is in not in a sleep mode. Having said that we can still do some basic tests and checks to determine why the starter will not crank the engine. Unfortunately, even with multiple computer systems, I don’t know of any vehicle made that can self diagnose itself completely and tell the owner why it won’t crank the engine so let’s see how we can gather information and with the help of some service information and wiring diagrams come up with a plan to diagnose the problem.

On a 2010 Impala we have many components and systems that can make the starter motor not crank the engine so the first thing I would recommend is getting a copy of a wiring diagram and service information so we can determine what needs to take place for the driver to turn the key to the start position and the starter to actually crank the engine. First initial test we can do is see to see if there is enough power to lock or unlock the doors as odds are the vehicle has power door locks and we can find out if they work. If the vehicle is equipped with a remote start option we can try that and see if the starter cranks the engine by pressing the start button. Once the doors are unlocked we can assume that the battery has some life in it and turn on the headlights and see if they are nice and bright and see if anything lights up or if we get any messages showing on the dash if we turn the key to the run or start position. These tests are similar to what we did in the older vehicles to gain initial information on the state of the battery and whether it is dead or appears to be charged and connected electrically to the rest of the system. In the end we find the door locks work and we do get a mileage display and other signs of life on the dash when the key is in the run position but when we turn the key to the start position the starter does not crank the engine over. We also observed that the headlights are nice and bright and that they don’t appear to dim any when the key is turned to the start position and the transmission is in park and moving the selector from park to neutral does not change anything. We also tried the remote start and it did not make the starter crank over when we pressed the start button. It is now time to consult a wiring diagram to see what all needs to happen for the starter to crank the engine. In most modern vehicles the wiring diagrams are not just three, four or five pages of drawings so it can take a bit of time to sift through the various wiring diagrams as a lot of them are broken down by the task that they perform – in this case we would like a wiring diagram that concentrates on the starting circuit. We would also like to have a good understanding of how the starting system functions and in this case we do have some diagnostic information available for a no crank –solenoid does not click. Attached is a pdf of the diagnostic information and a picture of the starter circuit as it appears on the Operation Charm website. The diagnostic information mentions a scan tool which we do not have access to right at the moment but it also explains how the system works so we will concentrate on that information and examine the starter circuit wiring closely to get a good understanding of things and how we may proceed from here using a volt meter and an incandescent test light.

 

[29 Chev]

Now let us concentrate on understanding how the starting system works and for that we will first look at the circuit description – here it is as stated in the service data.

“When the ignition switch is placed in the START position a discrete signal is supplied to the body control module (BCM) notifying it that the ignition is in the start position. The BCM then sends a message to the engine control module (ECM) that crank has been requested. The ECM then verifies that the transmission is in Park or Neutral. If it is, the ECM then supplies 12 volts to the control circuit of the starter relay. When this occurs, battery voltage is supplied through the switch of the starter relay to the starter solenoid.”

Let us examine the first sentence and we see the words “a discrete signal is supplied to the body control module (BCM)”. If we do a Google search we learn that “discrete input or output signal is a two-state signal which is either on or off, true or false, one or zero”. After doing a bit more research we have learned that in the automotive world (just as in the computer world) there are basically two logic levels when it applies to modules and logic circuits and these are an “ON State” which is when 5 volts is present on a logic circuit and there is an “OFF State” which is when 0 volts is present on a logic circuit. This indicates there are logic circuits involved and one thing I will mention about logic circuits is they use very little current so there is very little chance of a wire or terminal connection being damaged from overheating the way a regular 12 volt circuit can when it is supplying the current to a load such as a headlight or electric motor but the possibility of corrosion or rodent damage can still happen.

This might appear as strange language to an old school automotive person or someone with little automotive electrical and computer / module experience so let us examine the wiring diagram and see what is happening and maybe we can learn more. I have added some letters to indicate various points on the diagram and made things a little easier to read.

If we look at the drawing of the BCM we can see at point (A) we have a connector identified as X1 and at terminal 4 of this connector we have a 5 volt logic signal so we know this is in a normally “ON State”. We can also see that this logic signal is connected via a white / black wire (WH/BK) to terminal 6 on the ignition switch at point (B) so we can assume that the 5 volt signal wire at the BCM is a logic output and should always be at a 5 volt level when the modules are awake. If we study the ignition switch drawing we can see that there is a normally open 4 position switch that is show in position O – the other positions are I, II and III. If we do some more digging in the service data we will eventually learn that O = Off, I = Acc, II = Run and III = Start. If the switch is moved from position O to position III (the start position) then the 5 volt logic signal will now be applied to a 1.3 Kohm resistor inside the ignition switch and to a white (WH) wire which is connected to terminal 5 of the ignition switch. This wire is connected to terminal 2 of connector X1 of the BCM. If we look at point (D) of the BCM we can see a drawing of a pull down resistor and this tells us that terminal 2 of the BCM is a logic input that normally is at a low logic state (0 volts) but the circuit it is connected to inside the BCM can be forced to a logic high level when the key is in the Start position. This should explain the first sentence in the information stated in service data.

In the second sentence we see that “The BCM communicates with the ECM and tells it that a request for the engine to crank over has been received” – this line of communication is shown by two arrows going in opposite directions at points SD1, SD2, SD3 and SD4 so we now know that the BCM, the ECM and the IPC (instrument panel cluster) all need to have good clear communication lines so they can talk to each other so the components involved all know the engine needs to crank over and they must work together to make this happen. In the third sentence we read “The ECM then verifies that the transmission is in Park or Neutral.” This tells us that the ECM has to know that information before it will allow the engine to crank over. If we look at the drawing of the ECM module we can see that terminal 57 at a connector identified as X2 is connected to an orange / black (OR/BK) wire that goes to a switch somewhere inside or on the automatic transmission at point (F). Looking at point E of the ECM we can see that this is probably another logic circuit that uses a pull up resistor so it is normally at a logic high state (5 volts) but the circuit it is connected to inside the ECM can be forced low when the transmission is in Park or Neutral. If we look at the internal mode switch (X) we can see that when the switch is in Park or Neutral point F will be connected to ground by a black / white (BK/WH) wire that is connected to a ground connection point at point (H) G111. This explains how the ECM will know that the transmission is in Park or Neutral.

In the fourth sentence we read – “If it is, the ECM then supplies 12 volts to the control circuit of the starter relay.” If we look at the ECM drawing again we can see that at point (I) there is a normally open switch inside it that is connected to terminal 32 of the connector identified as X2. This terminal is connected to a yellow wire that is connected to the fuse block at connector X1 terminal J1 which is connected internally in the fuse block to the starter relay terminal 86 which is one side of the lightly wound coil of wire inside the starter relay at point (K). Since no voltage level is specified and there is no pull up or pull down resistor shown a point (I) we can assume that when the switch at point (I) closes this will be a 12 volt output and this is confirmed by the fourth sentence wording. If we look at the starter relay we can see the other side of the lightly wound coil of wire at point (K) is connected to the starter relay terminal 85 which connects to terminal C2 of connector X1 and from there to a black / white wire (BK/WH) that is connected to a ground point G111 at point (H). In the fifth sentence we read “When this occurs, battery voltage is supplied through the switch of the starter relay to the starter solenoid.” If we look we look at the drawing we can see that there is a red wire (RD) that connects to connector X5 terminal 1 which connects internally in the fuse block to terminal 87 of the starter relay which is connected inside the relay to a normally open switch at point (O). Terminal 30 of the starter relay is connected to the other side of the normally open switch and there is an internal connection in the fuse block from terminal 30 to one side of the starter fuse at point (P). The other side of the starter fuse at point (P) is then connected inside the fuse block to a terminal F1 at connector X1 and from there a purple wire runs to connector X2 terminal A of the starter motor at point (Q). We can also see looking at the starter motor drawing there is a battery cable that is connected to the positive battery post terminal and it goes to the larger stud on the starter solenoid at point (S). As part of the starter motor there is a starter solenoid with internal connections to a ground point at point (V) and to the positive side of the actual motor circuit at point (T). We can also see that there is a normally open switch (part of the starter solenoid) that feeds the starter motor at point (U) and the actual starter motor also gets its ground at point (V). If we look at the battery we can also see that there is another cable that connects the negative post of the battery to a separate ground point at point (W).

Hopefully this makes sense to the reader and they now see that multiple circuits and connections must work together to allow the starter to crank over the engine. Some of the problem areas that can make the starter not work are strictly electrical dealing with 12 volts and can still be diagnosed with an incandescent test light and voltmeter. Other areas that can prevent the starter from cranking the engine are the logic portions and the actual data communication lines of the system. If you are reading this and there is something you do not understand please ask as that is how we all learn – in my opinion there are no dumb questions other than the ones that were never asked.
Attached is some electrical symbol information that can be found in service data for this vehicle.

 

[29 Chev]

Since we now have a good understanding of how the starting system works we can now begin to formulate a plan to try and diagnose why the starter will not crank the engine over. We know the battery is in a reasonably charged state since the headlights come on when switched on and are bright so we will assume the battery and terminal connection at the battery are ok for now. Since the starter relay and the starter fuse are relatively easy to get to since they are located under the hood then that is where I am going to start. It is easier to pop the cover of the fuse block than it is to crawl around under the dash or under the vehicle trying to get to the actual starter – just my opinion.

One thing we can do that is relatively easy is to remove the fuse block cover and locate the starter relay and the starter fuse. The service data information actually gives us a layout of the fuse block and its components as shown below.

The first thing I will check is to make sure the 40 amp starter fuse is not blown – for this we can connect the clip of the test light to the negative terminal of the battery and then touch the probe of the test light to the positive terminal to verify the light illuminates which it does. We can remove the fuse and connect one terminal of the fuse to the positive terminal at the battery – will probably need a jumper wire for this. Once that is done we can touch the test light probe to the other terminal of the fuse and see if the light illuminates. It does so we can assume that the fuse is not blown – before plugging it back into the fuse block we will have a good look at the terminals in the fuse block to make sure they are not rusty or corroded looking. They look ok so we will move on and remove the starter relay where it is plugged in to the fuse block and have a good look at the terminals on it and where they plug into the fuse block. They also look ok so I will now identify which terminal on the relay is 87 – they are usually but not always stamped with small letters. In this case I can identify the 87 terminal so I will first verify the test light is still clipped to the negative battery terminal and then touch the test light probe to the terminal in the fuse block that terminal 87 of the relay plugs into since it should be hot at all times. We do this and to our surprise the test light does not illuminate. We touch the probe of the test light to the positive terminal on the battery and the test light now illuminates – we now know the test light is still working and there is a problem in the circuit that feeds the terminal in the fuse block where terminal 87 of the relay plugs into it - there is either an open or excessive resistance somewhere in that portion of the circuit. The problem may be in the wire between point (M) at the positive battery post and point (N) where the wire connects to the fuse block or the problem may be in the actual fuse block between where the wire ends at point (N) and the terminal that pin 87 of the relay plugs into. In my experience the wires and terminals used on the underside of the fuse block can get corroded and damaged over time if the vehicle is from a four season climate where salt and other chemicals are used to melt snow and ice. We will still need to verify where the problem is exactly but at least we have narrowed the problem down to an electrical problem and we can also verify that restoring power to the fuse block relay terminal 87 will likely fix the problem. To do this we can connect a temporary fused jumper wire to the positive battery terminal and then momentarily touch the other end of the jumper wire to where terminal 30 of the starter relay plugs into the fuse block and see if the engine tries to crank – before doing this make sure the transmission is in park, the parking brake is applied and all wires, tools, hands, fingers, etc. are clear of the engine compartment. If the starter attempts to crank the engine over we can be fairly confident this is the only problem but the relay may still be a problem since we have not tested it. Since we know how it wired internally with the wiring diagram we can use a test light and / or an ohmmeter to verify it is working ok.
I have attached the image of the wiring diagram one more time to make the circuit testing a bit easier to follow.

In this scenario of a no start / no crank it may be easier to purchase a new 4 terminal relay connector pigtail and splice it into the wiring circuits at the four points that the fuse block terminals connect to rather than trying to locate where the circuit is open or has excessive resistance but that is entirely up to the individual and what their skill levels are. If this is done make sure the wiring gauges are as heavy as or heavier than what he original wires are so they can handle the current flow required in the circuits.

As a final note I will mention that had this been a communication problem between the modules involved the problem may have required more equipment and a good understanding of how the modules talk to each other. A good scan tool will be able to show if the modules involved have the ability to communicate and also the user may be able to view whether a module (such as the ECM) is receiving the command to crank from the BCM. A relatively inexpensive oscilloscope can also be used to view the communication “chatter” between modules on the data line wires and determine whether the data communication “chatter” looks good or appears to be garbled – there are several good youtube videos that can be viewed to determine if the “chatter” looks ok or not when viewed on an oscilloscope if you happen to have such a tool. A good thing is that even without a scope the communication wires can usually be found and identified using the service data information – it may take some digging and sifting to find the information but it is usually there. A visual inspection and a good nose (sniffer) can also be beneficial to find a spot in a wiring harness or connector that rodents have decided is their new restaurant or home to hang out at.

It is my hope that these posts have helped give a better understanding of how the dreaded no start / no crank can be changed from a “what do I do now” scenario to a “let’s figure out how to diagnose it” scenario.

 

[29 Chev]

In this post we will discuss a starting system for a 2017 Impala 3.6L without the start stop option as per the diagram I have attached. I am not going to diagnose a no start / no crank situation in this post but rather examine the system diagram to see what may be different or perhaps what might be considered missing information.

We can see that while the starting system has changed a bit in the seven years there are some similarities. We are still using a battery that has a negative cable and a second wire connected to ground points throughout the vehicle to provide a return circuit path for any electrical components. There is also a positive circuit that in the drawing appears to be a cable that connects to a battery fuse block – in reality the battery fuse block is connected directly to the positive post of the battery. The engineers have added a 300 amp fuse that supplies power to the larger stud on the starter solenoid via a battery cable and a 250 amp fuse that supplies power to the underhood fuse block. The drawing shows how the BCM (body control module), the ECM (engine control module), the TCM (transmission control module) (which is connected to the transmission internal mode switch) and the ignition switch employ various electrical connections and data transfer lines to decide whether the starter motor will power up and crank the engine over. There is still the starter relay and a starter fuse but it appears an ECM fuse has been added to the system so we can assume that fuse could play a role in whether the starter cranks the engine over.

If you remember a few posts back I mentioned that service data and wiring diagrams may not be identical to the vehicle a person is working on and in this instance that is indeed the case. The ignition switch shown in this drawing at the red arrow is a 4 position switch – OFF, ACC, RUN and START - I assume it represents a keyed switch. In my case the vehicles wiring is very similar but it uses a keyless entry system that requires the module in the remote control key fob to interact wirelessly with the anti theft module in the vehicle to do pretty much anything with the vehicle. The vehicle has a push button on the dash and if you press it once without your foot on the brake the response is the same as being in the ACC position of the keyed switch. If you press the button once and have your foot on the brake the engine normally cranks and starts. I looked through the service information I have and eventually found information on the push button and how it interacts with the KEM (keyless entry module) and the BCM (body control module) to establish whether the push button on the dash has been pressed and the press meant that the engine should crank and start. The point I am making is that while I eventually pieced together the information for my vehicle I did not locate a diagram that shows the starting system for a push button style vehicle like mine so it is sometimes necessary to read between the lines to determine how a system works on newer vehicles.

The other thing I will mention is that for the engine to crank over it is necessary for the driver to press the brake pedal down – I assume this is a safety feature. I know that the majority of GM vehicles around this time period required pressing the brake pedal whether the vehicle had a push button start or a keyed ignition switch - yet there is no information to indicate this and that the brake pedal switch plays a factor in whether the system decides it is ok to crank the engine. At initial glance this may appear to be an omission in the system information documentation and it may well be. However, it may not be an omission as I know on my vehicle if I press the button without pressing the brake pedal the system responds by turning on the ACC position but there is also a message on the DIC (driver information center) screen for a few seconds that says to press the brake pedal if you want to start the engine. Theoretically if the switch on the brake pedal or one of the wires that connect it to the BCM was damaged then this would prevent the engine from cranking but at the same time there should also be a message displayed to let us know that from the starting systems point of view the brake pedal has not been pressed and that if the driver wants to start the vehicle they need to press the brake pedal. I am not sure if a similar year vehicle that had a keyed ignition switch and the brake pedal was not pressed while trying to start the engine would display a similar message as I have never tried it. My point is that we must always do initial tests and observe all the results we obtain and then think carefully as to whether they indicate a problem that might give us a clue so that we can formulate a plan to diagnose a problem such as a no start / no crank situation - in this instance it could be as simple as the driver forgot to press the brake pedal. If they did press the brake pedal and got this message then that would give us a clue that the BCM thinks the brake pedal has not been pressed and that it would be a good place to start with our testing once we determined what was necessary to tell the BCM that the brake pedal is being pressed.

 

[29 Chev]

I'm having the no crank/ no start on my 2011 LT. When I turn the key on all dash lights except theCEL-check engine light (sometimes it very dimly lit- barely)are on in run position. This is when I get nothing. When I switch off and turn back to run the CEL is lit up and it will crank and start right up. Sometimes I have to cycle off and on a few times to get illuminated CEL and starts every time. Any ideas or help greatly appreciated. I also noticed only having the problem in hotter weather. Didn't happen last winter (Iowa)

What engine 3.5L or 3.9L? When the CEL disappears do you still have a PRNDL display and does it show that the transmission is in Park? What happens if you move the transmission from Park to Neutral - same problem or does the CEL suddenly become illuminated?

 

[29 Chev]

3.5 motor it's the initial times first time putting the key in when I don't have the cel it's showing that the car is in park I turn the key off turn it back on once or twice in the light ignites I will check when I get home if I shifted into neutral if the light will come on. It seems like a cycling of the key off and on a few times takes care of it

Did some research and I have attached a wiring diagram of the ECM (which controls the Malfunction Indicator Lamp (CEL)) and also a wiring diagram of the starting circuit.

Looks like the MIL / CEL is powered by the cluster and is grounded by a circuit in the ECM to make it illuminate and the ECM is necessary for the starter relay to be energized. I would suggest the next time it does it to check to make sure you have battery voltage at the two ignition feeds and the one B+ feed to the ECM connector. There is also a good possibility there is a corroded / high resistance ground connection at the terminal area where G111 is located. If you have a test light and a multimeter capable of measuring up to 15 volts DC you could do some voltage drop testing to try and determine where a connection on either the power or ground portion of the circuit has excessive resistance.
Hope this helps you.

 

[29 Chev]

Did some research and I have attached a wiring diagram of the ECM (which controls the Malfunction Indicator Lamp (CEL)) and also a wiring diagram of the starting circuit.
View attachment 169901
View attachment 169902
Looks like the MIL / CEL is powered by the cluster and is grounded by a circuit in the ECM to make it illuminate and the ECM is necessary for the starter relay to be energized. I would suggest the next time it does it to check to make sure you have battery voltage at the two ignition feeds and the one B+ feed to the ECM connector. There is also a good possibility there is a corroded / high resistance ground connection at the terminal area where G111 is located. If you have a test light and a multimeter capable of measuring up to 15 volts DC you could do some voltage drop testing to try and determine where a connection on either the power or ground portion of the circuit has excessive resistance.
Hope this helps you.

G111 Location areas under hood