[Shane C.]

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.
View attachment 169657
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.
View attachment 169658

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)

 

[Shane C.]

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?

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

 

[Shane C.]

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?

 

[Shane C.]

This is perfect thank you so much I will do that this weekend thank you for your help truly appreciate it