[29 Chev]

Thank you for the detailed information you supplied and I applaud you for trying to diagnose the problem rather than just replacing parts.
The first thing I notice in the one picture you posted is that two terminals appear to be suspect - I have identified them with red arrows. The one at the end appears to have the tongue portion of the terminal pushed up a bit and it appears to have a darker colour where it would make contact with its counterpart which could be causing an intermittent connection and the one near the middle appears to have a bit of tarnish as well. It could just be the quality and angle of the photo that the connector was taken at but I would suggest verifying that they are good. I think the one at the end is the power feed for the module.

The other thing I noticed is the tests you made and I wonder if these were done with the connector unplugged and with no load placed on the circuits? I will attempt to point out things that may show your tests as good when there could still be a problem - not trying to criticize but educate.

I pulled out the multimeter and follow along with the Service Manual steps. By completing the following

• Test the Red/Wht B+ wire for Voltage: 12.5V - If this was done using a voltmeter connected to the connector and the connector was not plugged in then the reading may not really prove anything. To properly test the circuit would need to have a load placed on it with something like the module or a 12 volt lamp. I would suggest using a light bulb that can draw about an amp or two since the circuit is fused at 10 amps - even an incandescent test light may show up a problem but they usually only draw around 300 milliamps. Sometimes you can clip the test lead clip to the negative lead of the meter and hold it to the negative terminal of the connector. Then tough the positive lead to the Red/Wht B+ wire and observe the meter reading - let us assume that you get 12.5V. Then touch the probe of the test light to the positive lead of the meter and the light should light and then observe the meter reading - it should remain the same. If the voltage drops at all downward (such as 12.3V) then the circuit has a problem and there is excessive resistance somewhere in that circuit and as the current demand goes up (such as with the light bulb being lit) the voltage level is dropping. I am assuming that you used the ground terminal in the connector as your ground point for the measurement. You can also use a jumper wire and test bot the positive and negative portions of the circuit separately by connecting a jumper to a battery ground or positive terminal and then observe if there is any difference in the readings versus what you got at the connector terminals. If you obtain anything larger than 0.1 volts difference then there is a problem somewhere.
• 
  
• Test the 2 blk/wht ground wires for Continuity to negative post of battery: 1.1Ω (Service manual states under 10Ω is good) - what reading did you obtain with the meter leads connected together? 1.1 ohms may not seem like a lot but it is very difficult to use an ohmmeter to verify a circuit does not contain excessive resistance. Here is a link to a voltage drop article I wrote that may help you understand how testing for voltage drop is a much more effective way to determine if a circuit has excessive resistance. https://www.impalaforums.com/thread...ical-problems.1893448/?post_id=3749248&nested_view=1&sortby=oldest#post-3749248
• 
  
• Test the pink IGN wire for Voltage: 12v (When key is in run position) - That is a 0.5 volt difference compared with the 12.5V reading you got when you checked the hot at all times circuit which suggests there is a half volt of drop somewhere in that circuit. Again, was this test done with the circuit loaded or with the connector unplugged and where was the ground lead for the meter connected?

I hope this helps you a bit with your quest to finding the problem as I doubt the module itself is the problem but I may be wrong. Intermittent problems can be a challenge but life is full of them I have found over the years. One last thought would be to do a voltage drop test between the ground wires and a good known ground point such as the negative battery terminal to see what sort of reading you get with the circuit loaded - anything more than 0.01 volts would make me think that there could be excessive resistance and ground point can corrode over time.
One other thing I will mention is that dielectric grease can actually act as a insulator if too much is applied on a surface. I find a product called Fluid Film is better to protect terminals from corrosion - just a suggestion.
Forgot to mention that you can often back probe a wire terminal if you are careful using a thin strand of wire or a T pin if desired so that you can do checks using a meter or an oscilloscope to see what is actually happening in a circuit in real time without damaging the wire.

 

[29 Chev]

The male pins do go in the outer cavities but it is imperative that the connection points where they mate with the terminals are clean and also that the terminals have some "drag" (or clamping action) on the pins so that there is always good conductivity and minimal resistance - resistance creates heat which can promote tarnish and weaken the terminal metals spring tension.
The wire stay should not have to be removed as often you can see the terminal where the wire is crimped on the back side - as long as the wire or pin is thin enough you can usually slide it in to make contact with the terminal metal. Sometimes it takes a gentle action and a little luck along with a couple of tries to get it to make contact.

The wake up reading at 11.3 volts would not be a concern to me at this point since it is probably coming from a serial data signal on a Bus line. That voltage you are seeing may be an average of a high / low waveform of serial data packets and it may not be a steady voltage such as what the module needs to work properly. To view the wakeup signal I would suggest using an oscilloscope which you probably do not have access to. I would concentrate on making sure the module has the proper voltage and ground circuits (powers and grounds) so the module is alive and able to do its job properly regardless of the system voltage of the vehicle. Something to keep in the back of your mind is that a vehicle electrical circuit is usually between 12.4 (at rest and everything shut down) and 14.2 volts (when the engine is running and the alternator is working properly). When the vehicle is starting and the engine is cranking over the starter current draw may actually lower the voltage of the vehicle electrical system to about 10.0 volts or slightly less depending on ambient temperature, condition of battery and circuits and this is when the power and ground connections to any module become critical so that they all still are alive so they can do their jobs and communicate with each other properly. Any excessive resistance can result in the 10.0 volt level that a module might see when cranking dropping below a threshold where the module temporarily looses its brain so to speak. Any tarnish or corrosion may result in the module voltage remaining below what it requires once the cranking engine starts and the system voltage recovers to normal levels - especially where moisture is present since electricity takes the path of least resistance and water is a much better conductor than air as far as resistance goes. Hope this makes sense to you.

 

[29 Chev]

The 12.12 volt reading on the ignition wire versus 12.48 volts on the B+ wire indicates there is a voltage drop of .36 volts at some connection point (or points) between the battery positive post and where the wire terminates at the TCM connector - to me this would be a concern as it indicates excessive resistance somewhere in that circuit. It may or may not be creating the problem with the TCM that you are experiencing but it will probably become a problem with the TCM or another module on that circuit at some point in the future. I would suggest that you measure the voltage drop between the transmission fuse (10 amp) and the positive battery post by placing the red meter lead on the battery post and the black meter on the fuse with the module plugged in and the key on - might need a jumper wire to connect to the positive battery terminal. If you have a wiring diagram for your car you can check and see how that circuit is fed from the positive battery post to the transmission fuse and then test at various points that are easy to get to so you can determine where the excessive resistance exists and then hopefully resolve the problem.

One other thing I would suggest is to actually measure how much the test light you used to do your tests actually draws for current when it lights - this can be done by connecting the test light and the amp meter portion of your multimeter in series and connecting the circuit to a 12 volt battery. If it is only drawing 200 or 300 milliamps my concern it is not loading or stressing the circuits you are testing enough to show up a voltage drop that may be present if the TCM module is drawing 3 or 4 amps which it can easily do in use.

 

[29 Chev]

One more suggestion would be to measure the voltage at the actual battery with the ignition on with your voltmeter to give you an idea of what the battery voltage is with a bit of load since the majority of modules will be powered on. Was also wondering if when you measured the B+ voltage at the TCM connector was the ignition on or off?

 

[29 Chev]

One other suggestion when doing the tests (voltage drop or ohm) is to wiggle / move the harness in numerous spots while observing the meter readings and see if you observe a change - if there is a break in one of the wires inside the harness this may help you locate it. Given the way that you described the problem it appears the problem is something that comes and goes fairly quickly which makes me think something is going open (wire, connection, etc.) (just for a millisecond sometimes) which will trigger the code and result in a loss of communication. Hope you find it.

 

[29 Chev]

The pin 15 test is what I would expect to see since the ACC line is actually serial data communication from what I have read - I think if you check your wiring diagram you will find that line connects to the BCM, ECM, CCM (chassis Module) and the TCM and transmits serial data between them using a series of highs (around 11.2 volts) and lows (0 volts) and if you looked at the signal with a scope it would be a non symmetrical square waveform. It is probably a very low current digital signal and using your load tester is dropping the voltage as the tester pulls it a bit closer to ground. Don't think that is your problem.
A voltage drop of 0.1 volts that you got when you tested pin 13 to ground is a problem - should not be that much as it is .03 volts higher than you got on the B+ test using the same ground (terminal 18). I would suggest checking the circuit from the battery positive terminal to that connection using your wiring diagram at easy access points where there is a connection point (fuse, terminal relay, switch, etc.) to see where the voltage is dropping. Use the power distribution wiring information to see how the current flows from the battery positive to get to pin 13 of the connector. Given the results of your B+ to the same ground point test (terminal 18) I would say the voltage drop is occurring in the positive portion of the ignition feed circuit somewhere - just my opinion. Note: this drop may actually be greater when the vehicle is running and in motion since the TCM is communicating with other modules and drawing more current than it would be using your tester.
You can also connect a jumper wire from the battery negative post (or terminal) to one terminal on your tester and connect the other terminal to your ground wire terminal 18 and use your voltmeter to determine how much voltage drop is occurring on the negative side of your circuit - just a suggestion.

If you check the bottom left of member GrizChev post he has a link to his help files. Hope this helps you.

 

[29 Chev]

Thought this might help you and others who read this thread understand the data signal voltage reading you are getting on pin 15 - found a youtube video that shows the type of signal that I would expect to see on the ACC wire terminal 15 if it was viewed using a scope. As you can see the waveform is going from just under 12 volts to 0 volts for very brief portions of time when data is being sent or received and in between the waveform remains high at just under 12 volts. Since a multimeter sampling rate is not fast enough to accurately catch and display such rapid changes of voltage levels and most of the time the voltage level is just under 12 volts the multimeter sampling rate averages the voltage level samples it sees and decides the voltage level in this circuit is just under 12 volts and that is what it displays (the 11.41 volts you obtained during yesterdays tests). The voltage level on this circuit is constantly swinging from 0 volts to just under 12 volts as the modules it is connected to communicate (jibber jabber as Eric O calls it) back and forth whenever the vehicle is in an awake state - the current level of these types of circuits is usually in the milliamps to keep power consumption on the "electrical grid of the vehicle" to a minimum to reduce load on the battery and alternator. As such if any external load (such as your tester) is connected to the circuit the voltage level will usually drop noticeably such as you observed. Hope this makes sense to you.

 

[29 Chev]

I would say you are getting close to finding the area of excessive resistance - a voltage drop of 0.28 volts using your tester is too much in my opinion. Can you post a picture of the battery positive terminal (where the cable connects to the positive post to the Fuse Box Main feed as there is probably another connection or two in between that is a problem or it may even be corrosion in the main feed wire.

 

[29 Chev]

Just so I understand things - the video that shows where you had a voltage drop of 0.28 volts - was the ignition switch in the on or off position and if it was in the on position what all was working - radio, blower motor, etc.?
If you tested with the load pro connected to the battery positive cable terminal and the negative battery terminal with everything turned off and got the results above (0.04 volt drop) that could be very possible - did some research and the load pro adds approximately 0.5 amps of load to a 12 volt circuit which is about the same as lighting two 194 bulbs (used in side marker or dash light applications on vehicles made in the 70's to 90's.
If the test you did in the video was done with the key on then all the modules would be consuming a bit of power (current) as well as any components such as blower motor or radio as examples. Adding the load of two 194 light bulbs to a battery that is already being loaded by the modules and other components should not create a large voltage drop that you have observed (0.28 volts) in my opinion.
I would suggest redoing your test that you did in the video of the battery feed cable terminal with the key in the on position (both at the terminal and also the stud as suggested by GrizChev) and see what voltage drop you get and then connect the probe to the positive battery cable terminal at the battery post and to the positive post itself and see what drop you get there - leave your ground connected the same during all four tests. I am suspecting that the feed wire from the battery cable positive terminal to the fuse box main terminal has excessive resistance in it and the more the circuit is being loaded the more the excessive resistance is becoming a problem - this may be even more true if the alternator output trying to charge the battery may have to go through that portion of the wire to try and recharge the battery once the engine is running. Not sure why the TCM is the only module that appears to be having an issue but it may be the weak link in the chain and most susceptible to lower voltage. Just my opinion.

 

[29 Chev]

I understand your frustration since nothing is jumping right out and screaming "I am the problem" - patience and the desire to not give up are two things that one sometimes needs in these situations.

My thoughts are that if it was a data communication issue only then when the ECM sets the code that it lost communication with the TCM then it should also loose communication with the ABS module since it is on the other side of the TCM and log a code about that (or at the ABS module) - this is why I believe it is a power / ground supply issue to the TCM where the voltage to the TCM is low enough that it can no longer do its job properly.

A 0.04 voltage drop using just your load pro is still significant voltage drop to me since it is only drawing 500 milliamps of current (as I stated about the same amount of current that is needed to light two 194 bulbs). A few suggestions you could try to verify or disprove there is excessive resistance in the cable running from the positive battery cable to the main fuse panel stud that you were testing at. Connect a small battery charger to the battery so the voltage remains constant at the battery. Then turn on the parking lights (leave the ignition off) and then do a test with your load pro and see what sort of voltage drop you get at the stud at the main fuse panel when the load pro button is pushed - I would expect to see only a drop of 0.01 to 0.02 volts if the cable running from the battery positive to the stud on the main fuse panel does not have excessive resistance. You can also then try this test with the headlights on and observe the results. As a last test you could connect a 14 gauge jumper wire between the positive battery cable terminal and the terminal at the stud at the fuse panel and then see if there is any difference between having the jumper wire connected or not connected when you test the voltage at the stud using the load pro - there should not be.

Just my thoughts and opinions.

 

[29 Chev]

I usually do the B+ and B- battery posts to verify battery voltage after turning something on (like the park lights or use a test light) to load the circuit I want to check out. Then I switch gears a bit and set the voltmeter to millivolts and go B+ post to B+ cable terminal. Then B+ cable terminal to FB + terminal, etc. That way using the voltmeter on the millivolts scale I can get a better idea of whether the drop is 0.01, 0.02, 0.03, 0.04 etc, volts - my meter will round if I use the 0 to 20 volt scale and all would show as say 12.5 volts on it until the voltage drop gets close to 0.1 volt level. I will then check out the ground portions of the circuit similarly.
Your method may be better plus you also keep a record so you can reference back to it in the future - good ideas. Here in Southwestern Ontario where salt and chemical ice melting spray is used during the winter time I use a product called Fluid Film to coat connections, terminals, etc. to prevent and slow down corrosion - it has capillary action (creeps / migrates) into things and does not conduct electricity according to the manufacturer.

 

[29 Chev]

Hard to fix a vehicle when it isn't broken which happens sometimes when problems are intermittent. Looking at your test results a few of the numbers are on what I would consider borderline to where there could be a problem but not going to say there is - just my thoughts. At least now you have some good data to work with if the problem reoccurs.
My thoughts on the CAN Bus video is something does not look right. It looks like you have the positive probe connected to CAN bus high and the negative lead connected to CAN bus low but cannot tell for sure. CAN Bus high should start off at the 2.5 volts scale and swing high to 3.5 volts - the positive probe of the scope should be connected to it and the negative to a good ground. CAN Bus low should start off at 2.5 volts scale and swing low to 1.5 volts again the positive probe of the scope should be connected to it and the negative lead to a good ground. If the two signals were displayed on the same screen the CAN bus high and low signals should be a mirror image of each other. I am assuming that your scope is only a single channel so what you could do is connect it to the CAN bus hi signal and then pause the waveform and take a picture or video. Then do the same with the CAN bus low waveform - that way you will have known good waveforms to compare to if the problem reoccurs. Will look to see if I can find some more info to show you what the waveforms should look like.

 

[29 Chev]

Here is some information regarding the CAN Bus high and low signals

 

[29 Chev]

Hope that the battery replacement fixes your problem. It is possible that the battery was allowed to become badly discharged at some point before you purchased the vehicle and it that was the case it may have had an intermittent connection at one the plates. I have seen batteries that will work ok for several days or longer under those circumstances but the right combination of a rough road and other factors can result in an unstable 12 volt supply. Having said that do not be surprised if the problem reoccurs as it would seem strange that just the TCM was being a problem - however it could also be that the wiring going to the TCM is very close to the alternator output and a bad battery as the main load for the alternator can produce some strange "hash" and "noise" in a 12 volt charging system which could create some strange signals on the CAN bus wires. Just speculation on my part.