2002 Duramax TCM No Comm
Duramax with Allison Transmission. No communication with TCM. Lose all network communication when shifted into neutral or drive, as seen in the attached video. The ECM on this truck has it's own network with the TCM and FICM separate from the rest of the network. So you need to communicate through the ECM to get to the TCM or FICM. There is no issue talking to the GPCM. I have load tested all power feeds and ground at the TCM, ECM, and GPCM. I have also verified communication lines. Donor ECM and TCMs have been tried. The video below is where I have left off my testing, you can see the communication bus come off ground when shifted into N or D. Unplugging the TCM will also correct the communication signal. Any thoughts?
Had a similar issue. Bad ignition switch. Voltage to Tom from ignition switch would drop down to 2 volts. Back probed wire and found issue.
I would disconnect the trans connector and see if the fault is gone if I am understanding your issue correctly. Would help to have a wiring diagram of network and TCM and transmission. Also I'm confused, in the beginning of your post you say when shifted too neutral or drive you lose all comm. However at the end of your post you say the signal “comes off ground” when shifted to neutral or drive…
I misspoke by habit, I meant to type com not CAN. It is Class 2. With the TCM unplugged, I do not have any communication issues. If you watch the video, you will see the signal change from 0v - 7v to about 2v - 7v when in N or D.
Travis. That truck has Class 2 communication at terminal 2 of the DLC. Follow the purple wire to terminal A of SP 205 which is higher up, but not far from the DLC, if it hasn't been messed with. It is up in the direction towards the headlamp switch. All of the Class 2 branch circuits culminate at splice Pack SP205, so you can check each circuit from there. The scop is showing Class 2 data…
I misspoke by habit, I meant to type com not CAN. It is Class 2. I did correct that in my original post. The PRNDL doesn't show anything since the TCM is non communicating.
Here are the ECM TCM FICM CAN network diagrams
So to clarify as Martin also mentioned. The ECM is on the class 2 network but the TCM and FICM are not. The ECM communicates with the TCM and FICM on a separate CAN network that is not connected to the OBD2 or any other modules. TCM is always dead Class 2 goes dead (scope capture shows 2v pull up from 0v) in N or D unless TCM is unplugged. My video scope capture is from the Class 2.
Thanks, I would disconnect PNP switch with it running and see what happens to the waveform then. If no change then disconnect the connector for the internal trans harness and see what happens.
Done a lot of testing to this one with a long break in the middle. If I remember correctly, I do not lose class 2 if either one or both are disconnected. I just don't see any connection between anything on the Transmission and the ECM but obviously there is a connection I am not seeing. Seems like a back feeding ground or poor ground. I will retest this though. Thanks.
I can add that with the transmission or the PNP switch disconnected does not bring back TCM communication.
As Caleb recommended, start at the NSBU switch and see what happens. As I mentioned before, the switches can be problematic and also the internal solenoid and NSBU wiring is going to be hard due to age. BTW, replacement NSBU switches have an alignment tool on the outside, that is removed once installed over the shift shaft. Same as on 4L60/65E and 4L80E FWIW, the trans pan has a drain plug…
Travis, I know you mentioned load-testing the grounds, but it sure seems like the PRNDL logic circuits are loading the ground shared with the ECM and TCM causing the ground shift on Class 2. I'd still have a look at it to make sure there's no greenies growing around it. Its on the driver's side of the block near the oil pan rail.
Done a lot of testing to this one with a long break in the middle. I believe I even backprobed and direct ground, load tested the ground, and watched the ground with a scope at the TCM and ECM but since I am starting fresh I will recheck.