06 Envoy Denali - Code U0101

HardTimes

Member
06 Envoy Denali 4wd - 150k - 5.3l. Under normal driving conditions around 45mph vehicle went into limp mode. Speedo stopped working and Gear Indicator stuck on "1". Limped back home and hooked up my scan tool to see if any pending codes were present as MIL was not on. U0101 Lost Communication with TCM is only thing present. I searched the web for ideas. Checked for obvious signs of broken damaged wires/connectors, all look fine. Pulled apart both fuse blocks to check the traces, all are fine and have continuity. Checked pin 6 and 14 on DTC, reads 60.8ohms. Have continuity on GMLAN from TCM to ECM. Replaced TCM and ECM with both used and Dealer New units and no fix. Changed ignition switch as well. I have traced all grounds and cleaned mounting points. I'm not sure where to go next, any ideas would be greatly appreciated.
 

Mooseman

Moderator
I would imagine you also checked all the grounds? Is power getting to the TCM? Do you have access to a high end scanner like a Tech 2 or Snap On Solus? Sounds like this is a hard one to solve.

Maybe try disconnecting the connector to the transmission and see if the TCM starts communicating again. Could be something shorted in the tranny.
 
OP
OP
HardTimes

HardTimes

Member
I downloaded the schematics for the grounds. Body and chassis and checked all. My scan tool is a basic unit that reads codes and IM Readiness. I will pull the trans connections to see what I get, I did pull the neutral switch plug and reseat previously. I failed to mention since it seemed the trans ran fine after limp mode engaged, I had the trans rebuilt not 2000 miles ago.

Thanks for the reply, I will update once I pull trans connections. At this point I would rather deal with a mechanical failure than electrical!
 
OP
OP
HardTimes

HardTimes

Member
When trans harness is unplugged code P0700 (Trans System MIL Request) shows up along with the U0101. Plug back in P0700 goes away. Just for kicks and giggles I swapped out the TCM I got used since it was non-returnable and I get U0100 code instead of U0101 but the gear indicator works as it should. While the trans will go into R N D when trying to accelerate in D the trans doesnt know what to do. Complete opposite of what it does with original TCM and the dealer new TCM. Truck drives fine on original TCM other than being in Limp Mode and without speedo or gear indicator.
 

Mooseman

Moderator
OK, try disconnecting the PCM and see if it's a comm loss to the TCM. How are they connected according to the schematics?
 
OP
OP
HardTimes

HardTimes

Member
They are connected by GMLAN SDS Tan and TN/WH. Pin 1 & 2, circuit 2500 & 2501 on ECM C1 and Pin 37 and 38 on TCM. I have only done a continuity test on these wires to see if they were broken. Should I be doing another test with these?20190915_093346.jpg
 

Bow_Tied

Well-Known Member
Wow, sounds like a tough one. I have no idea. Good luck, looking forward to hearing the fix.
 

budwich

Well-Known Member
you can try some other tests... first "looking one way" then the other. By this, I mean disconnect from the tcm, on the cable connector at the pins you identified, do a resistance measurement across the pins... this will check the path thru the cabling to and including the PCM... depending on what's inside the pcm circuit. In general, you aren't necessarily looking for an "absolute" value but to see if you see resistances of some form. You can do the same test, reversing the leads as this may change the reading IF there is some active component in the pcm (ie. diode or such). Note these should be carried out without powering. You can do the same tests in the "opposite direction" (ie. at the PCM end) and compare the results. They might show differences that may be obvious... ie. very high or very low readings in a certain directly. The test can also be done on one pin only to a body / good ground. All these kind of try to check an "unknown circuit design" / "black box". Further since you have another module you can do further comparing.

NOTE: because the circuits at the end are somewhat unknown, things like capacitors and such may cause the reading to slowly change as they charge / discharge... no a problem, but note / observe what happens.

Perhaps some other tests when powered can be done BUT the pins are close / difficult to isolate so you should be careful to avoid shorting and causing more problems.... probably leaving these types of tests as a last resort.


A quick search of the net shows this.... not sure if is applicable but some aspects may apply to help understand what your measurements are seeing.
 
Last edited:

seanpooh

Well-Known Member
With the new modules you obtained, did you have them flashed for your truck? They shouldn't be plug and play for your VIN. I always program a module if it's replaced.
Also a U0100 should be a history code indicating there was a loss of comm at some point in time.

Just as an example: I worked a Caviler (whatever model year it was) that had the same symptoms as you described, still can drive it but no gears & code was set lost comm with TCM. I go through all the things I mentioned below. Just to be sure, my group leader had a working spare TCM for a Caviler for whatever reason which I plugged in. Comm was restored but couldn't drive it because it wasn't programmed to that car.

Below is just an insight of what I do when I have comm codes for a customer's vehicle at the stealership.

After making sure the power and ground is good to the module, we had the dataBUS tool to show all of the modules on the GMLAN lines (high & low). It would monitor comm until a module drops out. It gave a general idea of what pin, + or - at what module is being lost. Wiggling harnesses, looms and connectors while watching the tool helped a lot finding the short.

Also, testing continuity isn't enough to tell if there is a break in the wire. It's my personal opinion and I've gotten burned on a few jobs because I believed 0 ohms was good enough. I'd disconnect both modules and see if the circuits will light up a bulb. Then I'd verify pin tension.

If you can't find anything, create an overlay backprobing the two comm lines. If there is no change with the overlay, it's the module.
 
OP
OP
HardTimes

HardTimes

Member
you can try some other tests... first "looking one way" then the other. By this, I mean disconnect from the tcm, on the cable connector at the pins you identified, do a resistance measurement across the pins... this will check the path thru the cabling to and including the PCM... depending on what's inside the pcm circuit. In general, you aren't necessarily looking for an "absolute" value but to see if you see resistances of some form. You can do the same test, reversing the leads as this may change the reading IF there is some active component in the pcm (ie. diode or such). Note these should be carried out without powering. You can do the same tests in the "opposite direction" (ie. at the PCM end) and compare the results. They might show differences that may be obvious... ie. very high or very low readings in a certain directly. The test can also be done on one pin only to a body / good ground. All these kind of try to check an "unknown circuit design" / "black box". Further since you have another module you can do further comparing.

NOTE: because the circuits at the end are somewhat unknown, things like capacitors and such may cause the reading to slowly change as they charge / discharge... no a problem, but note / observe what happens.

Perhaps some other tests when powered can be done BUT the pins are close / difficult to isolate so you should be careful to avoid shorting and causing more problems.... probably leaving these types of tests as a last resort.


A quick search of the net shows this.... not sure if is applicable but some aspects may apply to help understand what your measurements are seeing.


Thanks for the info. I was unable to dedicate much time this weekend to troubleshooting, I will try these things along with what seanpooh recommended.
 
OP
OP
HardTimes

HardTimes

Member
With the new modules you obtained, did you have them flashed for your truck? They shouldn't be plug and play for your VIN. I always program a module if it's replaced.
Also a U0100 should be a history code indicating there was a loss of comm at some point in time.

Just as an example: I worked a Caviler (whatever model year it was) that had the same symptoms as you described, still can drive it but no gears & code was set lost comm with TCM. I go through all the things I mentioned below. Just to be sure, my group leader had a working spare TCM for a Caviler for whatever reason which I plugged in. Comm was restored but couldn't drive it because it wasn't programmed to that car.

Below is just an insight of what I do when I have comm codes for a customer's vehicle at the stealership.

After making sure the power and ground is good to the module, we had the dataBUS tool to show all of the modules on the GMLAN lines (high & low). It would monitor comm until a module drops out. It gave a general idea of what pin, + or - at what module is being lost. Wiggling harnesses, looms and connectors while watching the tool helped a lot finding the short.

Also, testing continuity isn't enough to tell if there is a break in the wire. It's my personal opinion and I've gotten burned on a few jobs because I believed 0 ohms was good enough. I'd disconnect both modules and see if the circuits will light up a bulb. Then I'd verify pin tension.

If you can't find anything, create an overlay backprobing the two comm lines. If there is no change with the overlay, it's the module.

Makes sense about the flashing of the modules. I didn't do enough research other than taking the answer of "No, these are already programmed" of the dealership when asked "Do these need to be programmed to my vehicle?" I had contemplated making an overlay as you mentioned to see if that made a difference so I think I will go ahead and try that. Thanks for information, it is al greatly appreciated!
 
OP
OP
HardTimes

HardTimes

Member
I would imagine you also checked all the grounds? Is power getting to the TCM? Do you have access to a high end scanner like a Tech 2 or Snap On Solus? Sounds like this is a hard one to solve.

Maybe try disconnecting the connector to the transmission and see if the TCM starts communicating again. Could be something shorted in the tranny.

I think I may spend some time reading the write-ups in the scan-tool section. I'm leaning towards adding to my toolbox.
 
OP
OP
HardTimes

HardTimes

Member
I checked all wires again, everything seemed fine. Created an overlay like recommended and still the same. I'm leaning more towards the TCM is bad. I dont understand why my com issue goes away with the used TCM but comes back with the dealer new unit, that's my hangup. Will the unprogrammed unit from the dealer give me the same symptoms until its programmed?
 

Mooseman

Moderator
Possibly. It does need to be programmed. Should be doable in the vehicle.
 
OP
OP
HardTimes

HardTimes

Member
The used unit can be reprogrammed as well I'm guessing? If I can save $200+ utilizing the used unit that would be nice.
 

Mooseman

Moderator
Yes a used unit can be reprogrammed.
 
OP
OP
HardTimes

HardTimes

Member
You guys have been most helpful and I cant thank you enough! I will figure this out sooner or later.
 

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