[Realmsteel]

I've been chasing a possible can-bus issue on a '08 EXT without much success.

The truck seems possessed at this point.
Codes:
C0242, C0900
U0073, U0100, U0101, U0121, U0140, U2100
P0327, P0513, P0521, P0575, P069E, P0700, P0703, P0826, P0856, P0989, P1682, P2544, P2610
I know that one bad module or bad can bus wire can disrupt the entire system and trigger codes from everywhere.
I've checked the terminating resistor at the rear and checked the one in the ECU and got 120ohms.
When I check resistance at pins 6 & 14, I get something completely different than the expected 60 ohms. It started out around 25 ohms with everything connected and changes with the more modules I disconnect. I've removed every module under the truck including the terminating resistor, BCM under the dash and the connector under the driver seat and I think I'm sitting at 70 ohms. The resistance would change for each module disconnected. With all that disconnected, I tried connecting my meter to the plug with the terminating resistor and the pins for the ECM and it shows open circuit which I would expect with both terminating resistors disconnected. I think I still show open circuit when I reconnected the rear terminating resistor. Why am I getting a reading inside the truck when both are disconnected?

A couple times the truck would not shut off for several seconds with the key off and removed. I replaced the ignition switch with no change.
Another goofy thing going on is with the key removed you can hear a cycling high-pitched noise and then some solenoid is being cycled inside the transmission.
Occasionally, it will start cycling the rear shock air pump.
I removed a relay in the harness under the brake master cylinder and all that stopped. The relay checked out fine. There is something cycling it.
The truck will also ding the key in reminder randomly as well, again even with no key in.

During idle, the tach and temp gauges flutter and the truck idles poorly.
It won't shift out of 1st gear.
The key reminder randomly dings.
I'm getting about 7.5v at each pin 6 and 14 (hi and low can bus) I didn't write them down, so I'm going off of memory.

We've read that a common problem is battery cables, so we ordered them to be delivered Friday. The battery checked out fine.
Until then, I think I'm going to reinstall all the modules and reconnect the transmission connector and double check voltage at pins 6 & 14.
I found a Bulletin No.: 08-07-30-021G and will be going through that, though the truck always cranks and starts up.

Has anyone run into this and have any other ideas or specific items to check?

 

[Cameron0588]

I've been chasing a possible can-bus issue on a '08 EXT without much success.

The truck seems possessed at this point.
Codes:
C0242, C0900
U0073, U0100, U0101, U0121, U0140, U2100
P0327, P0513, P0521, P0575, P069E, P0700, P0703, P0826, P0856, P0989, P1682, P2544, P2610
I know that one bad module or bad can bus wire can disrupt the entire system and trigger codes from everywhere.
I've checked the terminating resistor at the rear and checked the one in the ECU and got 120ohms.
When I check resistance at pins 6 & 14, I get something completely different than the expected 60 ohms. It started out around 25 ohms with everything connected and changes with the more modules I disconnect. I've removed every module under the truck including the terminating resistor, BCM under the dash and the connector under the driver seat and I think I'm sitting at 70 ohms. The resistance would change for each module disconnected. With all that disconnected, I tried connecting my meter to the plug with the terminating resistor and the pins for the ECM and it shows open circuit which I would expect with both terminating resistors disconnected. I think I still show open circuit when I reconnected the rear terminating resistor. Why am I getting a reading inside the truck when both are disconnected?

A couple times the truck would not shut off for several seconds with the key off and removed. I replaced the ignition switch with no change.
Another goofy thing going on is with the key removed you can hear a cycling high-pitched noise and then some solenoid is being cycled inside the transmission.
Occasionally, it will start cycling the rear shock air pump.
I removed a relay in the harness under the brake master cylinder and all that stopped. The relay checked out fine. There is something cycling it.
The truck will also ding the key in reminder randomly as well, again even with no key in.

During idle, the tach and temp gauges flutter and the truck idles poorly.
It won't shift out of 1st gear.
The key reminder randomly dings.
I'm getting about 7.5v at each pin 6 and 14 (hi and low can bus) I didn't write them down, so I'm going off of memory.

We've read that a common problem is battery cables, so we ordered them to be delivered Friday. The battery checked out fine.
Until then, I think I'm going to reinstall all the modules and reconnect the transmission connector and double check voltage at pins 6 & 14.
I found a Bulletin No.: 08-07-30-021G and will be going through that, though the truck always cranks and starts up.

Has anyone run into this and have any other ideas or specific items to check?

Sounds either like a bad broken/corroded ground strap or one of your modules are bad. Are all or almost all lights lit up on the dash when things act up.

 

[Realmsteel]

I've checked a couple ground connections, but I do plan on removing and cleaning all I can find.
I need to stop and buy some more wire brushes as what I have are all worn out.

I don't remember all the lights that lit up, but will check that when I get everything connected back up.
I was thinking disconnecting one module at a time to see if it settles down.

 

[Team ZR-1]

I would look first at the ETBCM controller or it's wiring and then the PCM
also check the voltage is correct level at the controllers and any ground splice packs

One controller with a problem might be yacking on the Starbus/GMLAN or CAN networks causing the other controllers to have a U comm DTC

C0242

Faulty Electronic Brake Control Module (EBCM) EBCM Harness is open or shorted EBCM Circuit poor electrical connection Faulty Engine Control Module (ECM)

When traction control is active, the Electronic Brake Control Module (EBCM). sends a serial data message to the Engine Control Module (ECM) requesting torque reduction.
When certain ECM DTCs are set, the ECM will not be able to perform the torque reduction for traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.

C0900 CADILLAC Possible Causes

• Faulty Control Module Power
• Control Module Power harness is open or shorted
• Control Module Power circuit poor electrical connection

The vehicle control modules or sensors monitor the system voltage to verify the system voltage is within the normal operating range.
The Powertrain Control Module (PCM) sets the OBDII code when the Control Module Power is not to factory specifications.

 

[Realmsteel]

A little update:
We disconnected and inspected just about every connection we could get to under the truck and inside the engine compartment.
I removed and cleaned every ground connection I could find as well.
I couldn't get the ground strap bolt off behind the driver side head. I'll need a 15mm crows foot which I don't currently have.
Under the dash, I disconnected the BCM and about every connector I could find.
We inspected all the wiring we could see and found no evidence of rodent munching or chaffing.
I made sure to charge the battery overnight.
We reconnected everything back up and started the truck.
It was started to do it's goofy stuff and then settled down.
I read the codes and now the only ones left are Antolock C1100 and C0455
The stabilitrack light is on, but the truck seems to drive perfectly normal.

I just watched a video where a guy discovered a short caused by a stud on the engine chaffing the wiring on the driver's side. I'll need to check that to make sure that's not a cause in our case.

Tonight I will take the truck on an extended test drive to see if anything changes.

 

[Realmsteel]

Further and hopefully final update:
Last night we put everything back together.
Two of the 3 battery cables came in so we decided to go ahead and put them on since we hadn't bolted the engine compartment fuse box down yet.
We replaced the negative cable and the cable leading to the fuse box that included a new 175A fuse block and fuse. We're still waiting on the positive cable to the battery.
During the install of the negative cable, we discovered another ground connection at the front of the frame passenger side where that cable connects to.
It was pretty oily and dirty and a major pain in the butt to get to. We ended up pulling out the turn signal housing and it was still a pain.
All in all, I think there are 8 ground connections we cleaned up.
We also double checked the connections for the wheel sensors.
I didn't see any issues with the wiring over the top of the engine like the guy in the video I mentioned above.

We started it up, and it ran perfectly with no trouble lights on at all.
Took it for a lengthy test drive and it seems to run better than before the issues popped up.

Since we went through so many connections, I can't say definitively what the actual fix was, but I'm going to have to go with try replacing the battery cables first and then go through all the ground connections under the truck and in the engine compartment.
We joked that it may have been the hood ground wire since one of the connectors fell off when I went to remove it. Heck, maybe that was it.

If anything changes, I'll update this thread.
Hopefully, someone with a similar issue will find this thread and try the battery cables first.

 

[EricVonHa]

Further and hopefully final update:
Last night we put everything back together.
Two of the 3 battery cables came in so we decided to go ahead and put them on since we hadn't bolted the engine compartment fuse box down yet.
We replaced the negative cable and the cable leading to the fuse box that included a new 175A fuse block and fuse. We're still waiting on the positive cable to the battery.
During the install of the negative cable, we discovered another ground connection at the front of the frame passenger side where that cable connects to.
It was pretty oily and dirty and a major pain in the butt to get to. We ended up pulling out the turn signal housing and it was still a pain.
All in all, I think there are 8 ground connections we cleaned up.
We also double checked the connections for the wheel sensors.
I didn't see any issues with the wiring over the top of the engine like the guy in the video I mentioned above.

We started it up, and it ran perfectly with no trouble lights on at all.
Took it for a lengthy test drive and it seems to run better than before the issues popped up.

Since we went through so many connections, I can't say definitively what the actual fix was, but I'm going to have to go with try replacing the battery cables first and then go through all the ground connections under the truck and in the engine compartment.
We joked that it may have been the hood ground wire since one of the connectors fell off when I went to remove it. Heck, maybe that was it.

If anything changes, I'll update this thread.
Hopefully, someone with a similar issue will find this thread and try the battery cables first.

Thanks very much for sharing your troubleshooting. Electrical gremlins are a total pain, for sure. Intermittent electrical gremlins are the worst from a troubleshooting perspective because it comes down to "what is it not now?"

The engine bays in these trucks were never properly sealed from driving in the rain/snow/muck. I was frustrated with mine because I installed some wheel well curtains to cover the gaps between the frame and upper control arms. Only to find that the muck was still messing up the motor compartment due to crap getting past the openings of the poorly designed belly pan/shroud just below the crank pulley. Also put some additional hood insulation to stop muck from going over the top of the plastic radiator shroud and into the motor compartment. Long story but...

GM screwed the pooch. Weatherized electrical connectors are only weatherized until they are not. Then, all bets are off.

Hopefully your ghost stays away! Thanks again for sharing.