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Automotive Can Bus problem. Who knows about it?


JL Sargent
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I know we have some members in the automotive industry and I'm looking for some help with a weird problem. I have a 2007 GMC Sierra 5.3l  (not a classic) with a no crank no start after a heavy foggy type rain. This only happens after a heavy rain too. If it's sunny no problem what so ever. I have GMs tech 2 scan tool and when a heavy rain with lots of fog happens and truck acts up,  I lose communication on a lot of modules in the vehicle. I can check status of each module and look for codes on those and many go "no comm" after one of these rains.  I suspect the bus is going to ground or getting severly contaminated. I have checked powers and grounds at battery and ecm and those look good. My next try is to disconnect the EBCM which is under the truck and bolted to the frame since it is in the weather somewhat. Any help with it appreciated!

 

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Check the bulkhead at the firewall and all the connections related.

 

If you end up with a cannot duplicate ticket from the dealer use a garden hose pinpointed in small areas to try and get the problem to happen.

 

Beyond that its hard enough to diagnose many issues even with the vehicle right in front of you.

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11 minutes ago, jason str said:

its hard enough to diagnose many issues even with the vehicle right in front of you.

No doubt about it. I'm hoping maybe somebody has seen this problem before?

 

8 minutes ago, CECAA850 said:

See if all the modules lose communication with one particular module.

Yeah, so what seems to be happening is that when the problem occurs, the bus goes down and I lose about everything including instrument panel, radio, On star or comm, hvac module, brake control, etc. I guess I need to locate a data junction box and add each module by splicing them in one at a time until I lose communication again. Then that would be the bad module. Sounds good anyway.

 

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I know a little enough about this to be dangerous. Sounds like you are headed in the correct direction. One of the devices on the bus is screwing up communication on the bus when cold/wet. Do you know or have a diagram that shows all devices on the bus? When you're getting no comm start unplugging devices one by one

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16 minutes ago, JL Sargent said:

Yeah, so what seems to be happening is that when the problem occurs, the bus goes down and I lose about everything including instrument panel, radio, On star or comm, hvac module, brake control, etc. I guess I need to locate a data junction box and add each module by splicing them in one at a time until I lose communication again. Then that would be the bad module. Sounds good anyway.

 

You can't access the exact codes in each module?  They should show what module they lost communication with.  The suspect module probably won't have lost communication codes in it.

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2 minutes ago, CECAA850 said:

You can't access the exact codes in each module?

I can, but most modules are not giving any codes. I get 10 codes. These codes are the reason I think the EBCM could be the culprit.

 

The EBCM has 4 codes, U0101 lost comm to TCM, U0100 lost comm to ECM, U0140 lost comm to BCM, C0561 system disables info stored

 

The BCM has 6 codes, U0200 Lost comm to door module,  U0199 lost comm to door module A, U0198 lost comm to telematic control module, U0155 lost comm to instrument panel, U0184 lost comm to radio , U0164 lost comm to HVAC module.

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1 minute ago, jason str said:

Everything runs right through the junction box i mentioned in my first post.

So this junction box is in the bulkhead? Inside the cab or under the hood? I thought it might be near the DLC. Are you saying visually check those connections for corrosion, etc. or breaking the junction apart and test for communications on each wire to its respective module? I wish I knew which wire went to which module.

 

 

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2 minutes ago, JL Sargent said:

So this junction box is in the bulkhead? Inside the cab or under the hood? I thought it might be near the DLC. Are you saying visually check those connections for corrosion, etc. or breaking the junction apart and test for communications on each wire to its respective module? I wish I knew which wire went to which module.

 

 

You're looking for see water intrusion or corrosion.

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2 minutes ago, CECAA850 said:

You're looking for see water intrusion or corrosion.

 

1 minute ago, CECAA850 said:

The bus should go to each module and be the same color set of wires at each.

OK, thank you. Now if it would just QUIT RAINING!!! I'm sick of all the rain. We have gotten 12" of rain here in February.

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On 2/24/2020 at 1:02 PM, CECAA850 said:

Where is it located?

I do know where that is and it's under the hood and kinda mounted to the back of the ECM. It looks dry as a bone, the module itself, but could be.

EDIT: This is under the instrument panel near the DTC connector.

 

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There is a junction for the harness that runs through the firewall on many model GM's, not sure about the Chevy truck through the years.

 

There is 1 screw that holds the harness together, remove it and check the connections on both sides.

 

Follow the leads as mentioned and look for corroded terminals, rubbing harness wires & such.

 

Bad ECM's can do crazy things as well but if its weather related its good to look there first.

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