'03 Avalanche body module issues

I am helping my folks renovate a house and move into it. For the next few weeks I am using their spare pickup which is a 2003 Avalanche which has some issues I would love to help them out with if I can.

They took it to the Chevy dealership in their podunk town who diagnosed a bad body module, attempted to replace it and were unable to program it stating that their tool couldn't do so since the ECU would never finish the error scan and give the readiness signal to be able to set up the new body module, and that it was probably the aftermarket stereos fault. My folks paid their not insignificant bill and got the pickup back exactly as they delivered it.

As I have been driving it for the last week what I have noticed is that the seat heaters come on at random, the blower motor works only sometimes (when it does all speeds function correctly), about 70% of the time when you start the truck it will be in 4wd low and on one occasion switched from 2wd to 4 low while sitting at a red light. Pops says there are a few more funky things it does as well.

I have noticed things got worse after bombing down a particularly rough road and getting quite bounced around.  This makes me think more along the lines of a bad connection somewhere.

I plugged in the older Mac OBD2 reader at the shop and it said "no faults found". 

What I am hoping here is that this is a common enough issue that someone knows where I should start looking. Having never touched a CANbus system I am hesitant to just start tearing the thing apart.

Those can be fun.  Make sure your power supply and grounds are good. Unplug connectors at the bcm and look for green.  Follow harnesses and look for rub through.  Clean up the grounds at the frame, just beside the abs pump.  Make sure the battery is good, and the voltage is stable.

A bcm problem can come from any module it talks to, so that means everything... The seat heater and blower problem seem to suggest heater control, but the 4wd problem...

I did swap the body module using the 30 minute key on key off method. It fixed nothing. 

All grounds checked and cleaned.  Blower fan was a bad fan resistor pack. When the drivers window is cracked in the rain I can watch drips fall as if they are intentuonally channeled right on the window/lock/seat heater switch unit. Nice design GM! So now I am thinking more along the lines of a litany of small failures.  

I was foolish enough to actually buy this Avalanche from them. I..... I have no good explanation for this. I don't like the thing.

A.C. clutch doesn't engage. Have 5v reference at the low pressure cutout switch and jumpering it doesn't turn it on. I need to check for voltage at the clutch  but it is so damn deep in the bay. 

I wonder how hard it is to come up with a good wiring diagram, and trying to sort out a strategy, like going after items that are just "dumb" switches vs actual modules.

An aggravating CANbus thing I hadn't thougth about much is that you can't just remove an item to see if the other things stop misbehaving, right? Or maybe you can; presumably, as I think Keith mentioned in one OBD/CAN discussion, you don't want to have to call a tow truck if something HVAC goes splat; the question is, can something spouting gibberish and causing problems be removed and stop the problems? I guess finding out which bits are CAN modules and which are just switches would help think about it.

I wonder which category the cluster at the bottom of the rain drain is...

I'm thinking that seat switch might not be CAN: If it were, it shouldn't need all those wires; it would just send all its messages on the single bus. (Not sure whether yours has the adjustable pedals, but it seems odd that the entire switch type would be CAN or not based on that difference, or that the simpler version would be CAN and the more involved one not...)

I suppose it could be a bunch of wires from the switch to the BCM or a sub-board... Back to wanting a manual, but I'm crossing my fingers that individual splatted components can be switched out. Maybe try the seat heater switch and see if that one item stops misbehaving? Pick the cheapest misbehaving control?

Jumper K Balls said:

I have noticed things got worse after bombing down a particularly rough road and getting quite bounced around.  This makes

me think more along the lines of a bad connection somewhere.

Check the fit of all the module connectors.  Helped a friend track down a random misfire on his Subaru.  The ECU connector did not fit snugly and would lose connection on a couple pins if bumped.  Some tape to hold it in place solved the problem.  Another one of those 6 hours to find and 2 minutes to solve problems.