08 3500HD, Suspect BCM failure. not pulling things to ground

[bobejones]

Truck is an 08 3500HD, crew, 4x4, 6.6, LTZ

 

2 separate issues.  Will try to be brief so its easier to read.  

 

1.  Fog lights won't turn on.  All conditions met to turn on such as light switch on.  Removed push button switch and verified momentary switch operates normally.  Removed Fog lamp relay and confirmed 12V at two pins of the relay (cannot recall numbers right now).  One pin should pull low (to ground) by the BCM.  Verified that is not happening.  Anything else I may check?  Can a Snap-On scan tool access BCM functions as a Tech II will to see switch states and commands?  

 

2.  Further testing of devices revealed a RR window that will roll down but not up at the drivers door switch.  RR door switch will do neither operations.  Switch operation at the drivers door has an audible relay click in RR door for Up nothing for Down.   I originally suspected just the BCM but further inspection showed the BCM command going back through the RR door switch?  If so, does this sound more like a door switch issue?  Can I swap switches from the LR door for testing?  

 

 

If BCM is confirmed to have issues, can I find another BCM with the same part number to swap in or is it certain that a replacement BCM will need programming?  All other functions so far seem to work correctly.  

 

Also, just on a side note, I have removed the BCM and opened it for inspection.  Diagrams refer to the pinouts from 7 different connectors, X1-X7.  However, I cannot find labeling anywhere on the wires, plugs, or BCM.  Can someone confirm which is which?  I was going to inspect the Fog circuit on the PCB if I can determine the correct connector.  The pin callout for this on the diagram is plug X7, pin 11 I believe. 

[Jsdirt]

Possible you just have 2 faulty switches. The window switches are known failure points on the 900's - anyone who smokes is lucky to get 50k miles out of them.

 

You said fog light switch operates correctly. Did you hook it to a load (halogen headlamp) to verify that?

 

BCM most certainly needs programming if replaced - thank GM for that brainstorm. They have to be programmed to whatever options your vehicle has - they're all different. Especially if new (they come blank). Plus, I'm fairly certain that your VIN will need to be entered into it too. You need a J-box & a laptop to accomplish that - no scan tool has that capability. Any bi-directional scan tool can perform a crank relearn, or whatever else is necessary after programming. I'm a rookie in that area, so don't quote me on any of this, but I have done ALOT of research on the subject.

 

When I can't find connector labeling or wire numbers, I just go by wire color to verify I'm on the correct plug. Usually 5 wires across will be enough to let you know you're on the right one, if all the colors match.

[bobejones]

Well, I worked with it a little more today.  In doing some swapping of switches in the back doors, I determined the switches are the culprit.  So I disassembled the rear switches and surgically cleaned them.  That got me good functioning switches in the back.  That also got the drivers door switch running everything except the LR window up function.  I I tore into the drivers door switch.  a LOT more going on there.  The contacts were actually very nice, but I found a few suspects on the PCB.  Some corrosion on several resistors and transistors. I may try to reflow a few things but doubting it will come around.  So may be in search of a drivers door switch unit.  I am sure that will be cheap......not.  

 

On the Fog issue I jumped the wires at the switch harness, no go.  Then rang out the output wire from the BCM to the fuse block.  Everything checks out.  Appears to be a shit circuit in the BCM.  Unless I can chase something else down.  Maybe a previous owner had it changed and it was not programmed to operate the fogs?  

[Jsdirt]

Yeah you never know. That's a common mistake during programming, especially if the tech has 10 other things going on at the same time, and the truck wasn't his sole focus. Should be able to tell once the J-box is hooked up, and get on GM's site. Might even be able to tell with just the scan tool - if that parameter is missing, or dead, then that's probably why, aside from a BCM failure - that's not all that uncommon either.

[SpartanAutoworx]

Im very well versed in the BCM for the GMT-900 Truck.

If you need to replace the bcm it will have to be programmed to the car.  Get the new GM Part number for the replacement 25835966 which is also interchange 1290. Plug and play Solutions are available just have to do a little internet searching in order to save the trip to the dealer. ;)  Also 2008 requires no programming of the fog lights.