BCM options. It won't turn on fog lights. And prove climate control unit is the problem

bobejones · August 29, 2022

truck, 2008 3500hd LMM, LTZ, bought used

2 Separate issue. throwing them in the same thread since I rarely get much help on this stuff. Didn't want to waste the air space.

1. Confirmed that BCM will not turn on fog lights on. Switch confirmed to work, and BCM is required to switch a relay under hood. It will not send output to relay. Wondering if there is a way to reference the numbers on BCM to find a salvage piece? I won't pay to reprogram because I don't know if that is really an issue. I am in electronics but without the schematic, the PCB loses me fast so I can't trace the circuit otherwise to verify anything. I do know for a fact the dash switch sends power requesting lights to turn on.

2. For years I have fought with the vent selector actuator switching to default defrost. If in AC dash vent mode, truck VERY consistently starts up with the correct vents, then switches to defrost within 1-10min of running. If I stop the truck and restart it (drop power to climate unit), it will go back to the correct vent setting and will never change the rest of the trip. In many cases, I just leave the truck running to avoid the hassle. For this reason, I am sort of leaning towards the climate dash unit rather than actuator. As an aside, climate unit many times will switch to 'fresh air' mode instead of 'recirc' even though cabin temp is no where near controlled.

I will say that even in heating mode, the vent selection issues continue. I have not found anyone with similar symptoms to relate this issue.

davester · August 29, 2022

Did you backprobe the pin on the bcm to verify that the bcm is getting the signal from the switch, and that pin going to the relay isn't switching?

And perhaps check ALL the bcm fuses, that they all are good. The bcm gets multiple power and ground connections, and if they don't all get power/ground properly, the bcm may not operate properly.

davester · August 29, 2022

I would suggest getting a knockoff tech ii, both to see what the bcm is doing (it can show what signal the bcm is getting from the switch, and can directly command bcm to turn the fog lights on/off. And depending on how computerized the hvac hu is, it can get codes from the hu, and help with diagnosing what it's up to.

And a subscription to the full-service manual for your truck also would help, it'll have diagnostic procedures for figuring out both problems.

bobejones · August 29, 2022

I will have to eat shi* on the BCM deal. I determined the dash switch had issues and was not pulling logic power down to ground good enough. I reworked the switch and the fog issue is considered 'fixed', or at least the dash switch was the confirmed issue. Have had a LOT of switch problems in this truck. I have had to tear down every door switch in the truck.

Anyway, this now leaves the HVAC. I have never seen a knock off TechII, but would you have a link? I would certainly be very open to just owning one. I do have contacts to borrow a real techII, but always a hassle. Obviously most OBD goodies don't work outside of emissions related stuff.

davester · August 30, 2022

Here are some:

https://www.chinacardiags.com/wholesale/gm-tech2-gm-diagnostic-scannerworks-for-gm-saab-opel-suzuki-isuzu-holden-1011.html

They are also on ebay & amazon.

I can't recommend any particular setup, as I'm still researching it myself (I expect to soon get a second truck that would also work with a tech ii, and then I would get it, I currently use Autogenuity w gm enhancement which covers most diagnostic&testing stuff, just no programming).