Hey guys, having a strange electrical issue that I can't seem to trace, that jus started happening out of the blue. 2 of the back light bulbs on my instrument cluster (oil pressure and left side of the speedometer) randomly flash feverishly, and so does the rear window washer LED on the let side (not in the instrument cluster). They are not timed or synced, and some flash slowly and some flash vigorously. Also, I assume it's releated because it started at the same time, but the fan speed UP button does not work. Everything else in the HVAC cluster works, but if I want to raise the fan speed I first have to set my temp to 90 degrees then hit the auto button and wait for the fan speed to adjust to full speed, then I can lower it back down in manual mdoe using the fan speed down button just fine. Any thing I can check? I've jiggled all the wiring harnesses, reconnected that battery after 10 minutes, inspected all the fuses, looked for .obvious shorts, etc, but can't find it. If it was a short though, wouldn't ALL of the instrument cluster LED's be doing the same thing? I am pretty sure they are all wired together in series. Anyway, I'm out of idea on what to check, and not even sure if a mechanic would be able to trce it. Any ideas would really be appreciated. Thanks.

 

What you describe does indeed sound strange.

It could be a bad ground connection, fluctations in the12V feeds, check fuses, battery, charging etc. i.e. something that affects all the systems you mention. A low 12V feed or connection somewhere could cause strange things.

Water ingress somewhere?

Pretty sure the HVAC buttons operate via the ECU so why the fan UP button does not work compared to the others is strange, makes no logical sense unless there's is some other underlying HVAC problem?

If your engineer has acces to a GM TECH2 he can check all HVAC buttons including "UP" (just double-checked mine) plus many other systems such as battery voltage etc, it may well help you.

Oddly I dont have a rear washer LED on mine - what does it do? (water heater maybe?) but I do have a fog light button just below the washer but this is aftermarket.

Hope this gives you some ideas, good luck!

 

Thanks. From doing a little more research, it sounds like maybe a bad Body Control Module (BCM), would that make sense according to the symptoms? It only seems to effect electrical issues inside the cabin, not headlights or marker lights or anything, so sounds like that could be the issue. If it was the BCM, would only a couple of the instrument panel bulbs flicker, or would the whole cluster go out do you think? The strange part is, All of those instrument panel bulbs are connected in series to each other, so if one lights up they all should, and vice versa I would think, but who knows, lol. I can pickup a used BCM for around $150, but I've heard getting to the unit for removal and installation is a pain in the butt, and I can't get the exact part number without removing the old one, as I guess there are several versions of it, so I hate to try to remove it if that doesn't sound like the issue. Thanks.

Oh, and the rear washer LED... I don't mean there's a separate LED below the switch... I meant the factory installed light bulb that back lights the symbols (just above the switch).

 

The pdf contains detail the instrument panel, if you search for "Instrument Panel" you'll find details on its connector (P15) and many other things. This might help you diagnosis. I noticed connector A11 is the "Instrument Panel Lamps Diming Supply Voltage" also look for "Dimming"

It could be dimming related, have you tried the "dimming control" thumb wheel to see if its working correctly?

Not sure why some bulbs would flicker, makes no sense as I imagine they are wired in parallel with each other.

Personally I'd hiold back on the BCM until I'd checked out the basics, it would be costly if it turned out to be something else.

 

Thanks. I'll have to see what I can find. Yeah, the dimming controls work fine, even when the lights are flickering, the dimming works (makes it a dim flicker, lol). And yes, I meant wired together in parallel, lol. I was thinking BCM because there was one day where the door lamps and map lights were also flickering the same way, although those stopped doing it after 1 day. That combined with the 2 instrument cluster lights and the rear washer switch backlight lamp all being affected without anything on the outside of the truck acting up leads me think it's something with whatever is controlling the interior cabin lighting, which I think is the BCM (other than the obvious fuse boxes, battery terminals and ground points, which I've checked). I was thinking of getting a BCM from an Amazon seller which has a 30 day return policy, and if it doesn't work I should be able to return it.

 

The BCM is quite possible and that is also one thing that often gets wet from leaks. It is way behind the left dash vent against the firewall. If you remove that left dash panel with the vent you can see the wires and plugs on it facing you. No idea how easy it is to actually remove though as I've never tried.

Also - a BCM must be programmed for the vehicle. When new they are blank and a Tech2 is connected, the VIN entered for programming and that programs the vehicles specific options into it. If it is used or from another model vehicle not all features might work as the other vehicle might have had different options. I've seen this before with places selling BCM's - they think that because many different GM vehicles use the exact same BCM part number that they are interchangeable but that is not really the case. If one from a donor vehicle has the same (or more factory options) it will work.