GM fog light kit, ? about reprogramming BCM

1999GMCZ-71 · June 18, 2003

I have a 1999 GMC Sierra Z71 (new style). I am installing the GM foglight accessory kit and it says you have to reprogram the Body Control Module. What is involved in this, and can it only be done by a dealer?

silverado man · June 18, 2003

You have to take it to the dealer to reflash your BCM.

AZ-Z71 · June 18, 2003

I've already done this to my 99. First I hooked up the light kit then I tried them anyway to make sure I had to go. Unfortunately I had to go to the dealer. Supposedly what they do is down load some programing from your truck, send it to GM get a program back and upload it back to your truck. It only takes them like a half hour for the process and they did mine on a late Saturday afternoon.

The part that sucks, unless you are in good with the dealer is you have to pay them an hour worth of labor charge. An hour was the minimum charge the dealer I went to would charge. So it also cost me and additional $90. Ask me and that is highway robbery. Another dealer I asked to do this previously told me $60 but it was late in the afternoon and I wanted them installed now and didn't feel like driving the 20 miles to the other dealer, plus I needed an appointment for the $60 dealer.

Oh well that is what you pay for instant gradification.

Torp · June 19, 2003

My dealer told me last week it would be $128 parts and labor to add the factory foglights to my '03. I assumed that would be lock, stock and barrell, so I got an appointment for tomorrow afternoon.

GnatGoSplat · June 19, 2003

Before my full-power upgrade, I built a circuit that I soldered into the BCM to make the foglights work with the factory switch. It worked almost the same as factory, but the big difference is it had memory so when you turned off the ignition, the foglights will still come on next time you turn on the lights. It cost me about $5 worth of parts from Radio Shack.

I have since upgraded to full-power everything, so I ended up swapping the BCM out for a 2000 that I got off Ebay. It supported the foglights right away, although the fact that the foglights reset is pretty annoying, so I'm going to disable the BCM's built-in foglight circuit and solder mine back into the new BCM.

1999GMCZ-71 · June 19, 2003

Before my full-power upgrade, I built a circuit that I soldered into the BCM to make the foglights work with the factory switch. It worked almost the same as factory, but the big difference is it had memory so when you turned off the ignition, the foglights will still come on next time you turn on the lights. It cost me about $5 worth of parts from Radio Shack.

I have since upgraded to full-power everything, so I ended up swapping the BCM out for a 2000 that I got off Ebay. It supported the foglights right away, although the fact that the foglights reset is pretty annoying, so I'm going to disable the BCM's built-in foglight circuit and solder mine back into the new BCM.

i'd like to know exactly how you did that. Do you have schematic and a parts list from radio shack. Thanks alot! e-mail me

chevyman992002 · June 20, 2003

If I was to add an aftermarket set of foglights, would I have to have the computer programmed? I have a base model silverado which never had the fogs from the factory.

MattDewalt · June 20, 2003

GnatGoSplat, I'm interested in what you did too. I added factory foglights to my truck with the factory switch. They did not work. So, I removed the wire for the foglights that goes into the BCM and then the line that comes out of the BCM (I just removed them from the plugs). I then connected the two together in an attempt to bypass the BCM. Well, my foglights now work great. But the only problem is that the factory switch does not have both on and off positions. I gues it just sends a signal to the BCM and then the BCM actually turns the lights on and off. So, they work great but I have to hold the switch in to keep them on. As soon as I take my finger off the switch, they turn back off. I was thinking of wedging a toothpick in the switch to keep them on.

1999GMCZ-71 · June 20, 2003

GnatGoSplat, I'm interested in what you did too. I added factory foglights to my truck with the factory switch. They did not work. So, I removed the wire for the foglights that goes into the BCM and then the line that comes out of the BCM (I just removed them from the plugs). I then connected the two together in an attempt to bypass the BCM. Well, my foglights now work great. But the only problem is that the factory switch does not have both on and off positions. I gues it just sends a signal to the BCM and then the BCM actually turns the lights on and off. So, they work great but I have to hold the switch in to keep them on. As soon as I take my finger off the switch, they turn back off. I was thinking of wedging a toothpick in the switch to keep them on.

kinda defeats the purpose of the switch. My dealer said it would be 30 to 45 dollars to reprogram the BCM, and take about 1/2 hour. I didn't think that was to bad.

GnatGoSplat · June 20, 2003

I can't remember if I already drew up a schematic for it or not.

If not, since I have it out of the truck right now, I'll draw one up.

It's a pretty trick circuit and turns that momentary switch into a latching switch that latches on or off. Works like factory but has memory.

I had one "problem" with it - the foglights didn't come on in Park for some reason. It didn't have anything to do with the circuit, I'm sure it was due to where I soldered it into the BCM. I soldered it in the BCM because it's a bare circuit board and I didn't have any other place to put it, plus I didn't want to cut or splice the factory wiring.

I'll get some pics too.

cyclejerk · June 20, 2003

Gnatgosplat,

Me to, me to!

n8huntsman · July 18, 2003

Is there a factory switch for the fog's? I added them but there is no switch. I could just run a wire from the headlights and maybe install my own switch in line. But if I can just flash the computer that would be better.

obyone · July 18, 2003

I had purchased two sets of fog lights. The second cause the upgrade bulb melted the first's housing. Both factory kits came with a switch to replace the blank tab on the dash.

Dealer didnt charge for programming since the truck was in for warranty work anyway.

AZ-Z71 · July 18, 2003

The factory kits come with a switch. Actually you have to remove the old cargo light switch and replace with the new cargo light switch and fog light switch. (comes together as one) As someone above mentioned it is not a real switch per say where there is an actual on off position but sends a signal to the BCM to activate the fogs.

In order for the factory fogs to work with the factory switch the BCM has to be flashed by the dealer. Some dealers will charge you a half hour rate and some a$$es will charge you the full hour. (The one I ended up with was the prick) I don't know what year, but some year after I think 2000 they will come on automatically without flashing the computer.

Anyway, you could however wire the lights and bypass the whole system and connect them to the power surce and put an inline switch in.