Fog light head scratcher

Wingnutt · August 22, 2014

I've got a problem that I'm having a heck of a time figuring out. The truck is a '99 Silverado 1500 LS, extended cab (3 door) regular bed (6.5') with a 5.3 auto. My aftermarket fog/driving that had been spliced into the factory harness had been working fine since I replaced the factory lens droppers about 8 years ago. Fast forward to last week when they just stopped working. Of course I tried all the normal things, wiring connections, good ground, no blown fuses, relay replaced and lastly the switch (factory) was replaced.

Now here's where it gets interesting; needless to say none of the above cured the non-working status of the lights…HOWEVER, if I jump across the terminals at the relay (at the under the hood electrical center), they work, AND / OR if I jump across the wires where they plug into the switch they will work. But at the switch the jumper wire starts to get hot suggesting a grounding problem. Even though I can jump across certain wires at the switch, I'm not sure that I am jumping the correct wires because of the headlight high beam interconnect. I know that I can always by-pass the factory switch with an aftermarket switch, I would like to keep the factory setup if possible.

Soooo, does anybody have a schematic diagram for switch connection or has anybody ever ran across this problem before or anybody have any suggestions for what to try next?

Wingnutt · August 22, 2014

UPDATE:

I read on another forum that the cause is related to the BCM, and that the INT PARK fuse inside the truck needs to be good. It's a long involved process but surprise I found the fuse was indeed blown. BUT as soon as I turn on the headlights the fuse blows. Any ideas?

chupe442 · August 22, 2014

Get your meter out and start tracing that circuit.... You could be grounding in the lamps themselves. 8 Years old, that is a lot of time for aftermarkets and the elements.

Wingnutt · August 28, 2014

Sorry to have taken so long to reply, but work had me out of town for a week. Anyway I bridged the INT PARK fuse with a paperclip just long enough to verify that the fogs did indeed come on if the fuse was good and in the short time it took me to poke my head around the truck to verify the lights were on, the paperclip was glowing red! Definately a dead short! Now I just need to figure out how to remove the lower dash panels so that I can get to the BCM and check right circuit and pin-outs. I'm really hoping that the short reveals itself without looking!

bossier62guy · August 28, 2014

These may help your cause:

kneebolster.pdf

foglights.pdf

kneebolster.pdf

foglights.pdf

Wingnutt · August 29, 2014

Thanks, those were a great help. However, the mystery deepens.

Here's what I accomplished tonight. The INT PARK fuse does NOT blow until the park lights are activated…interesting. I disconnected the rear park lights from the circuit by unplugging the "pigtail" from the junction box where the taillight, trailer harness and tag light "pigtails" connect. No good, pop goes the fuse.

Next I located an aux junction block (under the dash to the left of the steering column) there is a park light relay (not shown in the schematic, but pointed out in the owner's manual. Removing said relay did NOT affect the parking lights which is not a good sign, but it did help by pointing out that there is probably a problem with the park light circuit. Lastly I located a 30A fuse in the power distribution box under the hood and pulled it. Out go the park lights.

Never letting a chance to blow a fuse go to waste, I replaced the INT PARK fuse, crossed my fingers and turned on the park lights…the INT PARK fuse remained whole and happy. Pushing my luck I turned on the headlights and still the INT PARK fuse remained in one piece. Lastly I tried the for light switch (new) but no joy on the fogs. I turned everything off and replaced the 30A fuse in the power distribution box and turned on the parking lights and pop goes the INT PARK fuse confirming that there is defiantly a problem somewhere in the park light circuit.

However, because the BCM is part of the park light circuit that hasn't been ruled out…yet. I think replacing that might be a good project for next week.

bossier62guy · August 29, 2014

The BCM supplies a ground to the fog lamp relay when it receives a request for "fog lights on". The norm is for the BCM to supply a "low" to activate a circuit. I don't see it causing your problem.

bossier62guy · August 29, 2014

Another schematic...as you can see the BCM only supplies a ground to the park lamp relay. The lack of a ground isn't going to blow a fuse.

parklamp,etc.pdf

parklamprelay.pdf

parklamp,etc.pdf

parklamprelay.pdf

Wingnutt · August 29, 2014

The schematics verified what I found out last night. However, and this is the mystery comes in, all the marker and park lights are on when commanded, and all the fuses are in tact. Not to mention that they go out when the appropiate fuse is pulled. Going back to when I quickly bridged the INT PARK fuse plug-in, the fact that the paper clip heated up enough to make it glow in a matter of seconds confirms that there is a dead short somewhere, but only the INT PARK fuse is blowing. The one last area that I can think of that needs investigated is that the chime was not doing its thing when the door was open and the park lights were on. Of note, the chime does work when the key is in the ingnition and the door is open though. Again

Duelin · August 29, 2014

The BCM supplies a ground to the fog lamp relay when it receives a request for "fog lights on". The norm is for the BCM to supply a "low" to activate a circuit. I don't see it causing your problem.

He might of cause a problem with the BCM when he put a jumper at the relay and switch. Hopefully he didn't. But you have to be careful when you energize a circuit. You have no idea where it might be going. More so with computers where a ground might a common ground for different circuits.

bossier62guy · August 29, 2014

All he did was to take the relay out of the circuit....no big deal.

Wingnutt · September 1, 2014

Ok, I hate to say it but I just can't figure this one out. I spent most of the weekend under, in, around and everywhere on the truck and cannot find the problem. Every circuit tests good every fuse is doing its job without blowing except for the INT PARK fuse. I guess Im down to three options; either throw a BCM in it or take it to the dealer and give them a specific amount of time for troublemshooting (basically in my opinion, that is just giving them $$ for them to say that they searched for the problem) or bypass the factory switch and wire the fogs up with an aftermanrket switch 'cause I am out of ideas.