Fog light problem

Buddaorf · May 16, 2004

Got a problem wondering if some one could help. Im getting no power to my fog lights and there is no power at the relay from the switch. The switch seems to have power though. Anyone know where I could buy a Wiring diagram for a 2002 GMC 2500 HD with the Duramax. Been looking for a chilten and they have seem now to put one out for that year yet.

Thanks for any help anyone can offer.

zorrro · May 16, 2004

I do not have a wire diagram either, but trouble-shooting switches, relays and loads (lights) is common place in my world.

Since you are measuring voltages I assume you are familiar with the voltmeter or light bulb you are using. It also looks as if you know where the components are and how to access them?

With that the case, this is what I would do:

Start at the switch, you measured voltage here right? Make sure you have 12v between one terminal and ground all the time (with key on?). Then turn the lights on and check voltage coming out. You should have this with a good switch.

Then go to the relay. The relay should, at a minimum, have 2 terminals (or one and ground) for the coil and 2 for the switch. Find the coil terminals turn on the lights and look for 12v supplied to the coil. If you do, listen for a click, the click is good. If you do not have 12v then there is a problem in the wiring between the switch and the relay coil. This not a normal place to have a fuse, so I would be looking for wire trauma. If the coil is getting 12v it should click when it first gets energized. If no click, you need to Ohm the coil and make sure it is not open. It should have a very measurable resistance (less than 100 ohms). If the relay clicks when you turn off and on the switch, then check the other terminals on the relay. With the lights off one of those terminals (assuming only 2 exist) should have 12v between it and ground. With the lights turned on and the relay "clicked" into position both terminals should have 12v between themselves and ground. If this is the case, the relay is good and getting and giving the proper signals. If the other terminals have no voltage present (the not coil terminals), this is a good time to locate the fog light fuse and recheck it.

With the relay passing voltage through it's contacts the fog lights should now have it and be lit. If the lights have 12v and are not lit, they are bad (both of them). If they do not have 12v at this point in time I would then search that wire for trauma.

I know this is basic stuff, but basic stuff wins the war on wires.............

Buddaorf · May 16, 2004

Thanks, ill play more at the switch. I tore the switch apart and have been trying to figure out the circurt board. They got powers and grounds for the fog and bed lights all over that switch. The bed light works just fine but nothing for the fog. All the power seems to be at the right places.

Had to back track to the switch from the relay has power at the switch part but for the accual coil half there is no power, even if you try to press the fog switch on. Not sure if this runs through the computer, I sure hope not, but if thats the case i guess thats why i have an extended Warrenty.

If ya got anymore Ideas, I will take anything at this point

Oh ya Fuse and relay Good

Mort · May 16, 2004

I read in a post that the fog lights are controlled by the BCM. If you add fog lights to a truck that did not have them as original equipement you have to program the BCM to recognise them.

Buddaorf · May 17, 2004

These are Factory

BlackSilver · May 17, 2004

In the 2002 I think you need to have your low-beams on B4 the fog lamps will illuminate.

Mort · May 17, 2004

These are Factory

Mine too. Mine is a Canadian market vehicle and an '03. So the fog lights are controlled by a computer being the BCM. When I turn my ignition switch to the run position or if I start the truck the fog lights will come on if I push the switch. Not only do the fog lights come on but so do all the running lights front and rear and the liscense plate lights. So the fog lights will come on without the low beam headlights. My DRLs will come on as well once I move the gear selector out of park but are off if the vehicle is in park.

So it could be a problem with your BCM or your wiring from the switch to the relay which runs through a connector strip somewhere under the dash. It is the same one that is used to put the diodes in to get all your lights to come on with the high beams. A little vague I know but I have no personal experience with it just what I read.