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2002 Sierra 1500 headlights make no sense, help please

trouttrooper

trouttrooper

Big Blocks ROCK!!!
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few months ago I bought a 2002 GMC Sierra 1500 for my son. Not a perfect truck at 300,000 miles, but everything works.....at least until last week. Just out of the blue when you turn on the low beams, the driver's side high beam and passenger side low and high come. When you switch to high beam, the driver's side high beam turns off and the low beam glows very dim, the passenger high beam gets brighter and the low beam goes really dim. The fuses and relay under the hood are good. I suspect there is a bad ground somewhere, but not sure where to look. I can't wrap my head around the logic of the high beams coming on when on low beam is selected, and why the driver's low beam only comes on when high beam is selected, albeit very dim.

Any thoughts? Hoping there's an easy answer that I'm missing
Thanks!
 
Welcome to the forum TTT.
That sounds to Me like a poor grounding problem.
Under hood, left side fender, should be a screw with some grounding wires attached to it. Remove that screw, using a piece of sanding paper, scrub the area where those terminals clamp to the fender itself. Next scrub, with the sanding paper scrub each of the terminals until they are bright and shiny. If You have some anti corrosion grease of some sort, give each of the terminals a light coating of that stuff really helps to give each terminal some protection.
 
Ya, I know it's been awhile. Life threw a big monkey wrench at me a few years ago with a divorce and priorities in my life shifted a bit. I don't spend much time on a computer after work anymore.

I suspect a grounding issue too. I was wondering if there was a specific ground wire location that was just for headlights. Reason I ask is none of the other lights have a problem. marker lights, turn signals, brake, reverse all work perfectly.
 
Ok, so here's the follow up. The cause was the 15amp fuse under the hood for driver's headlight was blown. At least I think it solved it......I'm not entirely convinced. I don't get to tinker on the truck much since it is what I gave my son to drive and it's about a 5 hr drive away. We've been working together over the phone trying to figure it out, but that only works so far. When I saw the truck last I know all the fuses were good. I didn't have much time, or the proper tools, to dive into it too much then. This weekend I took all my diag. stuff and was planning on switching out the turn signal arm since I was thinking it had to be either that or the bcm at this point. There's really no other location that controls high and low beam.

So anyway, while I had him start removing all the stuff needed to replace the turn signal switch, I started poking around under the hood with my meter when I discovered the blown fuse. I'm not entirely convinced I've solved the problem. I don't understand why removing a fuse labeled "left headlamp" will kill the low beam and then activate the high beams. I tried removing the "right headlamp" fuse to see what would happen and it just killed the passenger low beam. It didn't turn on the high beams. I still suspect a bad ground or loose wire somewhere, or possibly bcm starting to go. Fuses don't just blow for no reason. I even went so far as to have him turn the hazards on, low beams, pull back on the turn signal arm to have the high beams on as well, foot on brake to activate lights, and put it in reverse to have every single light on. I then went all around the truck smacking and wiggling every wire harness and connector I could find to see if I could blow a fuse again.... Nothing.

So at this point it's working. For how long??? not sure.
 
Not clear headed so someone check my thinking:
High beams getting ground through one low beam or vice versa.
All the lights on at once is too much amps so pops fuse.

Yes could also be switch or bcm- but bcm only if it has control of lights which I wouldn’t understand. Idk if it is possible to test lights and turn off. Then unplug bcm and test lights again if the lights still work then the bcm can’t do it- so it is then out of equation.
 
So here's the final follow up. I figured it out....for the most part. Before all this happened I had installed on of those aftermarket "all 4 on high beams" kit. basically extra relays plugged into an aux block to use the hot wire from the high beam to trigger the other relay to keep the low beam on. I've done this before on several trucks with zero problems. It worked perfectly on this truck as well when I first installed it. When this problem started up I immediately disconnected the fuse and unplugged the 2 relays off the aftermarket aux block so it wouldn't be a part of the equation. thinking maybe faulty relay. With the fuse and relays removed the wires running to that block are just dead end wires not connected to anything. But the truck was still popping the driver's headlamp fuse. It was never immediately, but within a minute or two after turning on the lights. I was able to physically see/work on the truck this weekend and I decided to just cut the wires running to the aux block right where I had tied into the factory wiring. No more popping fuse. My theory is the aux block molding developed an internal crack in the plastic that somehow allowed the hot wire to arc to ground.
 
MrMarty51 said:
Welcome to the forum TTT.
That sounds to Me like a poor grounding problem.
Under hood, left side fender, should be a screw with some grounding wires attached to it. Remove that screw, using a piece of sanding paper, scrub the area where those terminals clamp to the fender itself. Next scrub, with the sanding paper scrub each of the terminals until they are bright and shiny. If You have some anti corrosion grease of some sort, give each of the terminals a light coating of that stuff really helps to give each terminal some protection.
Click to expand...
I always take out the battery and add a ground to the battery box bolt.
I've had several issues with the fender screw. Never had an issue after adding the extra ground to the battery box bolt.
Just need a 5/16 eyelet or 2.
Or a 5/16 and 3/8" eyelet if you have done the battery bolt upgrade.

I use a 3/8 - 16 x 1-1/4" long Allen screw - plated, stainless or brass - with a flat end. I had cupped or pointy ends strip out like the OEM battery bolts.

I find this much handier than using a bolt.
I also use smaller outside diameter 10 mm stainless flat washers or brake line crush washers that fit inside the factory rubber cable ends, if still using OEM cables.
 
trouttrooper said:
So here's the final follow up. I figured it out....for the most part. Before all this happened I had installed on of those aftermarket "all 4 on high beams" kit. basically extra relays plugged into an aux block to use the hot wire from the high beam to trigger the other relay to keep the low beam on. I've done this before on several trucks with zero problems. It worked perfectly on this truck as well when I first installed it. When this problem started up I immediately disconnected the fuse and unplugged the 2 relays off the aftermarket aux block so it wouldn't be a part of the equation. thinking maybe faulty relay. With the fuse and relays removed the wires running to that block are just dead end wires not connected to anything. But the truck was still popping the driver's headlamp fuse. It was never immediately, but within a minute or two after turning on the lights. I was able to physically see/work on the truck this weekend and I decided to just cut the wires running to the aux block right where I had tied into the factory wiring. No more popping fuse. My theory is the aux block molding developed an internal crack in the plastic that somehow allowed the hot wire to arc to ground.
Click to expand...
When I did this upgrade I believe Glagulator made the kit plug and play for me.

I don't like cutting and splicing.

I did that on the first relay upgrades I did but got away from that later on.
 
when doing this headlight mod I would recommend using LED's for at least the highs due to the load all four halogen bulbs would pull. also use an inline diode so there won't be any back feed if a fuse or relay fails. on these newer rigs that have that fancy "leave the lights on while you exit" thing, this mod might cause some head scratching when something doesn't go right not to mention rigs with the new can bus system or bulb detection circuits!

Trust me, Can Bus can be a nightmare trying to diagnose light! I've had my fair share of dealings with that VW Jetta chasing the random bulb out detection. the dam thing was reading resistance and deemed standard American bulbs bad. found out I had to use specific VW bulbs in it!!!
 
I was able to completely turn off the DRL's using the software I bought for the jetta on the lappy top! I forgot the name of the software but it's similar to GMTDscan that works on the truck. you can do lots of mods and coding into the car.
 
That's IF the DRLs are controlled by the computer. On my 98 Burb, it's a circuit that runs out of the main fuse block on the firewall under the dash. I was able to divorce that circuit from the Hi Beam power when I installed my 50W HID Hi Beams and then fed it into the LED bulbs between the Lo Beam projector lenses and the Hi Beam reflector in the new assemblies I had bought. Did a write up on it here in the Electrical Section titled Divorced DRLs.
 
Wrong vehicle. The DRLs were a position on the headlight switch between OFF and the Marker Lights (or is it the Marker and Headlight position, I really don't want to go out in the snow just to look) but its circuit IS tied into the fuse/distribution box on the firewall under the dash where the flasher and various relays are located. I divorced mine from the Hi Beam circuit so I could use my 50W HID 9005 lamps and have the DRLs as separate lights so as not to kill the HID ballasts with too low of voltage. I uaed the DRL feed to trigger a relay that gave 12V power to the LED lights, instead of the 7V from the DRL feed that made the halogen Hi beams glow orangish as DRLs when on.
 
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