GMC Headlight ?

[blazerparts]

I have no low beams. When I switch to high beams three lights work, but the drivers side low beam is still out. Changed the offending unit but the condition still exists. Ideas?

[Classic Heartbeat]

A buddy of mine had something like that happen to him and it turned out to be the dimmer switch. Sounds stupid, but it salved the problem. WES www.ClassicHeartbeat.com

[Longhorn Man]

Unplug the dimmer switch. Take a jumper wire anmd jump from the one terminal that is alone, to one of the two that are side by side, see what lights light up. Then from the one single terminal, to the one remaining terminal, check out the lights again.
If they work the exact same way they are working right now, then your switch is fine. Then you'll need to keep looking. Another prime problem area is the plugs on the back of the lights... unplug them and look at them real carefully, look for green corrosion, rust, or the terminal missing all together.

[dyingdeadman]

check the dimmer switch then check the grounds, or vise versa.

[70GMCer]

As cheap as those switches are, just go and get a new one to begin with......I only paid around $5.00 at Advanced Auto for mine.....Then check your grounds and light connections.....

[Longhorn Man]

Since one light on each side works, and it is in the high beam selection, grounds won't be an issue here. The high beams require more amps which require a stronger ground, and since both lefts use one ground, and both rights use another ground, each side is obviously getting enough of a ground.

[blazerparts]

I'll start with the dimmer switch, thanks guys.

[blazerparts]

Ok, replaced the dimmer switch and no help. Added grounds at the battery(to the radiator support) and at the driver's side low beam that doesn't work(at radiator support). No help. Tested both low beam light plugs and have current at the plugs (horizontal connection on low beam, interior vertical connection at high beam). I would think on high beam, both the high and low beam slots, on the low beam plugs would have current. Anybody have an idea? Do I have the premisis for a thesis here?

[68gmcdually]

TTT for ya! I have nearly the same problem with mine, except for my low beams work but when I flip to high beams the two highs come and the the pass. side low but no driver's side low. I have checked the plugs and switch, it is really odd, seeing as though it works on low perfectly.

[Longhorn Man]

68daulie.. you have a burned out light... the low beam bulb has a high beam in it too...it is a hi/lo bulb, with the hi burned out.

blazerparts, you tried wiggling the plug on the light when your high beams is on? Try moving up and down, left and right, and unplug it and see it the light flickers at all.

[dwaite72lnghrn]

The wire diagrams show Different ground points for the left and right side. When you checked for voltage on the headlight connector that didn't work were you measuring from the tan wire to the black in the harness or the tan wire to the frame? The black wire on the high beam on the driverside goes to a splice and the outside lamp goes to the same splice on a different black wire. Take a sort jumper wire from a clean frame ground to the black wire in the connector and see if the outside lamp works.

[68gmcdually]

Originally Posted by Longhorn Man

68dually.. you have a burned out light... the low beam bulb has a high beam in it too...it is a hi/lo bulb, with the hi burned out

Thanks Andy; the part that frosts my corn flakes is that they are new halogen light bulbs plus I have those half chrome headlight covers and it is a half a day job to get the bulb replaced by the time you line up all of the screws. When the truck comes out of storage I am going to try replacing that bulb and I will also be installing those LED tailights like you did, Thanks for all of your help.

[blazerparts]

Longhorn, I wiggled the plug at the back of the light and still no light.Dwaite, I spliced a ground wire from radiator support to the black wire a couple inches from the plug to the offending bulb. Maybe this evening I can play with it some more, has to be a ground problem. Thanks for the help, guys.