1988 Surburban Headlight Problem

[trentc]

When I have the running lights on, all works fine. However after turning on headlamps, The right (passenger) side head light is dim, along with the park light and marker light on same side. Those lights are dim because there seems to be a short that is feeding the bottom bright lamps.

This has me confused because according to a diagram I have, the head lamps and running lights are not sharing the same wiring in the engine compartment. If one side is dim, then the other should also be.
Though I am using a diagram from 1985, the wiring seems the same, just some colors are different inside the vehicle (makes it almost impossible to trace wires on the inside).

I pulled out all the wiring on that side till the middle of the front of the vehicle and examined them all. Despite being old, they appear all fine.

Well..I went outside to to look again to verify something before I wrote more.

I turned on the running lamps with the headlamps on the right side unplugged. The running lamps did not come on that side until I plugged the top headlamp on. To me this means someone did some incorrect wiring, or defective wiring somewhere. I am thinking that under the dash is the next place to go, but without a color-coded wiring diagram I will probably be lost.

Anyone else got any suggestions ideas?


Update:

I went and played some more. It seems all these lights share a common ground wire. I disconnected the black grounding wire from the right headlamp and back to another grounding wire I found hanging loose. The light went bright, but the the other on the same side went dead.

So then I just connected all three and it all started working, but....

When bright light switch is turned on the left top lamp goes out and the bottom right never comes on.

Maybe a complete harness replacement, or run another black ground wire?

[rhsub]

Your problems really sound like a bunch of bad connections and grounds. To check for bad grounds use a volt meter set on 20vdc, connect common lead wire to a good ground (battery preferred), touch other lead to ground terminal on headlight, turn on lights, a good ground will have 0.0 volts, if you have a voltage reading work your way back toward the ground connection for the wire until you find 0.0 volts again, this is were your bad connection is. A test light will also work but a digital volt meter is the most accurate way of finding a bad connection especially if you are working on ecm/pcm circuits were a .5 volt reading will really mess up the computer
Electrical circuits are not as scarry as you might think. Electricity needs a complete circuit to operate "ie" bat + to switch to light to ground to bat-
Hope this helps:cool:
Ron

[JohnC]

Yes, the ground is bad. The headlights are grounding through the other circuit, which is why both need bulbs for either to work.

[Renntag]

I second the ground suggestion.

Did you find any creative wiring under the dash?


Another point is that there are upgrades for the lighting where by you install additional relays provided with good clean power. The increased amperage to the headlamps will allow them too burn brighter.