High beams not working properly

Trying to figure this out while I work out my transmission issues. Low beams both work, but when I turn my high beams on one of the low beams shut off. Not sure where to start to try to fix but I have notice that this tan wire with white stripe is just dangling around not connected to anything. It comes out of the headlight harness loom just behind the driver's side headlights. Any idea what this goes to and if it's related? I can find any wiring diagrams that indicate what this could go to...


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Figured out the random wire, it goes to the brake sensor on my proportion valve underneath the radiator. Got it plugged back in. Still trying to figure out the headlights though!

I suggest chasing/refreshing grounds to start. 1988 was a transition year model, meaning the "other than 1/2 ton" trucks tend to have features common to the before and after years. I don't recall if they had the "4-on" headlights, but I don't think so. If you have one low beam remaining on, it's probably been modified. Look for additional wiring with a diode between the hot wires connecting the beams. If one diode fails or loses contact, that low beam won't stay on.

Turned out to be a bad bulb. Put an indicator light on the power connector and it lit up. Swapped the bad passenger bulb with the good driver's bulb, and the good bulb still lit up (this time on the passenger side) but the bad bulb was still bad.

But, I will say, I didn't see any modified wiring.

Bad bulbs are easily diagnosed and replaced. Still doesn't explain your original problem.

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Originally Posted by AugustDiesel

Low beams both work, but when I turn my high beams on one of the low beams shut off.

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If the low beam worked, then didn't work with the high beam while the other side did, something else is amiss. Could be coincidental failure and a corrected poor connection while messing with it. Stranger things have happened.

Turned out to be a connector issue I suppose. Replaced the harness with a heavy duty replacement harness from LMC truck and updated the lights as well. Works great now!

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