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Probably a simple fix but figured I would ask so maybe I could save some time. Turn the headlights on dim both sides come on, switch to high beam the pass side comes on but the drivers side doesn’t and the low beam goes out. Go back to dim and it comes back on.
Thanks
Rookie suggestions:
Bad/corroded/loose ground wires?
Connect a voltmeter to the driver side hi-beam connector at the headlight...operate the lights and dimmer...watch the voltmeter to see when you are losing voltage.,,,then connect the voltmeter an inch or so Before the spot where the wires branch off from the main headlight harness to see if voltage is present...this will show if there is a broken wire in the driver branch.
Update us.
Last edited by doorgunner; May 14, 2021 at 08:19 AM.
Definitely sounds like a bad ground. start with a test light to see if your getting power to those high beams. not likely both bulbs on one side are burnt out. If you confirm you are getting power. Must be a ground.
The low beam could be grounding through the high beam wiring.
Last edited by 4-vettes; May 14, 2021 at 09:36 AM.
Different dimmer switch but this is how my 77 is wired. Same wire colors as your 75.
At your dimmer switch the HIGH Beam is a #16 LT.GRN wire.
At your dimmer switch the LOW Beam is a #18 TAN wire.
Coming out of the firewall bulkhead connector there are 2 #16 LT.GRN HIGH Beam wires, one feeding the right side, one feeding the left side.
The outside bulbs are duel filament a LOW filament & a HIGH filament.
There will be a jumper wire from the outside HIGH filament to the HIGH Beam bulb.
Check voltage at each bulb plug.
You have both outside LOW Beams so those grounds seem to be OK, but those grounds also jumper to each HIGH Beam Bulb.
Last edited by bmotojoe; May 14, 2021 at 11:19 PM.
Must be a wiring and/or dimmer switch problem. Since the low beam works, but doesn't light when you flip the dimmer switch, the switch and related wiring must hold the problem. When you select HIGH beams, all four bulbs should have the low-beam filament burning.
Last edited by 7T1vette; May 17, 2021 at 04:27 PM.
Must be a wiring and/or dimmer switch problem. Since the low beam works, but doesn't light when you flip the dimmer switch, the switch and related wiring must hold the problem. When you select HIGH beams, all four bulbs should have the low-beam filament burning.
Actually, I'm pretty sure the low beams only have one of the filaments lit at a time, so if the low beam bulb goes out when you turn on the high beams, it just means that the 'high beam filament' of the low beam light is burned out. If you put a test light on the dimmer switch wires, I think you should only have voltage on either the green wire for high beams, OR the brown wire for low beams - not both wires at the same time. Have you verified that the bulbs are good?
Actually, I'm pretty sure the low beams only have one of the filaments lit at a time, so if the low beam bulb goes out when you turn on the high beams, it just means that the 'high beam filament' of the low beam light is burned out. If you put a test light on the dimmer switch wires, I think you should only have voltage on either the green wire for high beams, OR the brown wire for low beams - not both wires at the same time. Have you verified that the bulbs are good?
Agree,
With headlight switch pulled to headlights on you should have 12vdc to the top prong BLUE wire on the dimmer switch.
You will have either 12vdc to the LT.GRN wire at the dimmer or 12vdc at the TAN wire at the dimmer but not both.
The wire colors on my photo LT.GRN & TAN may be swapped around on your 76. I own a 77 completely different style dimmer.
The dimmer shown will be the same as yours.
The OP's low beam goes out when the dimmer switch is depressed. It should stay on. Since is was illuminated on low-beam position, the ground is obviously OK. Switch and wiring are the concerns.
I'm sorry I just disagree about the low beam filaments should stay on.
The dimmer switch is either sending voltage to the Low beam filaments or the HIGH beam filaments.
A different drawing showing ground locations and dimmer switch.
But I do agree OP should also check the dimmer switch.
If OP need directions on how this is done, just ask...
Last edited by bmotojoe; May 17, 2021 at 10:28 PM.
I'm sorry I just disagree about the low beam filaments should stay on.
The dimmer switch is either sending voltage to the Low beam filaments or the HIGH beam filaments.
Correct - If one of the two filaments was burned out in the low beam headlight, and the single filament was burned out in the high beam light, the OP's description is EXACTLY what would happen.
Correct - If one of the two filaments was burned out in the low beam headlight, and the single filament was burned out in the high beam light, the OP's description is EXACTLY what would happen.
bradleyb66 you were right on! Went to the parts store and picked up 2 dim and 2 high beams, old ones were dingy anyway, and bam all 4 work now.
Thanks Everybody for the input!
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