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Old 04-03-2019, 01:49 PM   #1
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Join Date: Dec 2006
Location: Studio City, Calif.
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GMC Headlights (4 bulbs) all dead or dims only.

I can only run with dims (outboard ones). If I need high beam all are dark. I replaced the dimmer switch with a known good one, no change. What would cause this? Thanks. - Brian
'69 GMC C2500 Custom Camper, 8 1/2' bed (same as Chevy "Longhorn"), 350 ci. mild RV cam, NP 435 CR 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

Last edited by WorkinLonghorn; 04-03-2019 at 02:36 PM.
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Old 04-04-2019, 11:12 AM   #2
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Location: Kalamazoo, Michigan
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Re: GMC Headlights (4 bulbs) all dead or dims only.

The power wire for the headlights is the blue wire at the dimmer switch terminal. It connects to the brown wire for low beams and the light green wire for the high beams.
I would try jumping the blue wire and the lt green wire to see if the high beams come on.
If they don't then you might have a break in the light green wire between the dimmer switch and the left high beam head light. Check the dimmer terminal plug for corrosion.
You may also have relays in the wiring if a previous owner had installed them and the high beam relay may have failed.
It"s also possible that all four high beam elements have burned out. Since your two outside bulbs work on low beam I would rule out bad grounds.

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Old 04-20-2019, 07:39 PM   #3
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Location: Tucson, AZ USA
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Re: GMC Headlights (4 bulbs) all dead or dims only.

Check ground leads from frame to radiator bulkhead.
Check each bulb for continuity.
Check voltage to headlight plugs.
Check regulator and alternator output.

Every 25 years I like to rebuild that 292, whether it needs it or not.
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Old 04-22-2019, 03:28 PM   #4
michael bustamante
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Re: GMC Headlights (4 bulbs) all dead or dims only.

definatly check your grounds at the radiator support
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