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Old 09-18-2018, 12:20 AM   #1
kevmcrae
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GMC four headlight issue

I have a 72 GMC C15. Have the newer style halogen headlights with conversion pigtails. On high beam I was getting a short or something causing the lights to go out. If I shut the light switch off, then on again they would come back on but go off again very quickly. Low beam was fine. installed a new headlight switch-same issue. I just installed an aftermarket headlight harness with direct to battery power and relays thinking the higher power headlights were drawing too much power through the switches. Now on low beam all four are on. High beam is only the outside. I'm not sure if this was happening before I installed the new headlight harness though. So something is not right. Suggestions? Thank you, Kevin
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Old 09-18-2018, 05:02 AM   #2
'68OrangeSunshine
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Re: GMC four headlight issue

Bad relay.
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Old 09-18-2018, 08:12 AM   #3
Mike C
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Re: GMC four headlight issue

My own experience with similar headlight issues has been the high beam switch is bad or is going bad.
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Old 09-18-2018, 08:59 AM   #4
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Re: GMC four headlight issue

Trace it out. A test light is perfect for this.

I would start at the light.
Verify the power at the pigtail drops off when the lights stop working. If it has power, i'd blame the bulbs. No power, work your way back to the relay.

With the relay in place, test for power on the 'output' pin. If it has power, there is a broken wire. No power, move on.

Pull the relay out. Test the signal, battery, and ground slots on the pigtail. Signal and battery should show power (12v). clip the battery to the positive side of the battery (or junction block) and test the ground. If all of these look correct, replace the relay. Otherwise, follow your way back to the headlight switch, battery connection, or ground.

For the headlight switch, test the output side of the switch to see if you have power there. If so, look for a broken wire. If not, might be a bad connection or bad switch.

Schematics:
http://outintheshop.com/faq/wiring/w...s/image002.jpg
http://outintheshop.com/faq/wiring/w...s/image003.jpg

Another thought is to switch the high / low relays.

Quote:
Now on low beam all four are on.
Sounds like something is not wired correctly. Would expect this on high, but not on low. Might the wires had gotten mixed up some where? Unfortunately, this can be hard to troubleshoot without being there and seeing how it is wired.
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Old 09-18-2018, 09:03 AM   #5
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Re: GMC four headlight issue

My personal experience is that if you are running any wattage higher than stock the circuit breaker in the switch will trip, cool and reset. Sounds like what you have happening.
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Old 09-18-2018, 09:30 AM   #6
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Re: GMC four headlight issue

Is this perhaps the 4-beam GMC headlight harness from LMC? If so, it's wired all wrong and has been for years and if they're still selling it that way I'm shocked and dismayed but I guess not surprised.

Long run of yellow insulation across the rad support? That one?

If so, I had to start from scratch re-pinning all of the headlights and got it to work. There's enough wire and connectors and relays in there to build it properly. Just takes a lot of time and testing.
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Old 09-21-2018, 10:30 PM   #7
kevmcrae
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Re: GMC four headlight issue

I have the yellow LMC on my 71 Chevy K20 and it works great. This is a similar unit but is black and for the four headlight system. Wouldn't be surprised if that's the issue though. thanks for the advice. Kevin
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Old 09-22-2018, 03:49 PM   #8
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Re: GMC four headlight issue

I have Hella H4’s installed my 71 GMC. the high beams cause the circuit breaker to trip inside the switch. Low beams are find. Going to install a relay setup this winter.
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Old 09-22-2018, 10:47 PM   #9
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Re: GMC four headlight issue

I run halogens on the stock wiring with no issues. It has nothing to do with the halogens. Probably a bad dimmer switch or something to do with the changes you did.
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