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Old 07-20-2012, 08:57 PM   #1 (permalink)
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Default 1970 mach 1 headlight issue

This issue is driving me absolutely nuts. The headlights and sportlamps on my 1970 Mach 1 quit working. Every other light (brake lights, tail lights, parking lights, interior lights, etc.) still works. I've replaced the dimmer switch and the headlight switch to no avail. I can't think of anything other than a wiring problem. Any idea how to test the wiring, and where to start looking? Thanks for the help.
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Old 07-20-2012, 09:06 PM   #2 (permalink)
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A simple test light will tell you if your getting power at the headlight pig tails. I Believe the sport lamps get the power from the parking lights. I'll have to check with my dad to verify.


Check your fuses
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Old 07-20-2012, 10:22 PM   #3 (permalink)
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No fuses for the headlights - they've got a circuit breaker in the headlight switch. New switch didn't help. Tested the pigtails - got 12 volts on low beam and 0.2 volts on the high beam. Plugged in 1 headlight on low beam and watched the voltmeter on the other pigtail go to zero. Sure sounds like a dimmer switch, but it's brand new as well. What else can I check?
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Old 07-20-2012, 10:39 PM   #4 (permalink)
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Grounds?

3 pins on headlamp pigtail...... low beam, high beam, ground.

Check continuity of ground pin to battery -.
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Old 07-20-2012, 11:11 PM   #5 (permalink)
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Checked the continuity to ground on both pigtails - continuity is good. Brand new battery, so that shouldn't be causing any problems either. Stumped!
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Old 07-20-2012, 11:37 PM   #6 (permalink)
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Quote:
Originally Posted by 1970mach1T5 View Post
No fuses for the headlights - they've got a circuit breaker in the headlight switch. New switch didn't help. Tested the pigtails - got 12 volts on low beam and 0.2 volts on the high beam. Plugged in 1 headlight on low beam and watched the voltmeter on the other pigtail go to zero. Sure sounds like a dimmer switch, but it's brand new as well. What else can I check?
Did the first light work? Check where the other side is spliced into the harness for possible corrosion and then ohm out the "bad" wire while flexing it to check for a break.
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Old 07-21-2012, 05:24 PM   #7 (permalink)
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Neither headlight works. Both pigtails show 12 volts on low beams with the lights removed. Switch on the high beams and the voltage reads 0.2.

With low beams on and volt meter hooked up to one pigtail, plugged a light into the other pigtail and watched the voltage drop from 12 volts to nothing.

Could it be drawing too much current and tripping the circuit breaker when the bulbs are in? Is so, what could be the culprit?
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Old 07-21-2012, 09:22 PM   #8 (permalink)
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Just measured the voltage at the dimmer switch connections. There's 3 wires - 1 black and 2 red. On low beam, both red wires register 12 volts. Push the dimmer switch button to high beam, and both red wires read 0 volts. This is a brand new dimmer switch. Am I looking a short somewhere on the high beam circuit? If so, how do I find it?
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Old 07-22-2012, 12:40 AM   #9 (permalink)
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Sounds like you found your problem. One wire is the feed from the headlamp switch and should have 12 volts regardless of what position the switch is in. The other two go to the headlamps, high beam and low beam.

What I would do is remove the switch and using a jumper wire temporarily connect the feed wire first to the low beams and then to the high beams and see what the actual lamps do. If they work fine then you have a crappy dimmer switch (happens). If the lamps do not light then you need to trace the wires out to the bulbs. I'd first check to see if either is shorted to ground.

If the dimmer is okay you could have an issue at the firewall junction or somewhere along the way.
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Old 07-22-2012, 01:58 PM   #10 (permalink)
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OK, removed the dimmer switch and used a jumper wire to connect the wires. I tried all combinations of the three wires - no headlights. I guess that means the dimmer is OK, but the wiring is the problem. What's the best way to trace out the wires? Where would I look for a short to ground?
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