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1970 Mach 1 351 Cleveland, 3-speed manual

Hi all, so I have the dreaded headlight issue that so many always have. First off, I have sealed beam lights which have been replaced but are quite old and work but flicker after about 5 minutes.

I replaced the headlight/dimmer switch and the brights/lows floor switch....I even used the 1969 switch because of the stronger breaker...NOTHING, they just stay on longer but still turn off.
*(interior and all marker lights including grill mounted Sport Lamps stay on and Never turn off).

New alternator, wiring looks great, headlight plugs look great, ground wire near radiator looks great. So is it just a wire to headlights, or something in the light itself even though they turn on and are bright?
Thanks in advance. Neil http://mach1club.com/images/smilies/Smil...o_that.gif
(07-12-2016, 11:25 AM)neilsonnewby Wrote: [ -> ]1970 Mach 1 351 Cleveland, 3-speed manual

Hi all, so I have the dreaded headlight issue that so many always have. First off, I have sealed beam lights which have been replaced but are quite old and work but flicker after about 5 minutes.

I replaced the headlight/dimmer switch and the brights/lows floor switch....I even used the 1969 switch because of the stronger breaker...NOTHING, they just stay on longer but still turn off.
*(interior and all marker lights including grill mounted Sport Lamps stay on and Never turn off).

New alternator, wiring looks great, headlight plugs look great, ground wire near radiator looks great. So is it just a wire to headlights, or something in the light itself even though they turn on and are bright?
Thanks in advance. Neil http://mach1club.com/images/smilies/Smil...o_that.gif
Sounds like a loose/dirty connection that heats up. Body to engine ground. Maybe a partially broken wire leading to one of the lights or ground. Have you turned on the lights and jiggled all of the wires that you can to see if that may cause a flicker? Not sure how those lights being on all of the time factors in. I had a '76 Chevy pu that the interior lights and maybe dash lights would come on if the "accessory" fuse was blown. Replace the fuse and those lights operated normally. Weird. Dave R.
With the age of your wire harness. Everything Dave said is sound advice. I would be especially interested in the wiggle test, as old brittle wires can break, or rub through to ground, or through to each other and can cause all kinds of havick. Don't forget the turn signals and brake lights run through the turn signal switch as well and can ground out, or short across to the marker/ park lights. Which run through the headlight switch. Which could be the cause of overheating the circuit breaker in the headlight switch. Hope it helps.
(07-13-2016, 05:07 AM)JTS71 Mach1 Wrote: [ -> ]With the age of your wire harness. Everything Dave said is sound advice. I would be especially interested in the wiggle test, as old brittle wires can break, or rub through to ground, or through to each other and can cause all kinds of havick. Don't forget the turn signals and brake lights run through the turn signal switch as well and can ground out, or short across to the marker/ park lights. Which run through the headlight switch. Which could be the cause of overheating the circuit breaker in the headlight switch. Hope it helps.

Thanks! Only strange wire i found is that the oil pressure sender wire has almost melted through....strange to me...i have an oversized braided ground coming from block to firewall so engine should be grounded fine....afraid to relay the headlights because of fire risk until i figure this out...i have old style sealed beams although they may be halogen and stronger compared to the original.
Yeah the oil pressure wire being melted is strange, but it runs through other harnesses as well and is probably shorted across another wire or something, as it's a grounding leg and only vary's resistance, due to pressure in the oil system. Therefore it shouldn't have 12 volts to melt it. Sounds like it got 12 volts from somewhere else. Hope it helps. JTS 71 Mach1
Hi, thanks for the help guys. Another question, could a 'bad' voltage regulator have anything to do with my headlights fading out after about 2 minutes then flickering (as explained in my older posts above)?? And again, they do this whether the engine is running or not.
(07-15-2016, 02:12 AM)JTS71 Mach1 Wrote: [ -> ]Yeah the oil pressure wire being melted is strange, but it runs through other harnesses as well and is probably shorted across another wire or something, as it's a grounding leg and only vary's resistance, due to pressure in the oil system. Therefore it shouldn't have 12 volts to melt it. Sounds like it got 12 volts from somewhere else. Hope it helps. JTS 71 Mach1


Hi, thanks for the help guys. Another question, could a 'bad' voltage regulator have anything to do with my headlights fading out after about 2 minutes then flickering (as explained in my older posts above)?? And again, they do this whether the engine is running or not.
(07-23-2016, 01:10 AM)neilsonnewby Wrote: [ -> ]
(07-15-2016, 02:12 AM)JTS71 Mach1 Wrote: [ -> ]Yeah the oil pressure wire being melted is strange, but it runs through other harnesses as well and is probably shorted across another wire or something, as it's a grounding leg and only vary's resistance, due to pressure in the oil system. Therefore it shouldn't have 12 volts to melt it. Sounds like it got 12 volts from somewhere else. Hope it helps. JTS 71 Mach1


Hi, thanks for the help guys. Another question, could a 'bad' voltage regulator have anything to do with my headlights fading out after about 2 minutes then flickering (as explained in my older posts above)?? And again, they do this whether the engine is running or not.


Most definitely, I don't believe the voltage regulator has anything to do with it, as the only time it controls anything, is when the alternator is turning / engine running. Simple answer is unplug the connector from the voltage regulator and turn on the lights. If they still go dim, you've got some serious issues. I believe I would turn the lights on wait till everything starts going dim then "fingerprint" the entire harness. Put your hands on every possible inch of the harness, switches etc, looking for the hot spot. Something is getting hot, overloading, and pulling the current that should be going to the headlights. which causes the dimming. Something is using that current, whether it be a motor, blinker, lousy connection, or a short. It's going somewhere and should be creating a hot spot. Find the hot spot and you should find the problem. Hope it helps JTS 71 Mach1
(07-23-2016, 05:36 AM)JTS71 Mach1 Wrote: [ -> ]
(07-23-2016, 01:10 AM)neilsonnewby Wrote: [ -> ]
(07-15-2016, 02:12 AM)JTS71 Mach1 Wrote: [ -> ]Yeah the oil pressure wire being melted is strange, but it runs through other harnesses as well and is probably shorted across another wire or something, as it's a grounding leg and only vary's resistance, due to pressure in the oil system. Therefore it shouldn't have 12 volts to melt it. Sounds like it got 12 volts from somewhere else. Hope it helps. JTS 71 Mach1


Hi, thanks for the help guys. Another question, could a 'bad' voltage regulator have anything to do with my headlights fading out after about 2 minutes then flickering (as explained in my older posts above)?? And again, they do this whether the engine is running or not.


Most definitely, I don't believe the voltage regulator has anything to do with it, as the only time it controls anything, is when the alternator is turning / engine running. Simple answer is unplug the connector from the voltage regulator and turn on the lights. If they still go dim, you've got some serious issues. I believe I would turn the lights on wait till everything starts going dim then "fingerprint" the entire harness. Put your hands on every possible inch of the harness, switches etc, looking for the hot spot. Something is getting hot, overloading, and pulling the current that should be going to the headlights. which causes the dimming. Something is using that current, whether it be a motor, blinker, lousy connection, or a short. It's going somewhere and should be creating a hot spot. Find the hot spot and you should find the problem. Hope it helps JTS 71 Mach1

Ok I'm at my wits end....took out the dash and gushes...checked resistance everywhere, checked grounds, nothing. With one headlight unplugged (either one) the headlights stay on forever. My dad bought this car in 1975 as second owner and nothing had ever been rewired. I'll update if I ever figure it out.
By flickering... do you mean after a few minutes of being on the headlights will turn off, after a period of time, come back on and continue this cycle.
If so that sound like they are cycling on the switches internal breaker.
It may be the halogen drawing to much power for the switch - unplug one headlight and see if the other still cycles.
Or are they flickering on / off randomly fast - which as several members have said is probably a wiring, short or ground issue.
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