Topic: dash lights/Headlights surging bright and dim.

[priceless]

We drive our 69 Mach 1 apporximately 500-800 miles a year.  We haven't really driven it at night or got caught out after dark in a while. We had to get our Mach out to drive it to our annual Pony Run early before sunrise and my wife and I noticed the bright/dim surging of the dash lights and the headlights. I was told by a few that it could be the voltage regulator and have had a few tell me it might be in the headlight switch.  The voltage regulator is a reproduction about 4 years but the headlight switch, to the best of my knowledge, is the original. 

Any thoughts would help....Thanks.

[drummingrocks]

My first thought would be the voltage regulator too.  I've had several of them cause the same problem you're describing.

You might also check to verify that all of your grounds are good and clean.  I've had loose/corroded ground wires cause similar problems.

[Carl]

On the less conventional side (and what drove me nuts for a couple of weeks), the same can happen with an intermittant short.  In my case, the taillight harness was rubbing against one of the taillight bezel studs, when it contacted, it would short and pull the voltage down, and the headlights would dim. 
One way to help track the issue is to watch the voltage (like with a voltmeter), if its not dropping or fluctuating when the lights are off, that would help rule out the voltage regulator.  If it only fluctuates when the headlights are on, I'd start looking for a wire with the insulation worn off.

  Carl

[midlife]

On the less conventional side (and what drove me nuts for a couple of weeks), the same can happen with an intermittant short.  In my case, the taillight harness was rubbing against one of the taillight bezel studs, when it contacted, it would short and pull the voltage down, and the headlights would dim. 
One way to help track the issue is to watch the voltage (like with a voltmeter), if its not dropping or fluctuating when the lights are off, that would help rule out the voltage regulator.  If it only fluctuates when the headlights are on, I'd start looking for a wire with the insulation worn off.

  Carl

+1.  That's exactly what I was going to say.

[priceless]

Would, or I should say, could any of this possibly start a fire?  Should I disconnect the battery when parked in garage?...Thanks

[midlife]

Would, or I should say, could any of this possibly start a fire?  Should I disconnect the battery when parked in garage?...Thanks

Probably not, if the source is the bad headlight or alternator/belt/VR.  Both of them won't pass voltage with the key off.  But...it's always a good idea to disconnect the battery if there is any doubt as to electrical problems.

What am I saying?  Please...burn up your harness so I can sell you a new, refurbished harness.  *G*

[Carl]

A battery cut-off is not a bad idea, especially if you're still runnning 40 year old harnesses.  I agree that the VR wouldn't be energized with the key off, but the headlight circuit can be energized if you forget and leave the headlights on (i.e., they are not switched,) as can a couple of other circuits.   99.9% of the time we remember to turn off the headlights . . . (I know, you're more likely to remember to turn the headlights off than to hit the battery disconnect switch, but . . .)

  Carl

[priceless]

Thanks for replies, got a little homework to do....