I'm sure this has been asked (and answered) before, but I couldn't find any relevant threads using the search function.

My headlights and instrument lights will sometimes get brighter/dimmer as the RPMs increase/decrease. I don't notice it all of the time.

The alternator and voltage regulator are both new. I'm thinking maybe the dimmer part of the light switch may be at fault.

Anybody have any experience or advice on this?

Thanks.

 

I just think this is a common older car problem. Well i wouldn't call it a problem.. it's just how they are. Both my 1966 mustang and 1968 buick do this.

 

My does it, I believe its just because of the voltage drop over the wiring in these car... we don't have a whole bunch of relays and stuff like new cars. When your rpm's increase your alternator cranks out more power so in turn there is less of a drop in power and the lights get brighter.

 

Yes to all those.
- Older wiring impedes current flow
- A stock 55 amp alternator can barely keep up with the car. Tack on the headlights, stereo, any other goodies and you're withdrawing from the bank (battery)

There are some modifications you can do to combat this situation. Headlight relays so that the lamps pull directly from the battery help significantly. http://www.midnightdsigns.com/james/headlights.htm

Another modification is to step up to a higher performing alternator, such as a 100 amp 3G one wire alternator.

Running your stereo of a direct feed from the battery and not off the ACC source will also help.

 

Definitely get a 100-amp alternator, seems to be working for me. Except I didn't get the one wire one, but it's pretty much the same as far as I know.

 

I had the same problem and fixed it for $15.00. Your stock voltage regulator is mechanical. Buy a new electronic one ( no moving parts) and it should solve your problem. It fixed mine and its been fine for almost a year now. I picked one up at my local auto parts store.

 

^not too sure if that's it, I have a electric one now but my lights still dim and whatnot. Maybe not as much though, can't be too sure.

 

I agree with the above posts and I like the electric voltage reg vs the stock mech one but I think its the wiring (inc the ground) that should be replaced if it hasn't already been done. It won't matter if you have a new reg and up the amperage by getting a bigger alternator if the wires that deliver the power are in any way corroded or loose or bent. A heavier gauge wire will do the trick (the bigger the better). hope it helps (if anything food for thought)

 

Hiya,
if the headlights in the front get dimmer at the same time as the interior lights then the problem is not the dimmer. The dimmer does not affect the headlights in front.
A lot of good suggestions here, but I want to point out what James said about the relays. Just do it and replace wires as you go. Even if that does not cure your problem it is a great upgrade to any classic mustang as the headlights will end up being brighter. The worse the old wiring was the better. You won't route the full amperage through the light switch and back this way.

 

Thanks guys. All good advice.

I'm pretty sure that my voltage reg is already an electronic one. But I'll check to be sure. That would be an easy fix.

Good catch Kalli. It wouldn't make much sense to run the headlights thru the dimmer, unless you wanted to use them as mood lighting. (Although, that's probably not out of the question for the late 60's -70's crowd. HaHa!)

It's not too high up on the to-do list, but we'll probably re-wire and add the relays when we get around to it.