I have a 75 F100 390 4wd, I rebuilt it last summer and have been driving it to work and back and using for my weekend projects. Over this time I discovered that it was overcharging(visible from the acid that was boiling over from the battery). So I went to my NAPA and after testing the alternator, which was faulty, I purchased a new one and a new voltage regulator. Everything seemed fine for a few months, until now. I drove to a woodworking store that is about 50 miles from my house and not far into the drive I started smelling sulfur(really potent). After getting home I realized my battery was dry(I did not check it's level from the first time I had the overcharging) so I filled the battery with distilled water and have been driving and constantly checking it's voltage. So here's what I've come up with: with the lights on it is running at a range of 13.5 to 14.3, with the lights off it runs 15.3 to 15.9.
I have also had an issue with my lights since before the rebuild. They will shut off while driving. If I switch the high beam switch on and off it will turn them back on. Actually it seems that when I don't use the high beams they won't shut off to begin with.
Any help would be appreciated Thanks,
Evan

 

Is the voltage regulator grounded to clean sheet metal on the core support or inner fender?

 

Ditto, excellent suggestion. I'll also add to make sure there is a ground strap from the block to the firewall (thereby grounding the inner fender).

 

First off, thank you for the responses. So I cleaned up where the voltage regulator attaches to the core support with some sand paper. I also did the same with the firewall ground strap location. I had a friend say that a ground to the frame can't hurt(thought I'd see what you guys thought) so I put one on. And it still seems to be overcharging.
Something I noticed yesterday was that it seemed like every time I would fire up the truck to test each suggestion, it would gradually get worse as I would switch from the lights on to no lights. Just thought that could possibly help. Since I'm pretty lost on this stuff, I don't want to leave anything out that might be significant info.
Thanks
Evan

 

If the battery voltage is higher than 14.4 volts or so, then the only explanation really is that either (1) the regulator is switching the alternator appropriately, yet for a given field the alternator is producing too much current or (2) the alternator is fine, but the regulator is sensing the system voltage as too low, and mistakenly turning the field in the alternator on too much. I can't see how the first one would work. The second is much more likely, hence the reason for checking the grounds on the regulator. This makes sure the regulator is actually seeing the full battery voltage, and not minus some offset due to resistance in the return path.

The fact that turning on the headlights lowers the voltage simply means you're placing a load on the electrical system which is normal. I hate to risk recommending that you replace a part and it end up not solving the problem, but at this point, if it were my truck I'd replace the regulator, suspecting the new one was faulty. However, if I'm reading your post correctly then you stated it had this problem with the old regulator. If that's the case, then it's probably not the regulator. Can you try this with a different battery? Are the batteries in your meter fresh? Does the battery still boil over?

 

There is a place in my town that rebuilds alternators, starters, and voltage regulators. They can actually test them to see if they have a problem. You might do some looking and see if there is anybody near you who can do the same. I have found that most parts store people don't know too much about electrical systems and don't really understand problems beyond what their piece of test equipment tells them.

Many "brand new" parts store parts are also faulty. I've also seen plenty of bad parts test good, and good parts test bad because the people at the stores don't know what they are doing.