There's a thread on the C1-C2 forum about over voltage charging problems caused by faulty voltage regulators. Some have encountered voltages of as high as 17.5 volts which can cause battery problems and blown light bulbs. The common solution is to replace the voltage regulator with a solid state one that is rock solid. I originally ran into this problem with my '67 L79, but later, when I checked it, my base motor '68 had the same problem. Here's what I said on the C1-C2 forum; since early C3's are essentially the same mechanically as C2's, I thought I'd post it here as well..

After I installed the new regulator in my '67, I moved my little voltmeter to my '68 base motor car. I took it for a drive, and saw that it was also charging in the mid 15 volt range. Two for two is a little too much for me and the '68 is also getting a new solid state regulator.

Detecting an over voltage problem is quite easy to do. You can get a digital voltmeter that plugs into the cigarette lighter for $5 to $10 at your FLAPS or Amazon, etc. Simply plug it into your lighter socket and go on your merry way. You should be able to check the voltage in your driveway with a regular multi-meter, but that doesn't always work. In the case of my '67, it would charge in the normal range for the first 4-5 miles, and then slowly climb as high as 17.5.

Even though the cheap plug in voltmeter is probably made in China, mine was extremely accurate. I checked it in my neighbors new BMW and it was right on. I also ran it in my GF's new Kia and my pickup and it worked flawlessly. Again new technology.

It's a cheap and easy check, and it sure beats replacing T3's and dash light bulbs.

Here's the gimmick.

 

Two things...Don't leave the voltage monitor plugged in for long periods if you don't drive often.

And, make sure to connect the black ground wire to an external voltage regulator!

 

I am guessing the art of adjusting a mechanical voltage regulator is a lost one...but they can be adjusted to work properly. no need to replace with solid state unless you have the desire to build one (they're easy to make with a zener diode, transistors, etc.)

More than likely the voltage regulator's coming out of asia are just poor quality and not mfg'd to proper tolerances. kinda like the temp sensors we get...