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I have a 93 honda del sol. I noticed a while ago while driving the headlights would occasionally brighten for a few seconds/minutes, then dim down. (I actually thought it was the other way around at first). The battery is a few years old, and has been overdischarged at least once, but has otherwise been fine lately.

I found a cigarette lighter voltmeter, used that, and saw that when the headlights do brighten voltage is upwards of 16V. Otherwise when running it's the normal 14ish.

Replacing the alternator is painful, but I'm 30% sure that's the only thing that could cause over voltage while running, but I want a second opinion. The only alternative I can think of is an intermittent connection to the battery (possibly internal to the battery) that occassionally disconnects it while running, and the 16V is just a 'no load/minimal load' setpoint of the alternator- I wouldn't think that'd be the case with the load of the headlights though.

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Virtually every alternator has a built-in voltage regulator never exceeding 15vdc. Average output is around 14.5vdc. Be sure battery cable connections are free and clean of corrosion (wet/dry powdery residue from battery acid venting depositing on battery posts/cable connections. Dried battery acid (white/blue powder deposits) will eat copper wires underneath battery cable insulation, preventing full current for starter use and intermittent voltage issues. A warm water solution of baking soda and toothbrush will dissolve and neutralize deposits immediately, bubbling up as indication. Plain water rinse. Do not allow this solution into battery refill ports as it will kill the battery. Plain grease on terminals to minimize future buildup. Good batteries measure 12.5vdc, engine off. Engine running, alternator output should be between 13.5-14.5vdc. Have the battery tested at your favorite auto store selling car batteries (usually free). They should be able to test the alternator too, without bench testing battery or alternator (Autozone is one place). If your alternator is original and car has high mileage, the alternator may be worn out (brushes worn, one or more diodes faulty). A worn alternator can be rebuilt or replaced as necessary.

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I ended up replacing the alternator (it wasn't quite as bad as I expected, but it wasn't fun either), and the over voltage issue seems to have been fixed.

I'm inclined to say "overvoltage can only come from the alternator, and it suggests a fault with the alternator regulator", (probably). In my case the alternator has an integrated regulator, so replacing the entire thing was the answer. If you have an external regulator, it might be only the regulator needs replacing, could be wiring issues, could be a dozen other things.

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  • On a lot of cars, the integrated regulator could be replaced without removing the whole alternator and it is a lot easier.
    – fraxinus
    Commented Sep 3 at 6:24
  • Not this one :-p, but that is worth keeping an eye out for if other folk run into the issue. (for curiosity sake I disassembled it last week- getting to the regulator involved a lot of hammering to get the rotor out and detach the windings from the casing- not exactly trivial to service)
    – CoderTao
    Commented Sep 3 at 23:57

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