I've been having some strange low voltage problems lately. I was messing around, and decided to turn on as many electrical loads as I could, which meant the radio, high beam headlights, defroster, and fan blower. Before doing that my check battery light was on and I was reading about 13.75 volts on my multi-meter. Turning all these loads on caused my voltage to drop to about 11.8 volts, however, after turning all the loads off I noticed that my check battery light was off and that my base voltage had jumped to about 14.05 volts at idle.

Does anyone have any idea what just happened?

  • I'd want to know what open-circuit battery voltage is in the morning, and immediately after a 15 minute highway drive. Those things would suggest whether the issue is a bad battery, or a bad alternator (with, i'm assuming, a built-in voltage regulator which would be the real culprit)
    – SteveRacer
    Jun 2, 2016 at 20:11
  • @SteveRacer Battery is almost new, just bought it a few months ago. I'm not sure if voltage regulation in my car is done by an external regulator or by the ECU. It's a 98 Mazda 626 GF 2L. Jun 2, 2016 at 20:15
  • I still don't have a clue why your battery light would come on at 13.75v ... this should be plenty fine to keep the battery charged. Does the voltage rise when you rev the engine a little? If there is a problem, I'd suggest it's with the regulator due to not keeping up with demand when the load is put on it. Jumping up in voltage is fairly normal after a big load. This is to recharge the battery. Jun 2, 2016 at 21:17
  • How many wires on the alternator plug? 3 wire systems are ecu controlled.
    – Ben
    Jun 2, 2016 at 22:54
  • @Pᴀᴜʟsᴛᴇʀ2 If I rev the engine without any loads on the voltage stay constant, if I rev it with all loads turned on it rises. Maybe I'll put up a graph later from my log data. Jun 3, 2016 at 5:35

1 Answer 1


It turns out that I had what appears to have been a faulty field control unit on my alternator. Voltage regulation is done by the ECU via the field control unit. I changed the alternator and now every thing's OK.

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