I have been doing some unrelated auto electrical work on a 1964 pontaic bonniville .I checked the charging voltage with the giant 389 at normal idle .Measured 15.9 Vdc on trusted DVM .When we lifted engine revs to about 2700 and turned on the headlamps the voltage remained at this seemingly too high 15.9 Vdc.I have seen 15Vdc on older classic cars with generators .This detroit iron has an alternater with external volt reg which is normal for its year of manufacturer .The car is really origional and I want to keep it that way .I could rebuild the internals of the volt reg setting up at say 14.8Vdc keeping the car good for open bonnet concource .Is this voltage normal?

1 Answer 1


No, that is to high for that era.

13.7 to 14.4V maximum - newer cars have fancier control (I have one) and it charges up to 15.3V but the ecu will control to a set charging algorithm.

Those regulators will just control the voltage to a given point.

Some of those external regulators had an adjustment - even some alternators had 3 connections to give low, medium and high outputs - usually based on use: low for continuous output ie lots of highway and high for stop / start situations.

  • Reassuring .I did warn the owner not to go on a big trip because the battery might spew over the nice internal paint .I think i will deal to the reg .
    – Autistic
    Sep 11, 2021 at 8:24
  • 1
    @Autistic - The voltage you've stated will boil the battery (literally) given enough time. If you measured the voltage as stated, they shouldn't run the car at all on it. You can try to rebuild the regulator, but I'll bet you won't have much luck doing so. You can still buy an original GM regulator for the vehicle. While it would look brand new, it will be stock, which still fits into an "open bonnet concourse" vibe. Sep 11, 2021 at 11:53
  • This is all good Paulster .I will discuss options with the owner .I only do the rebuild thing when the customer can not get the part .These rebuilds are only for special ocassions.
    – Autistic
    Sep 11, 2021 at 12:39
  • Before you repair or replace the regulator, carefully trace the wires between the alternator and regulator to confirm that there is no short to ground, perhaps from contact with a hot manifold. Some alternators produce maximum voltage when a regulator wire is shorted to ground, though I don't know if your alternator is that type. If it's easy to disconnect the wires at both ends, check ohms to ground. Shouldn't be any.
    – MTA
    Sep 11, 2021 at 17:07

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