Alternator dropping volts when engine is hot

So recently I’ve been thru 3 alternators.

I had my OEM one (12 years old, go out) I had 2,000 watt sound system on 80amp alternator. Pretty sure that killed it.

Got a remanufactured one off amazon , sold the sound system. Got a new battery too just in case. And after 2 months, when the car gets super hot after boosting a couple times. I would say around heatsoaked temp where the turbo is just hot as hell, the alternator goes to 12.2v at stopped light or while I’m pressing the pedal. If I let off the volts jump back up to 13.7v (according to the turbo timer)

Anyways since it wouldn’t stop going to 12.2v I returned it and just bought a brand new alternator instead of remanufactured

3 days after getting it the exact same thing, runs fine while engine is cool/warming up. Once it’s operating temp the volts go wacky.

In park the volts are normal.
Coasting without foot on pedal normal.
Stopped light, or barley pressing gas = 12.2v
Hitting the gas normal volts

So now I’m starting to think something is draining to much, or something is wrong electrically. Any advice on what I should check or solutions?


Sounds like you have a voltage regulation problem, not an alternator problem. That points to the ECU.

Where are you measuring the alternator output voltage?


How would you know that?

The OP didn’t provide the year of this Civic.


Sure he did.

That puts it at around a 2009. However, looking it up, I see there is a separate voltage regulator. That is likely the problem.

There’s a voltage meter on the HKS turbo timer.

For the regulator is it best to order one online or would one at a junk yard be fine. First time replacing that part

I’m guessing either alternator belt is slipping, or voltage regulator problem. Shops have equipment that easily measures the alternator’s current output, if you think the alternator is being asked to produce more current than it is designed to handle. I presume you’ve turned everything off that is possible to turn off and still keep the engine running still, right? No effect when doing that?

Turning off the heater fans, radio turned all the way down , unplugging the ac charger port will maybe ride it .1V so not really much

One thing that is important I noticed, if it’s sitting at 12.2V and I turn on the AC the voltage will stay at 13.3V with the ac on which doesn’t make sense , since that pulls power .

Would a high performance car with high torque have a higher risk of belt slip?

This is how the Honda dual-mode charging system operates. If it troubles you, switch on the headlights and see if it stays in the higher voltage mode. Don’t replace the alternator again.

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Well what do you know on my way home from work on a quiet street I decided to shut off my lights and bam volts drop, second I turn them back on we’re charging again… lol so 1. I didn’t have to replace any alternators just something funky with the charging screwed me?? 2. Just drive with lights on ? :joy: 3. Any chance I can hook up the sound system again knowing this?

The charging decisions are probably done by a computer, and it uses a complicated algorithm to decide whether to charge or not. It keeps track of how much current has been used vs how much the battery has taken during charging periods. So it knows more or less how much charge the battery holds at any point in time. It prefers to avoid charging b/c the alternator loads the engine and this can lead to stalls when coming to a stop sign, etc. If computer doesn’t think charging the battery is necessary it will lower the alternator’s voltage automatically, to a no-charge state. If you then turn on something that requires electricity, it will automatically begin charging again. That may be what you are seeing. But 12.2 volts in the no-charge states seems a little lower than I’d expect. Seems like it should be something in the 12.6 - 13.0 volts. Are you certain 12.2 is an accurate voltage measurement? Suggest to connect a DVM – or a different one – directly to the battery for a reference measurement.