Voltage Spikes
 

My Voltmeter will occasionally read very high voltage while under load, and for reasons I can't figure out. The alternator, while rebuilt, has basically no mileage on it and, when bench tested for an hour, resulted in no abnormal numbers. The battery is a new Optima yellow top, and the starter is used (but replaced long after the spikes started occuring). All connections are either brand new or wire brushed and bolted down solid.

It wont happen all the time; rather, I could be driving for an hour without an issue, only to stop th ecar for a minute, start it again, and suddenly see the voltage start peaking at 19v+. It moves up in conjuction with the RPM level, originally leading me to believe that it was the alternator, yet even after replacing it and the belt it persisted. The battery was replaced as well (an OEM Mazda-branded one) with a new Optima. At the same time, I replaced or fixed all the poor connections I could see that relates to the charging system. However, the problem persisted even after I looked after everything.

Thoughts? I figured that since there are a few electricians on the site, I might be able to get a couple of good ideas.

How did you bench test it? was it underload too? swap in a new voltage regulator to see if the problem persists?

The shop I had it rebuilt at bench tested it underload for both long periods of time with occasional short starts and stops to mimic a typical drive around town including running into a shop or two. I went one better than swapping the regulator and simply put in a different alternator. It seemed to get better for a little bit, but then the spikes started occurring again.

Stop the car as in when the car is idle? Have you check the rest of the belt system? fan/ belt clutches?

Next time you see the gauge spike toward 19v. If you can, pop open the hood and put your own volt meter across the battery just to confirm that it is actually is over charging. If it the volt meter moves along with the tach, i wonder if there's an intermittent short between the two.
At 19v, you might notice bulbs going brighter than normal or burning out

Yeah, I've checked with my multimeter a couple times when the voltage starting spiking. As well, I had completely unhooked the fan system to see if there was an issue with those.

At anything higher than 15v, things definitely start going brighter. Haven't burnt anything out yet, but I don't want to chance it. The alternator warning lights on the dash go on once it goes over 15v as well, which is why I originally thought it was the alt. But even after swapping in a different one, it was still occuring.

Godwin, I had checked the belts and tensioners, thinking that maybe something was either slipping or going haywire. It all seemed to be within oe specs. And stopping the car as in both keeping it at idle RPM (650rpm) and turning it completely off.

If you are sure that the charging and belt system is okay.. I think the candidates left is the electrical systems like some relays getting stuck.

You mentioned that the problem comes on when you are in idle. You might want to start on circuits that come on at that moment eg when coolant fan kick in etc?

I would suggest pulling all the fuses and put it in one at a time after the same operating conditions had been met.

It is going to take a long time, but I think that's pretty much the only methodical way unless you spot something like a loose wire or a bad cap etc.