I have a 1989 trans am carb swap. My car won’t restart after a while of driving if I make multiple stops. However AZ tested the alternator and battery and both tested good. I checked voltage at the battery terminals while the car was running and it showed 20+. I replaced the alternator with a reman unit and it still read high voltage so I took it back to the store. I could get a better quality unit but is there something I should check before I pronounce the old alternator dead.

 

A battery cannot produce 20V and I'm not sure the alternator can either even if regulator fails high (maybe 16V max). I suspect there is something wrong with your measurement technique or your multimeter. Maybe borrow a multimeter from a neighbor and see if it gives same results. Maybe the multimeter needs a new battery.

One sanity check is to measure across the alternator itself, battery terminal to frame, to see what the alternator is actually doing.

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Maybe also check the sense wire going to the alternator. If its reading really low, it may fool the alternator into thinking it needs to put out a higher voltage.

 

I'm not familiar with these alternators, does it actually have remote voltage sense? Or is it an excitation circuit? Huge difference between those two.

 

I'm not sure. Good question though.

 

Yes the alt has remote voltage sense.

However, not all cars hook it up that way. Some hook it straight to the big terminal of the alt, which is LAZY for the factory to do it, and vastly less effective than doing it RIGHT.

If I was the betting kind, I'd bet the OP's problem is his meter. 20V across the battery terminals would make the alt charge lead get EXTREMELY hot, the alt itself hot, and would let the magic smoke (sulfuric acid, in this case) out with a huge messy explosion. If he REALLY had 20V across his batt terminals his post wouldn't be about "high voltage"; it would be about why his battery blew up and sent lead shrapnel through the fender, hood, headlight, etc.

 

Maybe the alternator case isn't grounded properly somehow? Causing its ground to be at a higher potential than chassis ground?

maybe also put the ground probe on the alternator case and see what the voltage is there as well?

 

The problem of

Still remains.

See my signature, and carefully consider and apply every word it contains. Particularly regarding that proposed explanation, the word "all".

 

I can not see the standing Battery Voltage as 20+ Volts.
No way.

If we were working with the more modern GM PWM Regulated Alternator/ Charging Systems...
The Battery Voltage while Charging (Engine running) can read up to about 17.5 Volts.

 

I think you just had 4 engineers reply to your question.

 

It was the multimeter giving me the wrong reading. The HF multimeter let me down who would have thought . After using a slightly better multimeter the voltage across both terminals bounced between 13 and 14 when the car was cold. After driving it around and letting it get hot I turned off the car and turned it back on and it struggled to start. The voltage at the terminals was 13 ( pocket multimeter won’t read precise voltage). The engine bay gets HOT. I believe the issue is not over charging but undercharging when the alternator gets hot. Is that a possibility?

 

Your alternator is charging. If alternator could not keep up then voltage would immediately drop below battery "open circuit voltage" as the system would have to be carried by battery. In other words, you'd see something around 12.7V or less.

A depleted battery will drag down system voltage and the voltage will gradually climb to specification over time (could be a long time) as the battery charges. I'd suggest putting battery on charge overnight (it can take many hours) and do the measurement again with battery known to be topped off. If voltage is still lower than ideal, or starter motor is still laboring then report back and we'll talk about next steps.

 

Sounds like the starter is heat soaked, causing the struggle to start.

Check the distributor timing (base and under ECM control). Need to find out why the engine bay gets so hot.

Check that the air dam is in place.

RBob.

 

Voltage regulator may be failing.