Hey all. I've got a '91 Miata, and the other day my 1.5 year old Super Start battery super stopped. Haven't replaced it yet (thanks, hills!) but noticed my alternator is putting out about 17 volts(!) while running. This is probably what finished off the battery, and I assume it must be the result of a bad voltage regulator, which I understand to be located somewhere inside the alternator. Anyone have another theory I should consider before swapping out parts?
One related question: of course, anyone less cheap would go out and get a new alternator, but I was wondering if anyone has tried or would recommend just replacing the voltage regulator--$50 vs $100 for the whole unit, probably.
Most likely the regulator. I assume the battery voltage just keeps going up as engine speed increases?
I second the voltage regulator.
Good question, I only checked at idle. I also discovered that the super cheap multimeter (Cen-Tech) I was using was not accurate! A fresh battery seems to have helped, now it's only reading 0.2 volts difference off my lab meter at 14V.
Cautionary tale: beware of uber cheap multimeters, especially when they have tired batteries.
something else to keep in mind.. that alternator was trying to charge a dead battery...
mad_machine wrote:
something else to keep in mind.. that alternator was trying to charge a dead battery...
A regulator in good condition should still limit the voltage to 14.5 volts or so, no matter how dead the battery is.
stuart in mn wrote:
mad_machine wrote:
something else to keep in mind.. that alternator was trying to charge a dead battery...
A regulator in good condition should still limit the voltage to 14.5 volts or so, no matter how dead the battery is.
Some alternators will go up to 15.5, but that's still a far cry from nearly 18!
I'd still suspect that regulator. Take the car to Autozone or something. They can test the alternator on the car. Better bet would be to pay the 1/2 hour charge for a competent shop to test it.
Well, surprise surprise!
Turns out my multimeter was just lying to me. Voltage from the alternator is a (fairly stable) 13.5 or so. More like 13.2 with the lights on, but it looks like my alternator is okay! (Sorry, metalman.)
O'Reilly's will test my battery tomorrow before they fulfill my free (within 2 years) replacement. Hopefully that's that!
I'm surprised to see a multimeter so wildly wrong. The Fluke from my workbench took all the wondering out of the equation, but I bought this cheapo meter for the purpose of keeping in the trunk and diagnosing things like this...without a brand new battery, however, I'm not sure I can trust it further than I can throw it.
AutoXR
HalfDork
3/5/12 8:37 p.m.
and here I thought this was about the surplus of GM volts and the 5 week delay of production
Interesting...those cheap meters aren't as accurate as a good one, but they're usually not THAT far off.
