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Alternator at 20 volts

97 Mercury Grand Marqis

Alternator is under 6 months old. Shop had said the regulator was inside it.

Last night when it started, the dash lights wouldn't go on (but they still work when it's on battery).

I noticed the voltage indicator was at max so I turned off the radio. I had a DMM with me and measured 19.5V in the cig lighter, going up 0.1V every few seconds. It was only running at high voltage for a minute or so.

Fortunately I was already parked around the corner from the shop, but they were closed, so I left it there.

I guess the dash lights didn't work because they have variable brightness and the dimming circuit was confused. They were on when I parked just minutes earlier, so it seems likely the voltage wasn't high before it was parked.

The shop will have to start it again, so how long before that damages the battery or something?

Reply to
Tom Del Rosso
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Why would they _have_ to start it up?

I would remove the alternator, and test it on a bench setup with a known good battery, both without load (except the fully charged battery) and under some load. No need to start the car and potentially damage lots of expensive electronics from overvoltage. If the alternator tests bad, probably the internal regulator needs to be replaced. If it tests good, I'd look to the wiring in the car, especially ground straps/braids.

I would also question the health of the battery, I'm surprised that the alternator was able to overdrive it to more than 19V.

Reply to
Mark Olson

I would expect any shop to look and see for themselves before believing the owner.

Is it common for a shop to have a setup that can spin up an alternator?

Should I tell them to replace it anyway?

Reply to
Tom Del Rosso

You need to find a real auto electrical shop.

Yes. Any auto electrical shop will have a test rig for alternators and starters.

The battery? No, but I'd definitely check the level. If it has been run overvoltage for a while, the fluid will be way low. And remember, if you are seeing 19V at the buss, and the battery is trying to pull down the voltage to 14V or so, then you likely have well over 20V coming out of the alternator.

Pull the regulator, check for shorted SCRs. If the board has been rebuilt with cheap substandard parts, it'll be evident when you have it out on the bench. If that's the case, check the bushings and brushes too to make sure that they are of reasonable quality.

--scott

Reply to
Scott Dorsey

Almost every auto parts store by me has a tester for battery & alternator. a. If the car can be driven, you drive it there. b. They test the battery in the car with the engine not running. c. They test the alternator in the car with the engine running.

I generally go to multiple stores (if I can) because they're kind of sleazy. They want to sell new batteries and/or new alternators.

Nonetheless, _all_ will test the battery and alternator off the car too. a. They have a machine that runs the alternator on a belt. b. They plug in a special plug (different for each alternator). c. And the readout tells them what it finds wrong.

My advice is before you go, charge the battery overnight if you can. Otherwise they'll put it on a charger for you but that takes a couple of hours as they can't reliably test a battery without it being charged.

Be advised, there are core charges for both battery & alternator. Generally you get a new battery if it needs one. Generally you get a rebuilt alternator if it needs one.

Reply to
Andy Burnelli

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