Checking my alternator is working

longwalks1

How do i do the above please?

TrickyWicky

If you have a volt/multimeter you can check it quite easily. Put the probes over the battery when the engine is not running.. it will probably read about 12-13V.

Start the engine and do the same thing, this time it will probably measure about 14V. If it doesn't change from when the engine was off, your alternator is dead.

Wig

If your alternator was dead you wouldn't be able to drive your car more than a couple of miles.

To check it is working properly do the above and check the voltage across the battery when running is 13 - 14V then switch on the rear demister the aircon the full headlights and the radio and the wipers The voltage should remain constant and not fall.

Hammyman

Wig wrote: »

If your alternator was dead you wouldn't be able to drive your car more than a couple of miles.

Rubbish. I once got nearly a week when I had one fail. Was doing about 30 miles a day.

TrickyWicky

Contrary to popular belief a dead alternator doesn't mean the car won't go far, the spark is typically 20-30,000V and is just a small spark with little current. A 465Ah battery will take quite a while to drain like that.

It's the use of headlights that will kill it.

ariba10

And the starter motor.

Wig

I'll believe it when I see it, all I know is I had a dead alt and it drained my batt pretty quickly.

I'd like to take the belt off my alt to see how far I get to test your theories but
A) it's difficult to get to on the current car
I don't want to ruin my battery.

TrickyWicky

Well your battery may well be weaker than you realise then.

If you have your lights on, the heater blowing, the radio etc then of course it will drain quicker.

Trebor16

The alternator drive belt on an old motorcycle of mine broke once just as I was leaving Heathrow Airport. I made it as far as the Hanger Lane Gyratory System before I was forced to pull over as the electrics were failing due to a lack of power from the battery and call out the RAC.

anotherbaldrick

I once had my Alternator drive belt disintegrate on the M25. I made it home on the battery some 20 miles through the rush hour traffic and that was in a Volvo with the daylight running lights you could not switch off.

epninety

Wig wrote: »

I'll believe it when I see it, all I know is I had a dead alt and it drained my batt pretty quickly.

Chances are you fried a diode in the alternator (or the brush pack disintegrated, or similar) and caused an internal short circuit. That can leave the alt. drawing battery power all the time.

More commonly the alt. is open circuit, not charging the battery but not draining it either. In that situation, you might easily get many hours of driving, depending on the spec of the car, use of lights etc.

Often people don't find out the alt. is dead or dying until the car won't start, so have not much idea how far they got. Warning lights don't always work, aren't always understood, and don't always mean what they say.

The other possibility is that you don't find out how tired the battery is until the alt. doesn't take the strain anymore. I just measured the delivered capacity of the 8yo battery I recently removed from my car. It managed less than 20% of it's original spec capacity. There were no obvious symptoms, and I had no idea how bad the battery was, until the alt. brush pack jammed and stopped charging. I still completed another 50+ miles of varied driving, I just didn't shut the engine off.

Once I repaired the alternator, I carried on using the same battery for another three months, before I got around to changing it.