Dud alternator

I take it this is with the alternator not turning and the ignition on, so the warning light is off, but should be on?

The last time I had an alternator expire, it was the diode pack that went. The symptoms were that the warning light was off (+12V) all the time the ignition was on (engine running or not) and on (0V) when the ignition was off. It also sunk about 5A into its main output terminal when not turning, but charged OK otherwise. Presumably, one or more diodes wasn't blocking too well.

My money would be on the diode pack (also cheap) especially if the rest of your symptoms match mine...

Cheers,

Colin.

That takes me back - our Herald had a generator and positive earth.

Gareth.

Yes. Actually, a bit more than that ...

When the ignition is off, the warning light comes on, running between

Thanks for the advice ... a new diode pack is cheap and can't hurt.

Ian

Probably the diode pack, you might as well change th reg at the same time though.

Oops - yep, I got the symptoms right, but cocked up the cause. The warning light output would have stayed at +12V all the time as you say, with the unpowered ignition loads pulling down the other side of the warning light with the ignition is off.

Cheers,

Colin.

Those symptoms could also be caused by a bad earth to the alternator.

It's brand new you said? Get it changed.

Steve

I'vetruled that out because it's got two ruddy great bolts holding it to the engine, which is properly earthed. But I'll have a look for a missing earth inside the casing ...

Ian

Bought five+ years ago in a sale...

Ian

I've seen those two big ruddy great bolts not have a good earth though...

Me too, especially if it's old and been sat for a while. Take the ruddy great bolts off, sand/file the faces clean and ruddy do them up again.

Have you checked the earth strap connections to the engine block too?

I'm pretty certain you can get at the individual diodes (after you remove the pack) to check them with a DVM which has a diode test function - ie most. The reading should be the voltage drop - ie about 0.7v when connected correctly and zero with a reverse connection. Alternately with a simple resistance test a failed diode will almost certainly show a short circuit when connected either way, but near open circuit one way very low resistance the other.

There are usually 6 main diodes and 3 field ones.

Clean shiny alternator.

Yup. No problem. And it turns over just fine.

Ian