Alternator not charging at low rpm.

[tomstickland]

I was driving along today when all the electrics decided to have a go slow.

Windows wound at half speed, indicators ran at odd speed, radio went off. After a while it just went. A few hours later it happened again. I tested the battery voltage and it was 6v! With the engine idling it was still only 6v, so the alternator clearly wasn't charging. I managed to nurse the car home by revving it a lot, and after 2 miles suddenly all the electrics came back to life.

So I guess that the alternator isn't charging at low rpm. Is this a classic knackered alternator sign?

[rj]

Possibly but check the voltage regulator and the carbon brushes attached to the regulator first.

[Brianmoooore]

Unscrew the two little cross head screws on the back of the alternator, and remove the regulator/brush pack. I expect that you'll find that one of the brushes has worn too short to make proper contact with the commutator. Give it a tug to stretch its retaining wire a little, and put it back into the alternator.
This will normally make it work again for several months, while you source a new pack.

[Brianmoooore]

I'm getting slow!!

[tomstickland]

OK, good info there, thanks. I'll take a look tomorrow morning.

[tomstickland]

Well that was an adventure!
I made it about half way to the scrap yard when the electrics died.
I connected up the spare battery with jump leads, there was a bit of a spark, but I couldn't get the engine to turn over.
So I phoned up the breakdown service.


With nothing to lose, I removed the alternator from the car. I then undid the two screws that hold the brushes in place. Looking inside I noticed that one brush was shiney, whilst the other one had a darkened look to it. I guessed that this mean that there was poor contact, so I wiggled the brush aroud to encourage it out of its hole a bit more.

I put it all back together and tried starting the car. It was a bit slow turning, but it fired up and then sat there in steady idle.

I quickly put all the other bits back on, dumped the tools in the boot and drove to the breakers yard. It ran absolutely fine all the way there.

They had a completely reconditioned alternator for £85, so I bought that and swapped them over in the car park.

[Brianmoooore]

If you'd done this to the brush pack before you left home, you wouldn't have broken down - the alternator would have run fine for months.
All you needed was a new brush pack to 'recondition' (whatever that means!) your own alternator. Don't recall what they cost, but it's a lot less than £85!

[tomstickland]

Yes, but I also noticed a strand of loose wire on the commutator and couldn't be bothered to find out whether that was bad news or not. I've got a shiney alternator now, which obviouslly makes it work better.