Alternator advice

[StuartCJones]

Hello, my original alternator died on my 1981 1.9 petrol air cooled westy. I bought a replacement, reconditioned thing from eBay (couldn't find the part in supply anywhere else). Exact same product number, but the fittings are slightly different, meaning that the original fan housing doesn't fit on (that's not the main problem, but I thought it worth mentioning). Anyway, fitted fine without the original fan housing, belt nice and tight following tensioning adjustment and the alternator spins nicely when the engine is running. However, the alternator kicks out next to no charge, 0.15v and 0.35v. Not a connection problem, I'm testing the alternator at source.

One thing I have noticed is that the new alternator (and the old) have the three pin plug connection, but also a single pin connection separate to this. I have no idea that what this is. The single pin wasn't connected to anything on the old alternator, and I don't seem to have any loose wires.

The change is volts from 0.15 to 0.35 happens when I connect the mystery pin to an earth/engine block. No idea what this is about.

So I figure the new Alternator is bust... but I can't be sure. I'm probably missing something important. Please help!!!




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[Mocki]

Where is the blue wire ?
Without it the alternator won't "energise " and no out put will happen
You need 12v present on the small spade in the three pin plug before the output can happen

[Oldiebut goodie]

Laying on the tinware by the look of it!
You can fry an alternator by running it unconnected if that is what you are doing there. ( if there is enough residual magnetism for it to kick into life)
The 'spare' terminal is usually for a suppression capacitor if it isn't the rev counter terminal - what is it marked?
How did you arrive at the conclusion that your original had died?

[StuartCJones]

Where is the blue wire ?
Without it the alternator won't "energise " and no out put will happen
You need 12v present on the small spade in the three pin plug before the output can happen

Blue wire is part of the 3 pin plug (see pic) this is the only thing I have plugged into the back of the alternator. Where does it 'energise' from?




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[StuartCJones]

Laying on the tinware by the look of it!
You can fry an alternator by running it unconnected if that is what you are doing there. ( if there is enough residual magnetism for it to kick into life)
The 'spare' terminal is usually for a suppression capacitor if it isn't the rev counter terminal - what is it marked?
How did you arrive at the conclusion that your original had died?

The spare terminal is marked M (see pic)

I arrived at the conclusion that the old one was bust because it wasn't charging the battery... much as this one isn't!

Am I missing something really obvious here? Not sure what you mean by laying on the tinware!

Thank you for taking the time to look at this... I'm baffled.


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[CovKid]

resistance across ignition light on dash prompts alternator to kick off - in a nutshell. I'd be checking the trigger wire, particularly as you have same prob with a replacement alternator

[StuartCJones]

Laying on the tinware by the look of it!
You can fry an alternator by running it unconnected if that is what you are doing there. ( if there is enough residual magnetism for it to kick into life)
The 'spare' terminal is usually for a suppression capacitor if it isn't the rev counter terminal - what is it marked?
How did you arrive at the conclusion that your original had died?

The spare is marked M (see pic)

The old one wasn't charging the battery, much as this one isn't! I'm clearly missing something!


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[CovKid]

W

Normally provides signal for rev counter on a diesel.

[StuartCJones]

W

Normally provides signal for rev counter on a diesel.

Now's probably a good time for me to throw in the towel right?


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[Oldiebut goodie]

Replace the plug - it looks like the terminal is missing in it!



( there is a suppression capacitor already there - just didn't see it earlier)

[Oldiebut goodie]

Turn ignition on, don't start, and check that you have any where between 3v and 12v across the blue terminal and the alternator body. ( an alternator will excite from quite a low voltage, a 1.5v battery will do it usually) Are you getting the battery light illuminating on turning the ignition on normally?

[StuartCJones]

Replace the plug - it looks like the terminal is missing in it!



( there is a suppression capacitor already there - just didn't see it earlier)

I will, thank you!

If the alternator warning light on the dash is disconnected will the alternator fail to function?


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[Oldiebut goodie]

Yes, on most alternators, more modern ones will self-excite without the blue wire. Sometimes the old ones will if there is residual magnetism which is enough to give a small voltage to kick the alternator into life, unlikely on a new one that has sat on the shelf for years though.

[CovKid]

Sort the exciter wire and it should come to life. Chances are there was nowt wrong with the old one.

[StuartCJones]

Right, I'll give that a go. Thank you both!


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