No charge from altenater

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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No charge from altenater

Postby poweller996 » Tue Sep 19, 2017 9:24 pm

Hi all, I have now fixed my oil flow problem but now I seem to have no charge from my altenater.
when I put the lights on the amps meter goes down to 1/4 negative which it never did before I put it all back together.
I have tested the 3 wire coming from the alternater Orange, white and light green as stated in the Owners manual with a voltmeter And am not getting any Volts coming out of the wires.
I have also replaced all the electrical connectors from the altenater as they were all old and thought it might be a bad connection.
Is their anyway I can test the altenater coils.?
Any advice would be greatfull before I look to buy a new altenater.
Many thanks mat

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Re: No charge from altenater

Postby Ozmadman » Wed Sep 20, 2017 12:02 am

When I put my lights on my ammeter goes to the negative for a while until the alternator charges up the battery a bit. If you are testing the wires direct from the alternator then the voltmeter/multimeter needs to be set for A/C and not D/C. Have you tried testing the charging at the battery terminals with the bike running (D/C) it should charge up around 7 volts with some revs.
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed

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Re: No charge from altenater

Postby poweller996 » Wed Sep 20, 2017 7:08 am

Hi Paul, thinking about it I can't be sure if I had my voltmeter on AC as I was in a bit of a rush.
But would I not see some readings on my meter even on AC. ?
Many thanks mat

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Re: No charge from altenater

Postby Janet » Wed Sep 20, 2017 7:28 am

Does it make any difference if you rev the engine? With the lights off, does the ammeter show a charge? With the engine off, does it show a discharge when the points close. I know it sounds silly, but I was wondering whether you could have crossed a couple of wires somewhere.

Mine is different from Paul's as my ammeter shows a higher than normal +ive charge if the battery is low which, as the battery is charged, then reduces to just above zero whether or not the lights are on. It will show a negative if on a very slow tickover. Otherwise, negative only occurs when the engine isn't running and the lights are on or the points are closed. I do have a regulator/rectifier on it which may affect it and perhaps it shouldn't do this. An expert who knows about these things would be able to say.

Don't bother looking for a new alternator at the moment. If it was charging before, I suspect it hasn't just failed but that you were looking in the right place first. If you're confident that all the wires and connectors are sound, my advice is to check the voltage across your battery. Then run the engine fairly fast ( not thrashing it, but above tickover) without any lights on. After a few minutes, turn off and check the battery again. If the alternator is charging, it should be showing an increase in the voltage. Now do the same with the pilot light/tail light on. Finally, repeat with the headlamp/tail light on.

If the battery voltage increases each time, it shows the alternator is charging although it is always possible that it isn't working to full power. Finally, finally, leave it standing for a day or two and check the battery again. If it's showing near as dammit the same reading as when you left it, there is unlikely to be a short in your wiring and your battery is probably good. If it's down, take it off, charge it up, test and then leave it unconnected to anything. Later, check it. Then you should be able to decide whether the batter isn't holding a charge or there is a problem in the loom.

As the person who earlier this year rode 200+ miles in the dark with barely a light showing*, I can understand your frustration. In my case the cause of the problem appeared to be two suspect connectors but it took me a long time finding them. Often the simplest (and cheapest) answer is the correct one.

*I'd say I had witnesses but they couldn't see me. :rofl:
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Re: No charge from altenater

Postby Expat » Wed Sep 20, 2017 11:47 am

[quote="Janet"]

Mine is different from Paul's as my ammeter shows a higher than normal +ive charge if the battery is low which, as the battery is charged, then reduces to just above zero whether or not the lights are on. It will show a negative if on a very slow tickover. Otherwise, negative only occurs when the engine isn't running and the lights are on or the points are closed. I do have a regulator/rectifier on it which may affect it and perhaps it shouldn't do this. An expert who knows about these things would be able to say.


No way could i be accused of being an expert but my G3 behaves exactly how Janet has described and gives no trouble, so assume everything's A ok. Also have regulator/rectifier fitted. :)

Regards, Steve
Keep shiny side up.

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Re: No charge from altenater

Postby poweller996 » Thu Sep 21, 2017 4:58 pm

Many thanks for all your help everyone , I think I have now located the problem.
The altenater direct is only delivering 5.7v it should be around 6.2v-6.8v and according to the workshop manual this could be due to a demagnetised rotor.
Does anyone know where I can get the rotor re magnetised.
Many thanks mat.

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Re: No charge from altenater

Postby Ozmadman » Thu Sep 21, 2017 9:37 pm

Just try putting something metal (steel, iron) next to the rotor it should attract it pretty strongly, you should be able to tell from that
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed

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