Is it the alternator?

kommando · Apr 4, 2010

When you upgraded the stator did you do the rotor too, if its the orginal rotor then low magnetism can give low charging voltage, I also last autumn fitted a new battery but it had a dead cell so only got 10V at rest and needed 3K rpm to get the assimilator to register 12.5V with the lights on.

DavidT · Apr 4, 2010

Kommando,

I replaced the rotor and stator in 2006 (although I suspect they had been in stock for some time as the rotor has a 02/03 stamp on it). I did suspect the battery, but testing the output straight from the alternator eliminates that. The replacement rotor I tested with is from about '84 but it didn't show any less output by comparison, suggesting, to me at least, the stator is the culprit.

David

L.A.B. · Apr 4, 2010

DavidT said:
I have a ’72 750 fitted with a Lucas 3 phase high output alternator setup – RM 24 alternator & Podtronics voltage regulator/recticifier.

I believe you said on page 1 that it is actually a Sparx high output alternator, not a Lucas RM24, although presumably they are both similar units, so it could be worth contacting Sparx (Tri-Cor England) to see if they can give you any test data for it?

rond944 · Apr 4, 2010

Not the same poster as the one who started the thread LAB .

L.A.B. · Apr 4, 2010

rond944 said:
Not the same poster as the one who started the thread LAB .

Yes, sorry, I thought it was the same poster again.

DavidT · Apr 4, 2010

Sorry for the confusion! Just seemed more logical to post in an existing thread.

Diablouph · Apr 4, 2010

15073jrmd

When you did the rebuild, did you pull the 2MC? If it's still in, did you check it? My understanding is that that they get old and leak (don't we all).

L.A.B. · Apr 4, 2010

The late Triumph twins workshop manual charging system test data says the RM24 AC output measured between any two stator leads (with the 1 Ohm load resistor connected across) should be 4.5V minimum at 1000 RPM, and 6.5V minimum at 5000 RPM.

Did you connect an ammeter in series with the battery? If so, did it register a charge with the engine running and the lights on? And did it change when the revs were increased?

If a rotor has sufficient magnetism that it will hang off a steel rod or a screwdriver blade when applied to each magnet in turn without it dropping off, then it's probably still OK.

Flo · Apr 4, 2010

Just to add my tuppeny ha'penny worth, I had excactly the same problem years ago. Apparently, the rectifier was giving out the volts, but not the amps. Worth looking at.

DavidT · Apr 5, 2010

L.A.B. said:
The late Triumph twins workshop manual charging system test data says the RM24 AC output measured between any two stator leads (with the 1 Ohm load resistor connected across) should be 4.5V minimum at 1000 RPM, and 6.5V minimum at 5000 RPM.

Thanks for that - those figures would suggest that I am only getting the barest minimum output. Am I correct then in saying that the total power output at those RPM would be 20.25w and 84.5w (from my schoolboy physics W=V*I)? I think I will order up new kit. If it doesn't sort the problem its not going to go off under the bench (well, not quickly)

David

Is it the alternator?

kommando

DavidT

L.A.B.

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rond944

L.A.B.

Moderator

DavidT

Diablouph

L.A.B.

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Flo

DavidT