CB72 - Crazy readings when testing charging.

[Phil-UK]

Hi

This is probably one for any electrical experts ( e3steve ? ) but if you have seen similar issues please comment.

I've just fitted the Sasche elctronic ignition and noticed the neutral light seemed to be pulsing with engine revs; as the battery had been reluctant to take a charge ( using an Optimate ) when connected, I thought I'd check the charging.

Tried to use my multimeter but got rapidly changing readings for DC voltage across the battery with the engine running. Thinking it might be the rectifier dumping AC I changed the setting on the meter to AC but unfortunately its a cheap one and just doubles the DC voltage, so I got a reading even with the engine stopped.
While doing this I noticed that I got flickering readings ( DC or AC ) if I just connected the Common lead to the -ve terminal of the battery with the other lead in mid air - turn off the engine and readings stabilised !

Today I borrowed a better multimeter that shows no AC voltage across the battery with the engine stopped and used that to test the rectifier diodes on the resistance settings - seemed fine.
Then ran the engine just off the battery and tested for AC volts across the alternator leads getting readings between 12v and 20+v. Connected the alternator to the rectifier but left the output diss and tested this for DC to earth getting only @3v DC.

Thoroughly confused I connected everything back and ran the engine and tested for DC volts across the battery and again got readings jumping between 11v and 16-17v DC. Then tried connecting only the Common lead to the battery -ve and picked up flickering readings again even with this better meter.

The engine runs OK and I haven't had a chance to switch it back to points yet, but as the elecrtonic ignition only connects to earth and the ignition 12v at the coils I can't really see it producing these
crazy readings.

Q. - Are the readings seen with only 1 test lead connected due to some sort of back EMF ?
Q. - Given that there is no voltage control on the charging, what sort of DC voltage would be normal with the engine running ?
Q. - If the alternator rotor was too close to the stator ( I had to disturb it to fit the ignition ) could it be producing a very "peaky" output ?

Any thoughts or data much appreciated,

Phil

[Phil-UK]

Think I have found the cause of the erratic meter readings ...
Did a resistance check across the alternator stator wires and got a few ohms across the 3 of them, but then checked to earth and found an apparent short. The fault is in wiring within the stator where some of the insulation has baked itself into a brittle shell and cracked and fallen off in places.

Will swap for a spare but anyone every tried any of the paint on or spray on electrical insulations to cover exposed wiring in this sort of situation ?

Phil

[LOUD MOUSE]

The stator will be short to each other (in each winding) but none to the case. (ground). .........lm

Think I have found the cause of the erratic meter readings ...
Did a resistance check across the alternator stator wires and got a few ohms across the 3 of them, but then checked to earth and found an apparent short. The fault is in wiring within the stator where some of the insulation has baked itself into a brittle shell and cracked and fallen off in places.

Will swap for a spare but anyone every tried any of the paint on or spray on electrical insulations to cover exposed wiring in this sort of situation ?

Phil

[Phil-UK]

Thanks LM, thats what I thought; the "faulty" one had shorted to the alloy carrier.
Fitted a spare now which should fix it and will see about "repairing" teh old one or possibly getting it rewound if its not too expensive.

Phil