Alternator: Ignition Light not coming on

[NevSpit]

Hello everyone, looking for some sanity please…

The TR seems to have gotten through 3 alternators now rather too quickly for my liking (fails to charge) - not all failing immediately although the last one after only  3 months. I need to investigate whether the output wires are rubbing anywhere and shorting but that is not the subject of my question today..
 

I have obtained another (used) alternator and fitted it to the car to test but the ignition light does not come on. Now my brain is telling me the ignition light feed is positive and earths out the through the alternator to light the bulb and once running the alternator pushes 12v back to the bulb and so having positive on each side it goes out.  Checked the bulb circuit and the bulb lights if I ground the alternator plug end to the engine. So that is good. The alternator is earthing too as if I touch said wire on the alternator body it lights too. So, I used a continuity tester on the ignition spade connector and the body of the alternator and that works BUT only with the current running one way, the wrong way. 
 

Am I just misunderstanding how these work, is it possible to have a ‘positive’ earth alternator or has the previous owner tried to fix it and put a diode the wrong way round?

Any suggestions most welcome!

Thanks, Paul

[Pete Lewis]

sound like a(only needs one to fail)   diode on the rectifier has failed 

Pete

[NonMember]

I don't agree with Pete on this one. You'd need all three (of the nine!) to fail if they were involved, but the fact that you get continuity with current flowing "the wrong way" actually suggests the diode pack is mostly OK.

The symptoms you describe are exactly what I'd expect if the brushes are expired or the regulator is dead. The bulb earths through the field winding on the rotor, through the brushes, and down through the regulator. The diode pack provides the 12V to the top side of the field windings (and the bulb) when operational, which means those diodes must point downwards, hence your multimeter seeing continuity from a negative voltage.

[NevSpit]

Thanks both, I have put the alternator on the bench this afternoon and took the brush box off, the brushes seem ok. However, putting a meter across the two contacts of the slip ring it would appear I do not have continuity across them. Unless that is another problem I am presuming there is a break in the windings and that is the issue - does that make sense?

[johny]

yes unfortunately there has to be a circuit across the slip rings. Unusual to fail, any signs or smell of damage and I take it youre 100% sure of the open circuit reading? If correct theres no chance of the alternator working☹️

[NevSpit]

Hello Johnny, yes certain of the open circuit as I was running the same test on another alternator so I could compare. I also then noticed  that the slip ring was a little loose too. I shall return it as was bought as used but working. Looks like it has sat in a shed for a while. 

[NonMember]

I considered listed "failed field winding" in the list of possible causes but, as johny says, it's not terribly common.

[johny]

Sounds like a winding tail(s) might have broken where it connects to the slip ring which would be very likely if the slip ring has come loose. Possibly repairable but thats the sellers problem however I wouldnt admit to anything more than 'it doesnt work'😲