Spitfire Alternator troubles

[Greyherren]

I have a 1974 Spitfire and some alternator issues. All worked ok after a major rebuild of the engine, gearbox and overdrive…. This until I installed LED headligths. After driving some 30 miles, the ignition light came on, it went off after some few miles, but what happened is that the bulb overheated and melted the plastic mount in the speedometer housing ( I did find out this only later). I was able to drive another 100 Miles, this until the battery was “empty”. Called AAAa d the diagnostic was alternator failure….. got a new one, which lasted all about 30 miles…. By then got the car back via AAA. Had the original alternator repaired, some parts that connect the three wires had burned out.
After reinstalling the fixed alternator I only connected the fatter brown wire and the brown/yellow one.. not the thinner brown wire and all seems to work, although I am not driven with lights on….looking at circuit diagram, it does not make sense to me why the problem has gone and why there is this thin brown eitre going to the headligths witch, given the the headlights actually work without the connection to the alternator…. Any advice on what may have happened, or how to wire the alternator would be welcome.

 

[Sarastro]

That "ignition" idiot light is just a crude indicator to show whether the generator or alternator is working. One end is connected to the +12V, and the other to the gen/alt output. If the output is zero, it has 12V across it and it goes on. Even if it is on for some time, it shouldn't melt anything in the speedometer housing. Is it possible you have the wrong bulb?

The thin brown wire is probably for sensing voltage. That's why it is connected to that junction, which is where power splits off to the various circuits. It's not specifically connected to the headlights. If that isn't connected right, the alternator cannot sense the system voltage, and it might, say, put out full power when it doesn't need to. I suppose that could conceivably blow out something in the alternator, but I'd expect the alternator's circuits to be protected against that.

The only suggestion I can make is to be sure that everything is hooked up right, between the battery and that common point. Be sure everything is clean and tight. Dunno if that will help, but it can't hurt, and, without seeing it, it's hard to say more.

 

[Greyherren]

That "ignition" idiot light is just a crude indicator to show whether the generator or alternator is working. One end is connected to the +12V, and the other to the gen/alt output. If the output is zero, it has 12V across it and it goes on. Even if it is on for some time, it shouldn't melt anything in the speedometer housing. Is it possible you have the wrong bulb?

The thin brown wire is probably for sensing voltage. That's why it is connected to that junction, which is where power splits off to the various circuits. It's not specifically connected to the headlights. If that isn't connected right, the alternator cannot sense the system voltage, and it might, say, put out full power when it doesn't need to. I suppose that could conceivably blow out something in the alternator, but I'd expect the alternator's circuits to be protected against that.

The only suggestion I can make is to be sure that everything is hooked up right, between the battery and that common point. Be sure everything is clean and tight. Dunno if that will help, but it can't hurt, and, without seeing it, it's hard to say more.

Thanks Sarastro, good analysis and advice. Will go over the wiring and check that all is connected properly.