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doiron_dancer75 · Posted: Thu Mar 29, 2012 10:22 am Post subject: Alternator problems??

Last year I had a wire connected from the alternator to the fuse box/ignition switch/distributor/starter that burnt. What would cause this? Could it be my alternator overheating? I would really like to get the car running this year. Thanks in advance for the help

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'79 Euro 924

Khal · Posted: Thu Mar 29, 2012 10:42 am Post subject:

(Quick edit) After reading your previous posts, did you check the wiring on your starter?
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'80 924 Turbo

Jakkq · Joined: 29 Sep 2008 Posts: 810 Location: Omaha, Nebraska

doiron_dancer75 · Posted: Fri Mar 30, 2012 10:06 am Post subject:

We did check the wiring on the starter, everything was fine. Will be doing more tests to figure out the problem.
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'79 Euro 924

staticsan · Joined: 19 Jan 2009 Posts: 450 Location: Sydney, Australia

musicalannette · Joined: 21 Feb 2012 Posts: 413 Location: UK

If I have understood correctly.

To the light on question, it looks like the altenator field is getting a 12v feed from somewhere, it should normally get one as you turn the key. It goes out as the field voltage rises inside the alternator once it's up and running and generates it's own fileld, and then no current passes through the bulb. So there is probably 12 volts there when you connect the battery instead of when you turn the key. Its getting 12volts from somewhere it should not.

As for the burnt wire, you need voltage to create an arc. Quite a bit of voltage if the material you are tying to make the arc jump across is air. As with magnetics, motor/generator design and heating/cooling designs, most of this work was carried by Micheal Faraday. The man was a genius. You certainly won't get arcing across a gap of air with 12volts. You will have to touch the wires to get an arc. If you have a wire burnt out then it has probably touched something somewhere (if you do get a short, the fuse is supposed to blow) and had either no fuse in the circuit or the wrong one fitted. Once you have a shorted wire with no fuse, the current rises (amperes or the flow of electricity), when this flow increses there is a heating effect from all this current trying to get down a wire (which is the electrons darting about doing there work), if there are not enough of them in cross sectional area of the wire (if the wire isn't thick enough) the wire will heat up until it melts the plastic insulator. Then the wire becomes exposed and it shorts out somemore, it gets hotter and hotter, then the wire itself burns out acting as the fuse. Then your left with a nice black charred mess that looks like it has been arcing (which it has but only due to shorting out on things not due to sparks jumping across), so the arcing should probably be seen as an effect not a cause. For a cause, probably look for a short somewhere or something drawing too much current.

Hope this helps.
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....