Alternator circuit in 91 740: no charging, no light even!

Jan 13, 2010 3 Replies
E

This should be a simple one for someone that knows these cars...


91 740 has been sitting for a while in the humid tropics - usually this means a myriad of electrical connection problems, and this has been no different. I had to hotwire the fuel pump relay because it was sticky; the car starts and runs but doesn't charge the battery.

In addition, the alternator light doesn't ever come on - before car start (when all the other idiots lights show) or when it's running.



So, what should I check? there is a big wire and a little wire running to the alternator, but they disappear into wiring bundles. I'm assuming that one is for the light, and the other for the actual charging. Is there a ground cable as well?



I'm good with a voltmeter but I'm not sure what to test exactly. I could just throw another alternator in there, but I hate to spend $100 when it's just a corroded connection.



thanks in advance! elijah


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J

These are internally regulated alternators so the system is pretty simple. The big wire is the output of the alternator and connects straight to the battery. The actual route in these cars is over to the power terminal on the starter which is then connected straight to the battery. The small wire is field excitation and receives power from the accessory circuit whenever the ignition key is turned on. Ground is both through the frame, and an additional wire between the alternator and the engine block.

If the indicator never comes on, you probably have a burned out bulb or a dirty/corroded socket.

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B

Also, in some cars, the indicator lamp is required to provide "exciter" current to "start" the alternator. If the lamp is blown, the alternator will not start charging.

Ragards Barry

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R

You should check the battery, leaving a battery go low ruins it from sulfation fast, get it tested

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