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Can someone explain the alternator/charge indicator circuit to me ?

3140 Views 4 Replies 3 Participants Last post by  cowboyup151
I'm looking at a schematic in a Haynes Superduty manual.

The "Generator" has 3 wires coming out of it: B, A and "-". Additionally, the alternator is grounded.

So... B goes to the batteries. Makes sense. This is the big cable connection.

A goes to the battery junction box and has a 6A fuse.

"-" goes to the battery junction box but has the key and charge indicator in the circuit.

I don't understand the function of A and "-".

One more thing: I thought the ECM controlled the field in these alternators and I don't see a field connection anywhere.

Thanks in advance to anyone who can explain this to me.
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Here's the blurb from the factory service manual

With voltage applied to I circuit, regulator is activated, allowing current to flow from B+ circuit to generator field coil. Generator generates an AC output which is converted to a DC output by a rectifier assembly internal to generatro, and is supplied to vehicle through the B+ terminal.

From the wiring diagram, the A circuit is always hot, and is switched by the I circuit (probably the one labeled "-" on your diagram) thru the ignition switch. There's a little box inside the generator labeled "Sensing and Switching Circuits". It's like a relay, with the A circuit doing the work.

PCM (ECM) only controls the second alternator in a dual alt setup.

Hope that helps
You have it indeed.

The charge indicator light is in parallel with a resistor in the "I" circuit, and when battery and alternator voltage are the same, there's no voltage across the light. When there's a problem, there's voltage across the light and it lights up to indicate a problem. The resistor is there to let enough current thru to turn on the relay so the "A" circuit can energize the field.
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