Alternator Light
 

I looked up a Ford schematic of the G3 alternator to find out why my idiot light was not working. The light now seems to work fine after correcting a wiring problem, but I noticed the alternator output voltage does not drop back when the battery is fully charged. I noticed the schematic shows a 400 ohm resistor across the indicator lamp. I suspect this might have something to do with regulating charging since the alternator will not charge without the indicator line hooked up. Has anyone ran into this?

Chet

Hey Chet,

I've spent some time (still not solved. I now suspect a weak battery.) problem with my system and a few gracious people from this site sent me schematics and I'll offer what I know based on an assumption that you have an external regulator and that the idiot light with resistor is wired to the "I" lead on the regulator.

First, the voltage won't drop when the battery is charged because you are measuring the voltgae across both the battery and the alternator at the same time. Essentially they are equal and the alternator won't send any more current to the battery once it's charged because there is no more volatge DIFFERENCE between the two. This is Good.

Afar as the light goes, it's actually in the start circuit. Turn the key on to get the motor turning and voltage will return back to the regulator on the "S" lead, a relay will trip, or transsistor turn on and voltage will shunt around the idiot light circuit and the light goes out. Notice the light comes on when you turn the key on And stays on until you get the engine running. If the alternator fails, no juice to the regulator via the "S" lead , relay or transistor off and current from the battery now flows through the light letting you know something's wrong. As far as the resistor goes, my guess is it is there to get the regulator up and running when you first start the car should the light bulb be burned out. If you didn't have voltage on that lead at start up, the voltage regulator would not give the initial shot of field voltage to the alternator and it would never send a return voltage on the "S" lead.

There are varations to this of course but hopefully I haven't made a really simple thing sound to complicated. (Yes I Have - lol) - The Wiz

Wiz,

Thanks for the explanation on the voltage. I still like the old ampmeters better. I finally figured out the resistor. It's there in case the bulb burns out.

Today's project is getting an Autometer Tri-Alert unit to work in this circuit so I'll only have 1 warning light for 4 different alert conditions. I've already determined it will take something besides a blocking diode.

Chet