91 e250 w/300 six- batt went dead while driving, tested alt.@ autozone was bad, replaced alt. and regulator. batt tested OK. Still no charge, found melted wires in harness, some BOZO burned main fuse link and reconnected without link. I replaced harness, still no charge, tested all wires in harness, all OK except for excite wire that goes to S terminal, I think it fried " no voltage w/key on. Full fielded alt. and charged @ max. Question , can I run a new wire to S terminal on regulator from switched circuit "On when key is on" and do I need a resistor? Trying to avoid tearing into dash to find open. Any help will be appreciated, Thanks

 

I think you'd be fine but can't be sure right now, it's late. I've never seen a resistor in that circut, and as far as I can remember it's just switched 12v. The resitance is in the voltage regulator.

 

There's no signal coming from the 'S' terminal of the alternator unless the engine is actually rotating.

 

Some confusion here, caused mainly from the several different configurations Ford had with their alternators.

It sounds like he has the separate regulator/ alternator setup, with a dash mounted ammeter. I am going to go with this unless he writes back in and says differently.

The excite wire he is talking about going to the "s" terminal, goes to the "s" terminal of the regulator. It does come from a key-on source, through the "alt" light. If you want to rig it up so it works by running a new wire, I would go to the store and get a small 12 volt indicator light and mount/tape it under the dash. Make sure it has a plastic socket with 2 wires. Run one wire from a key-on spot in the fuse box to one side of the light, and run another wire from the other side of the light out to the "s" terminal of the regulator. That should work.

The resistor around the light in the factory setup is used in case the lightbulb burns out. If the light bulb burns out, you will lose the connection from the hot trigger to the "s" terminal. So if that happens, the resistor in the factory wiring keeps the circuit going so it will still charge when the light burns out. You can run without it as long as you keep an eye on the light. The light should light when the key is first turned on.

I like the light myself, because it brings better attention to alternator problems, as well as if the fan belt breaks.

 

Thanks Franklin2, finished wiring and everything works fine, turned out to be a cool mod as this van didn't come with a warning light, only ammeter. Appreciate the help.

Note to everyone, fuse links are there to keep your truck from burning down.