I've got a '77 F250 that has been a nightmare in the charging department. I replaced the fuse link, Vreg and Alt and it worked well for a few hundred miles. Then, my girlfriend goes on a long trip and half way through troubles (fuse link blows, she get a new battery, but still no charging.) I heard from another post that if the charging wire (w/fuse link) loses battery connection it will take out the fuse link. I replaced Vreg and Alt and no charging. I've checked the grounds and the wire from the Alt to Batt and Alt to Vreg. I haven't checked the Field to the Alt yet (that's next, I think this should have 12V with key on?) Any other help? She's about to put this truck on the block due to reliability and I love it...

 

The fuseible link only blows for two reasons;

1. You were working on it with the battery connected, and accidently touched a wire against a piece of metal or

2. You have a bare or melted wire somewhere that is hitting metal.

You may need to unwrap the harness between the alt and the reg/solenoid and make sure all the wires are ok. Make sure you have battery voltage on the output terminal of the alternator with the truck off. This will verify that fusible link is good.

Make sure you have voltage on the "s" terminal with the key in the run position(I am assuming you have a factory guages with an ammeter).

Get it running, and make sure you have some voltage on the "f" terminal. You need voltage on the "f" terminal for it to charge. If you want to test the alt, you can disconnect the "f" terminal on the alt, and run a jumper to the bat +. This will throw the alt wide open, and will test to see if it's good.

 

Thanks for the valuable info, dude. It puts me back on my original thought track which was an unfused short. This truck has a defunct dual battery setup that someone has taken the liberty to hang all kinds of accessories from. It's a spider's web. I'm adding more fuses. Two more thoughts. First, there is a ~12 ga wire tha runs from the battery side of the solenoid into a hole by the heater core. It had melted the insulation from it before and a shop replaced it. I'm not sure what this feeds. Is this a common source of concern? Secondly, the ingition switch looks as if it has had a melting experience. What could be unfused that runs through the ingition switch. Thanks for getting me to hunt for bare wires. I appreciate your input.

 

Since you have all this modified and added wiring, it's hard to say. But originally, the factory had a yellow wire of good size going from the battery side of the solenoid to a splice in the harness. From this splice hooks the alternator output, the feed for the headlight switch, and the main feed for the ignition switch. This mystery wire you are asking about may go to the ignition switch. Originally before it got to the ignition switch, there was another splice were it split, and one wire fed the ignition switch, and the other fed the hot all the time portion of the fuse box.

So there used to be one wire that fed the whole truck's power from the battery side of the solenoid, and it had a fusible link right where it hooked to the large terminal on the solenoid. Then after the splice where it spread to different parts of the truck, it had fusible links for each individual wire. You do not necessarily have to follow this wiring method exactly if you just want to get it running, but you should have everything fused to protect the truck from catching fire.

Also be aware of wiring practices. This is what usually causes problems. For example the wire going through the hole in the firewall. Where it goes through the firewall, there should be a grommet, or a piece of vacuum line, or something to protect the wire from the sharp edge of the metal on the firewall, and the wire should be tied back with wire ties or tape to keep it out of harms way.

 

Now I'm really baffled. I haven't done your component tests, but some troubleshooting has found that the wire to the S alt terminal has continuity back to a two pin plug on the firewall that has no mating connector, and 12V on the other pin. I think that this connector was for the electric choke. What is confusing is that the connector looks as though it hasn't been used since Moby was a minnow, but I know the Alt was working recently. Is there a way that an alt can charge without this stator supply? I'm going to test the Alt as you suggested and proceed from there. Thanks again.

 

If your truck has factory guages, you need no connection on the "s" terminal of the alternator. Here's a diagram.

 

The wire to the "S" terminal of the alternator is an "output" to supply current to a heated choke thermostat. Keep this in mind if your choke tube ever rusts away.(The other wire is for an anti diesel solenoid on the carb, IIRC).

 

Thanks for the clarification, guys. The simplified schematic is awesome. The one in my manual looks like a micro ink blot test. I have a factory Amp meter, but it doesn't work. This hasn't seemed to be an issue before. Hopefully I can get at it again early next week. Thanks, again.

 

OK, one more question. A couple of years ago, I replaced the fusible link with a Maxifuse. I had a 60amp fuse in it when it blew during the last episode. Now I'm rethinking my choice of fuse size. Does anyone know what the current rating of the fusible link for this truck would be? Am I looking to put a fuse just above or just below the output rating of the Alt? Comments welcomed. Thanks.

 

I would put one above the rating of the alt. You might want to try a 80 amp if you have a 75 amp alternator. If something catastrophic happens and that wire gets against some metal, the 80 amp will certainly blow.