I've got a 1G alternator in my truck. I had it wired using the harness from a '68 F100. From the regulator there was orange to the field of the alternator, white with a black tracer to the stator, yellow to 12V+, and green with a red tracer to the charge indicator light there was also a purple, aluminum, resistor wire running parallel with the green/red wire that I assume was some kind of voltage drop to excite the alternator. The wiring left alot to be desired, so this past week, I went to the pick n pull and got the charge harness from a pretty clean late 70's Ford Truck. The problem is that it had the same alternator, but different wiring. There's no purple resistor wire and the green/red wire connects at the regulator in the spot that the white/black wire did on my original wiring. You still following me? Good. I looked at some wiring diagrams for the later 70's trucks and they show the green/red in either spot, and the white/black either running from the alt. to the regulator or to the electric choke. I will probably wire the truck the way I have it now, but I'm still curious as to why the difference exists at all. i guess this really isn't all that important, I'm just curious. Thanks

 

Your 68 wiring harness came from a truck with a charge indicator light and the charging circuit wiring is like this:




The late 70's donor truck probably had an ammeter and was wired like this:



These drawings are from a late 60's Motors Truck Manual.

The 15 ohm resistor shown around the charge indicator lamp in the first drawing is actually your purple resistor wire. You are correct in that it provides a small (about 1 amp) current to the alternator field (rotor) to excite the alternator. Once the engine starts and the alternator comes up to speed, an AC voltage develops at the stator terminal (about 9VAC) which pulls in the field relay. The field relay is nothing more than an on-off switch for the regulator. It provides battery power to the regulator from the "A" terminal and shuts off the charge indicator light. When an ammeter is used as shown in the second drawing, the field relay is simply turned on by the ignition switch.

Some engines in the 70's era had an automatic choke wired to the alternator stator terminal. Current from the alternator would slowly heat a bimetallic spring and turn off the choke.

 

Thanks a ton! I've been searching for this info for quite some time. I'm running an ammeter. When I originally put the old harness in, it wouldn't charge. Eventually, I traced it back to the charge indicator light circuit and got it working. This has become one of those things where I keep asking myself, "What was I thinking?" I took something that worked fine, and just to make it look cleaner, gave myself a lot more work. Thanks again!

 

dmptrkr, thanks so much! I got it running and idling last night. If you care at all, I'm gonna tell a long story, I'm kinda bored at work and this'll kill some time . I guess I forgot to mention that I began replacing the whole harness with a new universal 8 circuit harness last summer. The problem was that it didn't include any of the wiring for the start or charging systems. A little free advice for anyone reading this that might be contemplating getting a new harness in the future, spend the money to get a complete harness. The money that I saved by being cheap has more than been spent on buying various colors of wire to make it look right, time, and trips to the Pick N Pull. Anyway, I have a 351W in my truck now. The truck that I pulled the engine from had a MSD 6AL box mated to a Duraspark II dizzy. To be blunt, it sucked. While I was in there, I pulled the whole Duraspark system from the truck at the Pick N Pull the other day. Last night was the first time my truck has idled with no assistance since I started this whole venture last Memorial Day Weekend. Now to ditch the tired, old C4 and get the T5 in.