i installed a gm alternator in my 79 and everything worked fine. i swapped it into my 75 wired the same way and it wouldnt shut off. even with both battery cables removed and the key shut off and removed from the ignition, the truck was still running, until i unplugged the alternator. it only has the charge wire from the alternator to the starter relay (battery positive) and the exciter wire going into the cab. Had to unplug the exciter to get it to die. It does not need that wire to charge above idle. but it worked fine in my 79. was the 79 factory ford wiring different that the 75 somehow? Sorry this might be kind of hard to follow.

 

Does one truck have an ammeter gauge, and the other truck have an ALT light?

 

i think they both have gauges. ill have to look.

 

First of all what are you thinking putting a government motors anything in a ford no wonder it doesn't wanna work LOL go to your local automotive electrical shop they should have a direct plug in diode that goes in line with the gm plug .plug it in and presto it'll shut off!!! they some time's feed power back through and allow the ignition to stay powered up

 

i used it because they seem to last longer in the mud and i got rid of about 20 ft or so of wiring to clean up the engine bay. That seems to be what it is doing, but why would it do it in the 75 and not the 79?

 

both trucks have ammeter gauges. the alternator is wired with the number 2 terminal jumped to B+ (the charge wire on the back of the alternator) and the number 1 terminal going to the exciter wire.

 

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Just my $.02, but maybe that GM alternator is just so happy to be on a brand of motor that actually runs, it doesn't want to let it shut off!

 

You have to have something crossed since the alternator has nothing to do with the ignition circuit.

Do you have the BROWN "I" post solenoid wire tied in with the + from the battery?

Josh

 

Allens77crew told you right !!! You need a diode. The way you have it wired, it will back feed and you won't be able to shut it off without a diode in the line.

 

I have had a GM alternator in my F350 for years.

It was sold to me as a stock item referred to as a 'one wire GM Alternator'
the diode was already built in.

Apparently used in some old farm tractor applications too, where generators were no longer available.

The wiring to the alternator was inoperable when I bought the truck so I installed this type of alternator.

 

If the ignition switch is OFF there is no way for voltage to get to the coil to keep the ignition going.

I am guessing the Brown "I" wire somehow is connected to full voltage.

With the switch off, reconnect the battery and see if there is voltage at the coil.

Josh

 

Thanks guys! I will have to do some more checking with the voltmeter this weekend when i get back to the truck. Tracing wires is simple now. I have removed everything i didnt want or need and have the harness unwrapped at the moment. Didnt get around to taping it back up last weekend.

 

I would think you've wired the alternator wrong. To make a GM internally regulated alternator work you just have to jump from the power wire to the field wire of the alternator.

Sounds like you've tied something to the I terminal of the solenoid which is feeding the ignition.

I've always run GM Alternators in my Ford so I have at least one reliable part in the truck. Actually one of them is running on a Chrysler ignition module after the Ford box failed.

 

also just a note that it would be safer to pull your coil then try and grab alt wires with possible AC or (high DC amp) voltage to kill it