78 F-150 Alternator not charging/Wiring info
 

Hello everyone!

I come looking for help with my 1978 F-150 Explorer. Yesterday when I went to start her up and was ready to go out for a trip I unfortunately was met with a lit alternator light.

The truck is now producing zero of it's own voltage. When I disconnect the battery the engine immediately shuts off.

Could this just be a bad regulator or does it definitely mean my alternator is toast? I checked the wiring for any lose connections and everything seems to be okay. What I did find were a lot of sketchy repair jobs. Regardless those had been around for years without any problem until yesterday.

How do I go about diagnosing the problem?

I'd also like to find the wiring diagram for the alternator, regulator, and solenoid so I can return it to it's original wiring and get rid of those weird repairs. I couldn't find much while searching.

Thanks a lot everyone!

have teh alternator tested. mite be teh regulator is shot.

I did another search and found a tip as to how to test the alternator. It involves removing the "F" wire connection from the back of the alternator. Then connecting the "F" post of the alternator directly to the battery positive terminal. If this causes the alternator to start outputting power it indicates that it is good.

I'm going to go try this now and will return with an update.

Okay update!

I performed the above test and with the multimeter on the battery the voltage did go up once I touched the F connection to the battery positive. I also heard the alternator whine and the engine sound change like it was charging. I hope this means my alternator is good.

Could this mean I've successfully narrowed the problem down to a faulty regulator? Or could I still not be out of the woods?

Yea that pretty much means the regulator is bad and the alt should be good. Just so you know though you shouldn't pull the bat wire off with the engine running. That was fine in the old days with a generator, but alternator can produce voltage spikes as high as 70 volts when you disconnect the battery. That can fry voltage regulators, radios, ignition modules, etc.

Hi Tim.

Thank you very much for the information. Also thank you for warning me on disconnecting the battery with the engine running. I knew it was risky but I didn't know the effects where that dangerous.

I also have another question. For some reason someone put a wire from the "I" post of the solenoid to the "S" terminal of the regulator. This causes one of the relays of the regulator to close when the key is put into the "on" position. However I can't seem to find anything about this on the schematic diagram.

Can anyone tell me what that is about?

This is a wiring hack by a previous owner, and is wrong. If you have an ALT light, then the only thing going to the S post of your regulator is the S or STA post of the alternator.

Applying power to the S terminal of the regulator closes the field relay in the regulator - but if you have an ALT light, keyed power isn't supposed to do this; power from the stator terminal (S or STA) of the alternator is supposed to do this. This is what shuts the ALT light off once the engine starts.

With an ALT light setup, the I terminal of the regulator is connected downstream of the ALT light in the dash. The other side of the ALT light gets keyed power from the dash.

Thank you guys for the help. I restored the truck to original factory wiring for this circuit and replaced the regulator. The truck now charges properly again! In fact before the repair it was putting out a bit of an over charge, but now it's to spec.

I did have one problem though.

At first my ALT light turned on before engine start as it should along with the OIL light. However when I went to start it on my way back from the store I noticed it only glowed dimly before start. After a couple more tries it didn't glow at all before engine start. I tried jiggling the connector to see if perhaps there was a bad connection but that didn't help. The bulb itself is also checked and is a known good bulb. The system is charging regardless of this ALT light malfunction going on right now.

What could be the problem here? Bad Ground at the regulator? Bad regulator?

I was wondering if it's possible to manually ignite the ALT light from the regulator's connector by either injecting power into the "I" connection or grounding it. This would serve as a test to see whether or not the problem lies in the cabling and connections that lead up to the dashboard.

Thanks again everyone for the progress so far, I'm glad my truck is charging again!

I would be weary of grounding the I terminal of the regulator plug to test the ALT light connectivity in this case. When debugging an electrical issue that you have not yet solved, it's never a good idea to pull connections up to 12 volts or down to ground. If something is sitting at a potential that it shouldn't be sitting at and you short it straight to another potential, this direct short can fry the wiring. In this case I would test the wiring for continuity, which is much safer.

Just a side note, my LTD (college car, now long gone) had the original electromechanical regulator that lit the ALT light while the engine cranked, and shut the ALT light out once the engine started and the alternator started charging. Over time the regulator had to be replaced, and I replaced it with a no-name electronic regulator from AutoZone that probably cost less than $20. I had the same issue - with the new regulator, the ALT light stopped working, yet the alternator charged fine.

Given this happened to both of us, this might be an issue with some aftermarket regulators. However, it could also mean that the ALT light shunt has shorted. There's a 15-ohm resistor in parallel with the ALT light to supply power to the regulator if the ALT light burns out. In sedans it's a resistor wire in the harness, and I know that in the 80-86 trucks it's on the back of the cluster - I'm not sure how it's implemented on 73-79 trucks. If this resistor shorts, then current won't go through the ALT light filament. However, this is far less likely.

That shunt is a resistor wire on these trucks. The actual wire itself is the resistor, same for the coil power for the ignition. I'd bet its the new regulator and here's why: Ford lists two different part numbers, one with idiot light and one with an amp gauge. The wiring is different in both cases. All the aftermarket regulators replace both styles.

Before I did the 3g swap on my fairlane I had the same thing happen, old mechanical regulator started overcharging to 18 volts. The part store replacement caused the alt light to be dim with the engine off.

Hmm interesting. My replacement is also a 20 dollar electrical regulator from autozone. I was going to be buy an original part for even cheaper but when I got to that part store it was closed. I decided to just go with autozone thinking that part would probably be better anyway. I'll try and see if I can get a refund or exchange.

im having similar issues right now. im getting no voltage alternator to charge battery. ( i know this is bad) but when i pull the terminal off the post of battery , truck dies. ive replaced alt, battery, and regulator (20 dollar one) im on my second alternator in 2 days. can't figure it out. i have an ammeter with no dummy light i can see. im getting voltage from the orange S wire on regulator when key is in run position. all fusable links ohm out, battery+ to alt ohms out, as well as pin F to Fld terminal on back of alternator. i tried jumping out volt reg with jumper wire to test output...nothing at battery. i have 3 wires that connect to my alt. fld, B, and sta. according to wiring diagrams, i dont connect sta because i have no electric choke (manual hot air choke)
however, i have 3 wires from my harness. and the original alt was wired in with all 3 as well. ive tried with and without the sta connected, no voltage at battery still. no voltage at FLD terminal on back of alternator to chasis ground either. Im stumped. been tracing wires and schematics for 3 days with no luck, just a dead battery that is 3 days old.
please help
1977 f-150
2wd auto
351m400
2 barrel motorcraft 2150 ( recently professionally rebuilt installed and tuned)

this wiring hack, though it may be wrong, what is the damaging effects by doing this as a means of it as a temp fix

this wiring hack, though it may be wrong, what is the damaging effects by doing this as a means of it as a temp fix

Where did you get the alternator? I had a problem with mine not charging the battery and had 2 remanufactured from 2 different national parts companies put on, first one worked sporatically and then died after only a couple weeks, second one worked but wasn't putting out enough juice to actually charge the battery (it would continue running if I pulled the cable off the battery so a "false positive" for me). I also suspected that some wiring was old and my fusible link was a little soft so I replaced all of them. I got a NEW alternator (Summit Racing) and have been running like a champ ever since.

If you've got an ohm meter you can test how much is coming off the alternator with it all wired up normally. (test the positive on either the alternator post or on the solenoid where the alternator wire ends up (I did both to ensure the cable itself wasn't losing anything along the way), anything