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  #1  
Old 09-29-2011, 01:41 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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!
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Old 09-29-2011, 04:01 PM
400 .040over 400 .040over is offline
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have teh alternator tested. mite be teh regulator is shot.
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Old 09-29-2011, 04:55 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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.
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Old 09-29-2011, 05:33 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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?
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Old 09-29-2011, 05:59 PM
Tim C Tim C is offline
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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.
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Old 09-29-2011, 06:24 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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?
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Old 09-29-2011, 10:18 PM
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fmc400 fmc400 is offline
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Originally Posted by CharlsChainCharlson View Post
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.
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.
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Old 10-01-2011, 11:36 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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!
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Old 10-02-2011, 10:09 AM
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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.
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Old 10-02-2011, 11:19 AM
Tim C Tim C is offline
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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.
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Old 10-03-2011, 05:18 PM
CharlsChainCharlson CharlsChainCharlson is offline
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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.
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Old 10-03-2011, 05:18 PM
 
 
 
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