Hey everyone, new to the forum

I've hit a wall on trying to figure out a charging problem on my 86 F-150 302 efi v8. Today i replaced the alternator, did some voltage tests..and the truck is still running off the battery and not charging. 12.20v running and 11.9 with lights on...which was the reason why i replaced the alternator in the first place.

My belts are fine, all the fusible links i could find aren't blown, not really sure where else i should look to fix this issue. Any help giving me some direction would be rad

 

Welcome to the forum kgodsey!

Is it getting excitation current to "turn it on"?

For Fords this is usually a light green wire with a red stripe that should have power with the key on..

 

ah, i don't know..but that will be the next place i'll check, thanks!

 

I have some troubleshooting help here;
Ford Truck Enthusiasts Forums - Grubbworm's Album: Troubleshooting help, for anyone that needs it.
Let me know if it helps.

 

Grubbworm,
Rep to you for archiving this valuable type of diagnostic information.

Direct link to that page:

Ford Truck Enthusiasts Forums - Grubbworm's Album: Troubleshooting help, for anyone that needs it. - Picture

 

Yeah! thats what i'm looking for..i'll try these out tonight when i get home from work and let ya know what worked

 

Yes, when the key is in RUN it's getting voltage to that wire and to the pin on the plug to the alternator.

Double checked the fuses, plugs, and connections...started and ran the truck for a while, lights on/off, blower on/off, revd the engine, etc..and the still no charge.

I didn't get a chance to do any of the current tests in Grubbworm's guide, i don't have an amp clamp right now, but i'll try em tomorrow!

I guess it's possible my alternator could be bad outta the box but, for some reason i doubt that's likely, but how would i go about testing that?

 

It very well could be.
The simplest way for you, without test equipment, would be to bring it into the store where you bought it and have them test it on the bench.

I figure you likely have a second generation alternator.
Does it have an internal regulator with 2 plugs on the back and a fan on the front?



If so, did you replace the infamous "Fire Plug" on the output side of the harness?



This is an ongoing problem with Ford trucks of this vintage.
The plug corrodes and that resistance causes heat which melts it and then it shorts internally.
The alternator is trying to charge into a dead short and can't provide power to the battery.
The alternator doesn't last long, and sometimes even catches the engine on fire.
Many alternator rebuilders include this plug with every alternator sold and won't warranty the unit unless it's been replaced.

They are available separately from company's like Pico. #5711PT fits 81-89 Ford and costs about $5.

 

That's the alternator i got, yep. I guess I'll just bring it back to napa.

I checked the continuity on the plug and it checked out ok, but $5 for peace of mind doesn't sound like a bad idea.

 

Here is how I would test this.

1: Remove both plugs on the ALT

2: Turn key to RUN to provide power

3: Test for 12v or battery voltage on the following, you can use a 12 volt test light if you don’t have a voltmeter:
Both large power output wires in the “Fire Plug”
The A wire on the Voltage regulator plug, top plug in the 2G photo
The I wire in the voltage regulator plug (light green w/red)

On your truck, EFI, the small white wire on the fire plug should just jumper/loop around to the “S” connector on the voltage regulator plug. You might have to un-wrap some black tape to verify this or just do a continuity test with an OHM meter if you have one. On my truck the small white wire on the power plug goes to the electric choke on the carb.

More info
The large wire on the voltage regulator, “A”, is the feed source for the field. If the field isn’t getting power the ALT won’t produce any output power.
If all this tests good and the ALT passes a bench check, you will need to remove the ALT wiring harness from the truck and visually inspect it. The harness disconnects in a 4 wire plug near the starter relay. I can post photos of the unwrapped harness if you get to this point. DO you have an AMP meter in the INST panel?

If you do change the “fire plug” just be sure you do a good job with the splices, each wire can carry 30 amps… point is… don’t just move the “fire point” somewhere else.
Jim

 

In "3" above both black/orange wires will *always* be hot unless their fusible link is toast.
No need for a key.
That's why a melted plug will kill the battery in short order.

 

i bought alternators for parts stores a few different times that where bad right off the shelf....i just ask them to test the new one before i leave

 

Yeah, i do..it doesn't move to indicate charge since and i'm not seeing anything with the meter either...last night i noticed voltage at both orange/black wires. But i'm gonna double check that and about the white wire that goes back to the S tonight when i get home and try to pick up another plug while i'm at it.

again, thanks for the help guys

 

The AMP meter should move to the discharge side with extra load, lights, heater fan... even without the ALT working.
Don't put a lot of faith in the AMP meter, it is a pretty worthless design.
The fat wire on the voltage reg plug that powers the field circuit should be hot all the time, be sure and check it also.
Jim

 

So i had the alternator bench tested and it tested to be good...nice to know. But when i unwrapped some of the taped up harness i think i found my problem, the white wire that should loop back to the S terminal is just capped off and looks burnt-up.



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I bought new plugs and wire, gonna solder n splice em tomorrow and hopefully that will fix this..