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
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.
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.
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.
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.
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.
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.