My son drives a 1997 F-150 with a 4.6L engine.


Ii stopped charging and we took the alternator off, took it to a parts house and they said the alternator was bad. Bought a rebuilt one and installed. The truck still doesn't charge.


Took the alternator off of my F-150 installed it on his and it still won't charge.


I have checked the maxi fuse, the mini fuse and both are ok. I checked the battery voltage at the B+ post on the alternator and there is 12V there. I checked for 12V at the alternator for the exciter voltage and it is there. I have checked every fuse in the truck, both under the dash and in the power distribution box under the hood and all are ok. I have checked the continuity of all the wires involved and all a good.


So what could be causing this truck not to charge. Thanks.

 

When u bolted up the alt to the block, that serves as your ground. Make sure it's a clean surface. If you are getting 12v at the alt with a good ground it's no good.

Ground the multimeter on the alt housing mounting bracket where it bolts up. Place the red plug of the dmm on the + bolt for the alt and recheck.

 

Make sure you check that inline mini fuse and holder real well. Screw around with it while the trucks running. Bend /push the wire entering the holder and make double sure the contacts aren't shorting. The 97/98's have this problem. They don't short completely, but enough that it won't maintain the battery. So , - inspect it, clean the contacts, move it around while the engine is running. When you get good contact, you'll know it. The alternator will sort of whistle, so listen for changes while your fooling around with wire/fuse holder and fuse.

MEGA fuses can crack inside as well. Best way to check isn't as simple as continuity. Easiest/fastest way to check that circuit is to bypass everything real quick w/solid wire/cable. Make your bypass from the alt to the relay. If anything changes in the charging circuit, - (should be about 14V) Then it has something to do with the mini and mega fuse within the circuit.

Like @ibd2328 posted, you need a good ground. Absolute ground is the Alt case/engine matting surfaces. It's a good idea to wire wheel at the engine side of that connection.

Good Luck

 

Thanks 1bd2328 and Jbrew for the responses. I checked all that both of you mentioned and all checked ok. There is continuity between the alternator housing and the block. The digital meter showed .2 ohms. I shorted the maxi fuse, no effect. I moved the mini fuse holder around, pulled out on it and pushed it in, no effect. This really has me stumped.

 

Bet dollars to doughnuts it's a bad alt. regulator anyways.

 

Ok, more info since I posted. We took the rebuilt alternator off and took it back to the parts house which tested it and said the alternator was good. Returned home and took the alternator off me F-150 and put on my sons and it won't charge. Put the rebuilt one from the parts house on my truck and it charges.


I don't know. I have been a mechanic for 45 years and have never run across something like this that I can remember. The alternator case has a good ground to the block. The wiring shows good. The fuses are good. All voltages are what they are supposed to be. Heck I even ran a jumper wire from the battery hot post to the exciter wire on the alternator and it still won't charge. Thanks guys.

 

Do the big 3 and I think ur issue would be fixed.

 

Yea, - he already has a 3G alternator. Wait, he doesn't anymore does he lol.

I still have the factory original on my 98, - Large Case 3G. I knew better than to give that one away for a 3rd party junker. There's only one place I trust on line to buy Alts from.. Stores are selling way to much junk out there now a days, - specially alternators.

 

I've always had good performance out of my Bosch Alt's. Factory alt I took off years ago still has resistance, so It is probably still good.

What after-market brands would you consider "reputable" brew?

 

Not really aftermarket, Ryan has a a site here, -

www.rjminjectiontech.com/?p=22

But the sites down, - it's been down for a couple weeks. I'll have to get a hold of him , see what's going on. He takes the Ford 3G's and rebuilds them for you. Sells them as well along with other products. He doesn't deal in the BS stuff and of course knows the difference (most important). I just got a bunch a misc ford blade ends, - crimp and solder type since my harness is deteriorating lol. But yea, the sites down. His prices are great as well.

He owns this site as well, -

http://oldfuelinjection.com/

But it's a tech site, - informational.

Not sure what's going on yet, - good guy, knows what he's doing.