OK, this thing id driving me up the wall and I cant figure it out or find anything helpful online. Pulled the 390 a couple months ago to rebuild and pulled the 3g alternator off it. I had done the 3g upgrade a year ago and have had no problems with it. Well the engine is done and back in and the same alternator was put back on, but it wont charge. Took it to the parts store and it tested fine according to the machine, but was putting out 15.6 volts. Put it back in the truck and still wont put out anything. Checked all the wires for continuity, the excite wire is getting 12 volts with key on, and the case is getting a good ground. Even tried running jumper wires from the pins to the connector incase a something was pushing in or not getting a good connection. Still nothing. Im baffled!

 

Make sure you have 12v on the large output post with the truck off. This will verify it has a good connection to the battery and no blown fusible links(if you are using the original wiring).

And for verification, with the engine running, take a short jumper wire and touch the battery + post, and then touch the "I" terminal(I believe it was originally red/green or green/red).

 

This is ho I have it wired. the only difference being its a 130 amp alternator and the mega fuse is 175 amp.

http://www.fordtruckclub.net/forum/p.../3ginstall.jpg

I terminal in plug reads zero with ignition off, 12 volts with key on. Zero ohms between +B stud and battery positive, so the mega fuse is intact. The only thing I can think of is possibly the voltage regulator due to the 15.6 volts when on the tester at the parts store, but im not electrically literate enough to know if that would cause these symptoms. I swapped the voltage regulator off another alternator I had laying around and nothing changed, but that was known to be a bad alternator so I might have just been swapping bad parts for bad parts. I have thoroughly tested every possibly thing I can think of in the wiring harness for continuity and 12 volts on the excite wire and everything checks out. I even used jumper wires from alternator terminals to plug terminals incase there was a loose connection in there and cant find anything wrong. My brain tells me it has to be something with the alternator itself but the auto parts store test (ran 3 times) passed it with flying colors.

 

So today I got a replacement (junk yard) alternator and new wire plugs. Still doesn't charge. I just remembered though that while rebuilding the engine I decided to rebuild the wiring harness as well in order to eliminate all the clipped wires from stuff I have taken off or added, especially under the dash. In doing this I removed the old voltage regulator, old fuse panel, and a lot of old wiring. removing the old voltage regulator shouldn't be an issue since the 3g is internally regulated, and I replaced the fuse panel with a pair of aftermarket ones that take the ATC fuses instead of the glass tubes. Everything is wired off these 2 panels, one always hot for ignition switch, horn, and similar, the other is switched power when key is on. This is the one that the exciting wire for the alternator is plugged into. In my head this new wiring should work perfectly and be much simpler (easier for me to understand anyways), and indeed every single electrical item on the car from lights to ignition to turn signals to radio, everything works flawlessly, with the exception of the alternator. Green wire has switched 12 volt, yellow wire has constant 12 volt, white wire jumps between plugs, and +B terminal is wired directly to battery by way of starter relay. Any new thoughts?

 

So I took the alternator, battery, and a spare alternator in to the parts store today and had them all tested. To my surprise they where all fine, including the old alternator I had sitting around that I had replaced almost a year ago. Now the chances that 3 different alternators had the same problem that made them not charge on car but tested good at parts store have got to be about a zillion to one, give or take, so I was pointed back to my wiring which I had tested repeatedly. I figured instead of trying to track down the culprit and get all frustrated I would just take it all out and start from scratch. With this in mind while at the parts store I grabbed a new 175 amp fuse. Low and behold when I started taking everything apart I found the old fuse was loose. Don't know if it blew or not, but with one end held solid, the other rattles around. Some how when I was testing it before I was able to get zero ohms, still don't know how that was possible. Anyways, new fuse, hooked everything back up and now it charges at 14.4 volts. Finally after 2 months I can enjoy this hopped up 390; that is as soon as I finish getting the carb and timing tuned in. Thank you Franklin2 for responded to my S.O.S., I appreciate it.

 

jchamp429, Glad you got her running. And now for my question...In post 3 (the diagram) My 3g looks nothing like that. I have only one 3 wire connector. One side is green w/ red stripe. The middle one is white w/ black stripe. It's a grey connector and plugs into a grey connector on the Alt. The other wire is orange w/ light blue stripe. Any ideas on how to wire it up? It's an 1978 Bronco. Thanks Tim

 

This thread is a year old. But to me, it sounds like you have a 1G, not a 3G. Where did you get your alternator from?

 

Thanks for replying Franklin2. Yes it is lol. I can't remember exactly which car. But I do know it was an Sable with the right year and engine. At least according from the list I got off of FSB. I'll try and post pics.

 

Ya if you could post pictures that would be helpful. I don't claim to be an expert, so I might be wrong here, but I think there are a few different mounting styles for the 3g resulting in different looking cases, but I believe they all have the same wiring hook ups.

 

Here are some pics. Where there is one hole. It's actually two.

 

Your alternator looks the same to me as the picture below. Has the same letters cast into the housing(ASI). The only difference I see is instead of a yellow/white on the "A", you have a orange/white.

 

Franklin2, Thanks for replying. So do I hook up the orange/white wire. Just like the yellow/white wire? TIA

 

I would. That's the wire that monitors the voltage of the trucks wiring, and tells the internal reg how much output the alternator should have. You can put it in different places on the truck and get different voltage levels from the alternator. Directly to the rear of the alternator will be the most conservative spot, since it's reading the voltage right at the alternator output. You could put it closer to the battery, and it would vary the output of the alternator to control the voltage at the battery, and compensate for any voltage loss through the output wire of the alternator and the fuse.

I would not put it way down in the harness far away from the alternator or the battery. That would increase the output of the alternator even more, but I would be afraid the battery might start losing water.

 

Definitely a 3g. Color of the wires doesn't matter as much as where they go and what they do. Just pretend orange equals yellow and you'll be fine. The 2 large holes indicates it's a 130 amp alternator instead of the 95 amp which had 4 smaller holes in that spot. The nub on the back may need to be hack sawed off to clear the cylinder head. I found that some of the wind stars from the same age range had the same alternator without the nub. Also there are a couple different output terminals, some are flat like yours and some have a long 90 degree thing that puts the nut outside the diameter of the alternator and is a little safer from arcing on the engine if the rubber boot wears through from vibration. Also, the 3 long metric bolt that hold the case together are spaced 120 degrees apart so you can re orient the casing to make the wires come out at a more friendly angle, just be sure to get it all reseated properly. I have done this before and never had bearing problems.

 

Thank you both. Gonna wire it up next weekend hopefully.