I know its been asked a million times, but i dont need help installing one, need help troubleshooting a failed one. I 3g swapped my 68 6 months ago and everything worked fine, last week i got the engine back in the truck after a rebearing. The alternator is not working at all and iv gone over it twice adn cant find whats wrong so ima get a few more sets of eyes on this because im stumped. everything worked before.

-I installed the alternator with the charging wire from the donor taurus through a 170 amp mega fuse and im getting 12 volt on both sides of the fuse so its good.

- the yellow wire coming off the alt that goes to the starter switch was never removed, the harness was laid up on the cowl during engine removal/install.

- the green wire coming off the alternator was tied into the green wire running to the alternator lamp that gets 12v when the ignition is on. when these wires are disconnected i get 12v from the trucks harness and the alt light is off. then i connect the wire the voltage drops to 4v and the light comes on. thats how i had it connected before the tear down and thats how i reinstalled it.

- the yellow wire coming from the trucks harness (same plug as the green wire above) is not connected to anything and is capped off with heatshrink to prevent shorts.

- i do not and never had a case ground wire and the engine block is grounded straight to the battery as it was before.

 

Got any pictures of what you've done.
And who installation instructions did you use?

Orich

 

this is the wiring guide i used.

 

Did you paint the engine all nice and shiny while it was out? If the mounts aren't grounded well, it won't work. Check voltage from battery positive to alternator case.

 

motor was not painted but i will check that. i actually have the battery grounded directly to the engine. i need to ground the chassis good but all my lights and junk work.

 

You should have had about 14.2 volts coming out of that alternator B+ wire. Even if there wasn't a "case ground" wire, the system will work easier with a seperate case ground.
If you don't have a case grounding point, improvise one.

 

Same here, must have found it at the same place. This is the simplest and just add a voltmeter

 

Ground the alternator case directly to battery with a jumper cable would be an easy test...

 

So, tripple checked all my wiring and am getting all the proper voltages at all the right wires, but they are all ~12 volt. the only other thing i can think of is that the battery was so dead the alt cant see it and isnt turning on. iv left it on the jumper cables to my car to get some juice in it for an hour or so. ima ground it with some jumper cables in an hour and if that doesnt work i guess the alt just took a crap sitting on the work bench????

 

Check the exciter wire (LG/red to starter hot on start and run )

 

Seems like someone else was having this problem a while ago. It ended up being he needed to put the block to firewall ground cable back on as I recall. You should have one there anyways so try that.

 

I had this exact same problem when I was putting everything back together on my truck this winter. I checked both sides of the "Mega Fuse" and had 12v at idle, but I didn't actually check where the (+) cable attaches to the back of the alt. It turns out, that while banging and yanking everything around under the hood, I had somehow pulled the large (+) cable out of the connector on the cable that runs from the mega fuse to the back of the alt, but not out of the heat shrink tubing. So, to look at the cable, it appeared fine. It wasn't until I went to remove the alt. to get it tested, that I noticed the wire had actually come apart. I re-crimped the connector back onto the cable, hooked everything back up and VOILA, 14-15v at idle!


I'm not saying this may be your problem, just that hopefully it is as easy to fix as mine!

 

2x with willowbilly,
Also did you first get the Alternator tested before installing at your loco auto parts store?

Orich

 

i just got the alt tested this morning and it failed. crazy how it failed when it sat on the workbench. worked fine before the motor pull, and worked fine on the first drive home. went out the next day.

 

I keep several of them around. Even the bolt down ones are good for parts, most of them free. It's a simple task to slap in the regulator and brushes. That being said, I have never had one fail, and all mine come from junk cars.
Yes it is odd it was working then not but believe me weirder things happened when I was in the repair business and try to explain to a customer how their car broke down while it was parked in your shop,lol.