alt not charging

[Rootie524]

i have all the bugs of the new engine almost sorted out but for the life of me i cannot figure out why my alternator is not charging the battery. voltage at the battery is not what it should be. so i checked to see what the voltage output of the alt was. right on the positive of the alt itself, it was zero. that only leaves one other connector and its the three wire connector going into it.

i thought maybe a fuse somewhere but i could not for the life of me find a fuse directly related to the alt. so i checked them all and it seemed fine. unless its some magical relay somewhere hiding from me.

it's probably something stupid somewhere and i hope one of you guys have run into the same problem and know how to fix it.

btw, it's a brand new alt. it happened after the motor swap on my old alt, so i just bought a new alt and i have the same problem. so its definitely wiring somewhere.

[jmgtp]

If you have 0v at the alternator post then you don't have a connection to the batt. There is a fusible link. On my car the alternator charge cable goes to the +12v post on the starter. Were it connects to the starter post there is a fusible link. I'd start there and make sure the fusible link isn't burned out. You should have battery voltage on the alternator post at all times, engine on/off, key on/off...always. I'm betting it's the fusible link or it simply isn't connected.

[Rootie524]

yeah but even so shouldnt there still be some voltage at the alt whether its connected to the battery or not? i mean if the belt is spinning the pulley it should be putting out some sort of voltage even without being connected to the alt.

which is why im thinking the 3 wire connector is bad or there is a fuse somewhere. although, like you said at the starter your alt goes there. on mine the only thing on the positive terminal of the starter is the one going directly to the battery. and from the battery it goes to the fuse box. that's about it. i cant see any wires that arent hooked up. but i didnt have too much time to check today.

[jmgtp]

That I don't know for sure, it might not put out any voltage at the post if not connected to a battery. I do know that if you dont have power at the harness for the alternator it cannot charge, that supplies the 12v for the field. An alternator does not have permanent magnets so you need voltage to create the field.

My recommendation:
Step 1: hook the alternator charge post up to the +12v of the battery and verify that you have battery voltage on the alternator post
Step 2: with key on you should have 12v in the harness (I think on pin L) this is necessary to create the field. If you don't have voltage here the alternator cannot charge.

That said, if you have battery voltage on the alternator post and 12v in the harness then it should charge. If it doesn't I would suspect a bad alternator. Remember, new part doesnt mean good part.

[Azrael]

The alt is fed a small amount of battery power from the voltage regulator into the field winding in the rotor to generate the magnetic field. That's what those 3 other wires are for. The + post on the alt and ground should be seeing battery voltage at all times. If not, then as jmgtp says, there's a break in that connection somewhere. If you can still start the car then the cable between the starter and alternator is likely the problem.