Mythbuster74

So like the title says I can't get my alternator to charge, even with the 470 ohm resistor in line to a 12v source. I've used my ordinal alternator from a 2005 GMC Yukon and a brand new one from a 2005 express van. Neither will charge. I've checked the voltage through the wire and at the connector it has 8.7v. The alternator is attached to the water pump with a bolt and another bolt with a turn buckle, I've also held a large gauge power wire to the case and head just to rule out a ground issue.

Mythbuster74

4 pin plug

Toddoky

Make sure your static battery voltage is between 12.4 and 12.6 volts before testing your alternator to rule out the battery being at fault. Do you have the resistive exciter wire connected to the "L" terminal of the harness/plug? Do you have the remote sensing wire connected (terminal "S")?

Mythbuster74

The battery is also brand new and will hold a charge somewhere in the 13 range with the motor off. I have the resisted wire in the L position, but no other wires connected. I'm running a megasquirt to give more info

Mythbuster74

So no I don't have a wire in S. I read that you don't need it but can add it if need be. It goes to a 12v source right?

Toddoky

The remote sensing wire/circuit is used by the voltage regulator to adjust alternator output based on load demand...I would recommend taking advantage of that function. You'll need to connect it to a remote 12V source that is within your central power distribution center (main fuse block or buss bar). Are you testing for alternator charge output at the battery, or off of the alternator output lug?

Mythbuster74

I didn't know it did that, I'll have to hook that up. And I as testing at the lug on the alternator. Positive on the lug, negative on the block. Also checking what the megasquirt was reporting.

Toddoky

I would hook up the sensing wire and retest the alternator again...if it still doesn't work, you can pull it off and have it bench tested.

Mythbuster74

12v switched correct?

Toddoky

Yes, that's correct.

Mythbuster74

Welp did that and still nothing haha. The alternator I've been using was new from GM and has been on a running car, so it's known good. Does it have to be above a certain rpm to excite? Anything like that?

Toddoky

At least it's not wasted effort as you'll want the benefit of the remote sensing wire being hooked up once you figure out what's up. No, the only alternators that need to be accelerated slightly to get them to excite are those of the 1-wire variety. If you're certain your resistive exciter wire going to the "L" terminal is supplying a 12v signal and is not shorted to ground, then I would have the alternator bench tested to validate its operation. It's either the alternator or the wiring that's causing your issue.