3G Alternator Charging Problem

COILDUP · Jul 18, 2010

Hey Guys,
I recently upgraded to a 3G alternator (130A) on my 93 Cobra and I am having a problem I was hoping you guys could help me with. I used the PA Performance 4GA power wire with 200A megafuse connected from the alternator output post to the positive side of the ignition solenoid. I'm not using the stock 10GA black/orange power wires. I connected the white wire from the old harness into the stator plug on the alternator and reused the D plug harness. I also added another 4GA ground wire. I have even had the alternator tested and it is putting out 130A.

The problem I am having is that it does not seem to be charging the battery. With the car running I would expect to see over 14V at the battery, but I'm only seeing 12V. With the car off, I am seeing 12V from the alternator output wire, but only 10.5V with the car running. I have tested the yellow A wire on the D plug and I'm seeing 12V with the key off. I am also seeing 12V from the green I wire on the D plug with the key on. Do you guys have any ideas where I should go from here?

Any help you can provide will be much appreciated!!

MustangLX-5.0 · Jul 18, 2010

It sounds like you have everything connected properly. Make sure the battery is charged, or it could be killing your alt. Also check to make sure there isn't some huge power drain going on, like a short someplace. Fuel pumps are notorious for this. And check to make sure your 4GA power wire is properly connected... and no bad fuse. Try testing the V+ at the very back of the alternator where the wire connects. This will rule out a bad fuse or connection... if that is the problem.

LarsD · Jul 18, 2010

Reattach the original 10ga wires (in addition to the 4ga wire) and see it that clears it up.

HISSIN50 · Jul 18, 2010

Since the A circuit is intact, you should be fine regarding not using the stock charge wire (it's still connected at the solenoid and functioning, or you'd not have 12V at the A wire).

As you know, using the two stock charge wires on both ends is a possible hazard. You did well.

The amp/battery light should be on and the resistance from this (and the parallel resistor if the light is burned-out) should knock the I-circuit voltage down to 1/4-1/2 battery voltage.
You could try a bench test of sorts where you connect the I-wire to a wire-splice going through a 500 Ohm resistor and then to switched 12V. If the alt charges now, the I circuit has an issue.

If the I circuit is ok, see if the stator loop shows any voltage. It should float about 1/2 battery voltage. Disconnect both ends and Ohm them out (looking for less than an Ohm of resistance). If the loop has continuity but no voltage, the I circuit is likely acting up.

LarsD · Jul 19, 2010

HISSIN50 said:
As you know, using the two stock charge wires on both ends is a possible hazard. You did well.

Whats the possible hazard? I did it on my old 89 and never had a problem. I'm about to do the 3G on my 91 and I'd like to know for reference.

HISSIN50 · Jul 19, 2010

On a stock alternator, the stock charge wires could catch on fire before the fusible links opened the circuit.

With a 3G, if the 4 AWG charge cable ever opens (broken lug, the cable chafes, shorts and blows the fuse, etc), all of the 3G current would run through those stock wires. If the fusible links don't open soon enough, bad things could happen.

COILDUP · Jul 19, 2010

Thank you guys for all of your help!! I will check the output from the post on the alternator to see what I'm getting, but when I checked the cable where it attaches to the solenoid, I was consistently seeing 10.5V. I'll also check the stator wire voltage. The weird thing is I'm not getting the amp/battery light, but the battery gauge is reading pretty low (like 10V).

If I had a short some place draining the battery, would this be the behavior I'd be seeing? I know there were some instructions around here somewhere on checking fuse by fuse. Just wondering if I should start checking that as well.

Thanks again!!

LarsD · Jul 19, 2010

HISSIN50 said:
On a stock alternator, the stock charge wires could catch on fire before the fusible links opened the circuit.

With a 3G, if the 4 AWG charge cable ever opens (broken lug, the cable chafes, shorts and blows the fuse, etc), all of the 3G current would run through those stock wires. If the fusible links don't open soon enough, bad things could happen.

Makes sense, never thought of that. So you can get by without having them connected at all then huh?

HISSIN50 · Jul 19, 2010

LarsD said:
Makes sense, never thought of that. So you can get by without having them connected at all then huh?

Yes. Just be sure the wire gauge for the new alt is large enough. For instance, 4 AWG wire is insufficient for a 200 amp alternator.

One can insulate the alternator-side wires and leave them be (and connected at the solenoid so the sense circuit functions).
Or one can re-create the sense circuit and remove the stock charge harness.

LarsD · Jul 20, 2010

HISSIN50 said:
Yes. Just be sure the wire gauge for the new alt is large enough. For instance, 4 AWG wire is insufficient for a 200 amp alternator.

One can insulate the alternator-side wires and leave them be (and connected at the solenoid so the sense circuit functions).
Or one can re-create the sense circuit and remove the stock charge harness.

I've got a 3G, and one of those PA 4ga setups on the way. So I should be OK. How do you recreate the sense circuit?

HISSIN50 · Jul 20, 2010

LarsD said:
I've got a 3G, and one of those PA 4ga setups on the way. So I should be OK. How do you recreate the sense circuit?

You could run a wire from the A-regulator-terminal to the battery lug of the fender solenoid. For circuit protection on this wire, you could use an 18 gauge fusible link (as is stock on the aero fox) or a 20 amp fuse (like 94-95 Mustangs have). Be sure the wire gauge chosen and circuit protection are appropriate.

You could also try to divorce the OEM sense circuit from the charge wire harness, thus keeping the OEM sense wiring.

Some aftermarket regulator kits have one loop the sense wire to the alternator's charge lug, but that defeats the whole purpose of having a sense wire (to accommodate voltage drop through the charge cable).

3G Alternator Charging Problem

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