voltage across battery when charging ?

[nitrous_bob]

not that it's apples to apples, but my battery on my bike w/ the key off was about 13.5-13.8

while running it was about 12.10-12.50

never checked the stang, never had charging issues.

[Hozer 88GTConv]

To back up what has already been said, this sounds a lot like my charging situation. I have underdrives, but the stock pulley on my Powermaster and it barely keeps voltage above 12.5 at idle.

What is the diameter of your crankshaft pulley? That will tell us all for sure if you have/or don't have underdrives...

[John Del]

If the alternator checks out on the bench, there's something wrong with the wiring, but it's not the guage of cables used to remote the bat to trunk. If the wiring can deliver the battery's current to start the engine, it's good enough to charge the battery. In any case, you should have a minimum of 13.8 volts at idle.

John

[Mustang_289]

We're running a stock crank pulley as once the motor is broken in, a Votech T-Trim awaiting our pony.

We put the grounding strap on - from the firewall screwed to the head on drivers side. We also put another alt. on the car and same low 12.3 volts. This clearly takes the alt out of the picture. We've also switched batteries as well no change.

[82 GT]

Well...that only leaves the wiring. Someone already said there could be a short somewhere. If it's not the battey, alternator, gauge of wire or lack of grounds then that only leaves the wiring and/or connections.
Can you tell us exactly how it's all wired?

[Mustang_289]

PA Performance 3G 130A alt, got their wiring kit with the 200A inline fuse which runs to the starter selenoid. Battery is located in the back hatch with a Morosso relocation kit, which has a 2G wire going from the starter selenoid back to the positive on battery. Negative from battery runs to a shut off switch, which is grounded to the frame in the rear. Up front, we've got an engine to frame ground, and an engine to firewall ground with the strap.

Is there a quick and easy way to check for shorts in the system?

[~The Jester~]

Sounds like a short to me. If you disconnect the battery, all of a sudden it's putting out the correct voltage at the starter solenoid. That eliminates the alt. and ITS wiring right there.

Not familiar with the PAP unit, is it internally regulated, or do you have an external regulator on that? I had that exact problem, and figured out my external regulator wasn't wired correctly. Fixed that, and buddah bing, 14volts at the battery in my hatch.

[Mustang_289]

yeah we disconnected the battery while the car is idling, and we saw 14.5v across the selenoid.

Are there any specific areas where a short could be, or do we need to check the whole flipping wiring in the car?

[~The Jester~]

D/C the postitive lead from BOTH ends, check it with an OHM meter to ground, see if it has a short in it. With it D/C there should be NO reading on the OHM meter.

This is a tough one bro, the more I think about it, the more confused I'm getting. This just doesn't make sense. It's gonna be something "stupid" when we finally figure it out.

Ahah! Are you running a disconnect switch in the rear of the car? If so, WHAT KIND? I had this freakin problem with my car too.

[82 GT]

Yeah, if it's Summit junk, get rid of it!!

[Mustang_289]

checked the + and - cables last night, no short between them. I even checked the disconnect switch it's working like it's supposed to.

We brought in an alt off a friend of my son's car. Put that alt on, still reading 12.3 volts. I told my son I'm a little concerned about the vortec supercharger bracket grounding properly to the engine block, as the block came painted from Keith Craft. So we ran another ground from one of the alt bolts to the frame. We took another reading and read about 13.3, this was very positive. Here's where it gets freaky. We took off that alt, put back the PA Performance alt on, and took another reading at idle and we're back to 12.3 volts.

I was also thinking that if we had something in the car loading the system, wouldn't a fuse pop, as all electrical runs through the fuse panel right? Only difference is some circuits are switched by ignition, and some circuits are live all the time.

[82 GT]

Now that you have a ground to the frame, is there one more alternator you can try?
That will tell for sure if the PA alternator is bad since you are now getting 13.3v with a different alt.
Sometimes an alternator with a bad diode will not always show up on a test bench.
I would try one more alternator, if you can, and if you get a good reading I would send the PA alternator back to them and let them test their own unit.
The guilty finfer seems to be pointing at the PA alternator. It sounds like you have it pretty much narrowed down to the problem!

[Unit 5302]

Just as an FYI, much below 12.3 volts, the battery probably won't have sufficient voltage to start the car.

12.4-12.6 volts is considered charged. 12.3 is pretty low, and 12.2 is discharged. Below 12.2 is pretty much dead and you shouldn't expect the engine to crank.

The only place I can think to check is at the union of the main power lines off the alternator. While two positive leads run off the alternator, they merge into one 10gauge wire, if I remember right (been a long time). That connection can get severely corroded which can definitely lead to low voltage coming from the alternator.

I apologize if this was already suggested, I didn't read the whole post.

[Gearhead999]

Have you put a load test against it with a load tester??

Also, it isn't a good idea to disconnect the battery cable while the engine is running to check the alternator. It will burn it up, you are making an incomplete circuit. Diodes don't like that. A good tester has a diode tester built in, I'd look there.

[Mustang_289]

Only load tests we've done is turning on lights, electric fan, all other accessories to drive load. What exactly is a load tester - how is it used?

We will be trying another alt this week.

[~The Jester~]

Originally Posted by Mustang_289

Only load tests we've done is turning on lights, electric fan, all other accessories to drive load. What exactly is a load tester - how is it used?

We will be trying another alt this week.

A load tester hooks directly to the battery, and supplies a "load" to it, and you can watch how fast the battery voltage drops right on the tester. Gas stations/Autozone pretty much all those type places uses them.

I think you're wasting your time changing the Alt. You've already proven that it works, ie bench test, and it works on the car with the battery disconnected.

Would this per chance be a CHROME alternator? A buddy of mine had his brackets powder coated, and the alt no longer grounded itself through the case. Cleaned off a little powdercoat, and all was well. A friend of my Dad's had the same thing happen with a chrome alt.

Just throwin shyt out there to check man. Like I said, this is gonna be something silly when we finally figure it out.

[82 GT]

Originally Posted by ~The Jester~

I think you're wasting your time changing the Alt. You've already proven that it works, ie bench test, and it works on the car with the battery disconnected.

No he hasn't really. He's only proven that the other alternator works correct.
Yeah, he did take HIS alternator to be checked and it checked out ok but I question the type of tester the place used. I say, at this point, his alternator has a bad diode and the bench test that was used couldn't check it under load or check to see if the diodes were good........or both
If the other alternator worked and got the correct voltage readings, why doesn't his PA alternator do the same?

[~The Jester~]

OK, I musta misread that then. I thought the PA read the correct voltage on the bench, and when he D/C the battery (WHICH may have killed the diodes right then.)

[Mustang_289]

Well the mystery continues. We put on a PowerMaster alt, same voltage. Never done a load test on the battery. Do I just put it across the battery with the car not running?

If something is loading down the car when key is on, wouldn't it blow a fuse in the panel, as all circuits run through the fuse panel whether switched on/off or hot all the time?

[Gearhead999]

To load test the battery. The car is off. The tester is hooked up to battery. The test puts an adjustable load on the battery so you can see what kind of reserve power it has with the load.

To test the charging system under load. You start the car and then load it up.

On a Load test for starter/charging system/batteries, you take the load probe [it's a clamp that you put around the wire to read amps] and put it on the bat. wire at the alternator, while the car is running. This will tell you what kind of amps the alternator is putting out, at the alternator before it gets to the car and battery.