voltage across battery when charging ?

[Mustang_289]

We're in the last phases of getting our 88GT on the road with the 331. The rolling chassis we bought had the battery moved to the trunk.

Got a PA Performance 3G 130A alt installed, we're getting 12.5v across the battery while car is idling. Is this where it should be, or should it be higher?

If this is low, any ideas on what to check, I've been checking everything in sight to ensure tight connection, fuselinks are good, good grounds etc.

[82 GT]

That's a little low. You should have 14.5 volts at idle.
I know on my power master alternator you have to rev the engine to 2500 before the field windings energize. Maybe it's the same for your pa-performance unit.
Also, you're going to have some voltage drop since the battery is in the trunk. What size wire is used from the alternator to the battery?
I would first have the alternator checked to eliminate that possibility and make sure the wire is heavy enough gauge they used to relocate the battery.

[Capri306]

I'm thinking it's a little low too, but then again, it's at idle. We haven't established whether there are underdrive pullies on it, which could be causing a lower alternator output. How is the voltage over the normal operating range of the engine? Does the voltage rise to ~14V when the engine's running around 1500rpm?

I think it would be great to find out how well that cable is transferring power to the rest of the electrical system, and also how well the battery's negative connection is made, i.e. is everything crimped & bolted down, or are things soldered, shiny, and ? This is one of the things that kinda scares me away from trunking my battery in the notchback: the gauge of the cables is going to have to be huge to ensure minimal voltage drop from the battery to the starter & alternator.

[Mustang_289]

Thanx for the input on this issue, please keep the thoughts coming as this is the last thing from getting this beast on the road.

No underdrive pullies, can't have them as we're putting on a T-trim blower
Gauge of wire to trunk #2
All ground connections are tight, clean, and free of paint

[GhettoPop]

my PA performance 130Amp alternator charges my stock stang at about 14.5V at 750rpm idle with no accessories on. with just say the lights on, it will charge at around the low 13's at idle. these numbers are not across the battery but from my autometer gauge reading from my ignition switch as i wanted to get a "real feel" of what voltage the accessories are getting. any time up raise the rpm's to 1000 or more, it will always charge at 14V or over. keep in mind that this is using a march 1000 underdrive pulley kit which includes a smaller (i believe) alternator pulley.

if it matters i upgraded to the cable running from the starter selenoid (sp?) to the alternator with the "premium" one that PaPerformance seels with the inline 200A fuse. i believe that is a 4 gauge cable. i also replaced my old positive and negative wires from the battery as well as the cable running from the starter selenoid to the starter with PaPerformance 4 gauge cables.

in your situation, what RPM are you idling at? what happens when you bump to 1000rpm? 1500rpm? post your stats.

if that cable going to the battery is really long you may want to upgrade to a larger size to be on the safe side and it can't hurt anything. you can always try shortening it if possible.

you may also want to condier changing the pulley on the alternator to a larger diameter model such as the stock size (i think it's larger than the pa performance pulley) and some manufacturers make pulleys larger than stock.

[mpj76]

I once had a very old style (1970's) 130 amp alternator with an external voltage regulator. During my heater core swap job, I drove the car without the instrument cluster connected for about 2 days. Turns out that the alternator was excited by a charge on the same ciricuit as the cluster. Without this circuit closed, I had to blip the throttle to about 4-5000 RPMS to manually excite the alternator. I could also excite it by running a lead from the + battery terminal and touching one of the terminals on the regulator. If you're running an adequate gauge of wire and have good connections, you may have a similar problem.

[Mustang_289]

I'll post the stats tonight on different idles
We're running the same PA Performance cable with the inline 200A fuse.

[Mustang_289]

Here are a few more bits of info from our charging issue

1) started the car at idle voltage across the battery was 12.03 in the hatch and 12.5 at starter selenoid.

2) At 3K RMP voltage readings didn't change.

3) Under load with lights, fan etc on voltage dropped below 12volts.

4) With the car idleing, removed positive lead from battery, car still ran. Voltage at the starter selenoid was 14.3. This is where it should be with the battery hooked up.

Does anyone have any other ideas or comments on this new info?

Thanx in advance for your input.

[82 GT]

I'm leaning towards a bad alternator. You should have had 14.5v across the battery at idle. I would get both the alt. and battery checked.

[Rev]

One more thing I had to do with the battery in the trunk and 2 gauge + cable was to run a 2 gauge ground cable all the way from the - terminal on the battery to the bolt on the starter motor. Grounding the battery to the chassis in the trunk was just not adequate, especially starting with the engine and starter motor hot.

Rev

[Mustang_289]

Well, here's an update on recent troubleshooting on this problem.

1) Took the alt. off the car took to an alternator shop, it's putting out 14.6 v and 152A at peak.

2) Put another battery in the car, and didn't change the readings off 12.3v.

Calling all experts - help WILL BE APPRECIATED. THIS IS THE LAST THING BEFORE WE GET OUT HOT-ROD ON THE ROAD. Thank you in advance for any help.

[mikew88gt]

Are you sure you don't have underdrive pulleys? Your alternator wants to put out 14+ volts like it should but when you're hooking up your battery it's dragging it down to 12. That's what my car does normally with underdrives. Even with a fully charged battery a car with underdrive pulleys will find itself trying to play catch-up with an electric fan, ignition, fuel pump, etc. If you want to check the integrity of your system try this. By placing a voltmeter along the entire length of your positive cable (from the battery terminal to the start solenoid) while the car is running you will read the voltage drop of that part of the circuit. It should be a lot less than 1 volt with the 2 gauge wire. If that checks ok try the same with your ground cable by going from your battery negative terminal to a ground point up front (usually on the left inner fender forward of the strut) and from the negative terminal to the engine block. These couple of checks will tell if your losing voltage from the wiring.

[82 GT]

If it was ME, at this point, that mofo battery would be back under the mofo hood!...LOL
Seriously, how many grounds do you have? You can never have too many grounds.

[Joel5.0]

Have you checked voltage directly at the alt output? Any difference if you have? A voltage drop of .5vdc is normal, anything over that and it's re-wiring time. I have underdrive pulleys in my '86 and had to rewire the alt. output 2 yrs. ago when I had the same problem.....and the battery is in the stock location, not in the trunk. Just my 2¢. GL

[mikew88gt]

Seriously, how many grounds do you have?

You should have your battery grounded in one place. The other "grounds" I mention are where other electrical equipment is grounded to the car. My point is, maybe his battery is grounded just fine but the ground for his alternator is not adequate or maybe the bonding strap from the engine/tranny isn't cutting it. These will cause problems that can be hard to diagnose.

Good luck. Hope you get it on the road soon.

[redsn95vert]

I've been running a stock alt, underdrives, and a Optima Red Top for years. The lowest my bat drops is 12.5 with all the bells and whistles. I just took a 8 hour class in starting and charging systems 2-3 weeks ago, and it seems like you have a short someware,,,,
Check the connections at the starter and your ground. hope this helps.....Good Luck

[Mustang_289]

Battery is grounded in one place in the back of the car to the frame. What is the bonding strap from the eng/tranny? Are you talking about the grounding strap from the block to the frame?

[82 GT]

There should be a ground from the block to the body(firewall) and another one from the block to the frame.
If the battery ground is all you have, then that's your problem for sure.

[Mustang_289]

so the strap goes to the firewall?

[82 GT]

yep and I would put another one from the engine to the frame.
That "should" solve your problem.....I said should.
Let us know how it goes