Charging Issues?????

PQTIII · Aug 12, 2007

1991 5.0, 5spd, new battery, stock 75amp alt

I took my car to the local parts store to have the charging system checked out. I have been having problems with the voltage when I turn the AC or headlights on. The store put a tool on the car while the alt was on the car. According to them, the battery was fine (since it was new) and the alt was charging fine at a little above idle. When I turned the AC and headlights on the voltage dropped to about 10 volts. They had me take the alt off of the car to bench test it. It passed the bench test. They said that it was charging very well. So I put it back on and it still has a problem. Where do I look now? Are there certian wires I need to check? What do I check for?

Thanks
Paul

HISSIN50 · Aug 12, 2007

Paul, you might be in the Twilight Zone of bench testing. An alt can be on the ragged edge of failing but it can still pass (I've had one pass with a dropped diode. That should have failed it immediately. I went across the street to another store and it failed the test).

Ensure your charge wiring and regulator wiring are up to par. And I was confused about the battery - new does not mean that it's good (though I dont think yours is the issue here).

It really does sound like a diode issue in your alt however.
Good luck.

PQTIII · Aug 12, 2007

I guess the easiest thing for me is to go buy another one and hopefully that'll fix it. The Twilight Zone for me is the electrical system. What are the color of the wires that I should check?

Thanks for your help.
Paul

jrichker · Aug 12, 2007

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Bligaderski · Aug 14, 2007

I had the same issue, I added a ground strap from the frame to the motor and it really seemed to help. I added mine on the drivers side from the bolt that holds the starter wire clip to the sway bar bolt, just clean off the area so there is no paint or grime and use a small piece of 4 gauge wire (about 2 feet) and you done. This really helped with slow starting, my 89 when warmed up would barley start, turning over really, really, really slow. Now it’s super fast to turn over.

BLOWN 5.0 FOX · Aug 14, 2007

3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G

STP72 · Aug 14, 2007

lol yes 3G for sure .. why bother with a stock Alt

ninety15.0 · Aug 15, 2007

Yup i agree...if you search around on the forum someplace you will find a thread that shows you exactly why the stock alternator is a piece of undersized pooop. From the factory they did not produce enough Amperage to charge the battery while multiple accessories were on! Having the AC on (Huge electrical draw) and the headlights on...its not doubt the car wont charge at idle. You need to upgrade to a 3G alternator. I did this a while back and now i maintain 13 volts with the lights, AC, and stereo on at idle!

hungrymonkey · Sep 15, 2007

I am having a similiar problem. At idle I will have 13 volts per the stock guage. with the headlights on It will dip down to 9 or 10 volts.
If I run the air and lights with the fog lights on, I will have some stumbling upon acceleration and backfiring. I am assuming there is not enough power for the ignition.

My question is (the reason I brought this back from a month ago) how accurate is the stock guage?

and would a poor ground cause this problem as well?

as for the 3g, is this the one wire alt? and what would I look for when I buy one? IE off of what year and make of vehicle.

thanks.

408gt91 · Sep 15, 2007

BLOWN 5.0 FOX said:
3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G 3G

+1

HISSIN50 · Sep 15, 2007

hungrymonkey said:
I am having a similiar problem. At idle I will have 13 volts per the stock guage. with the headlights on It will dip down to 9 or 10 volts.
If I run the air and lights with the fog lights on, I will have some stumbling upon acceleration and backfiring. I am assuming there is not enough power for the ignition.

My question is (the reason I brought this back from a month ago) how accurate is the stock guage?

and would a poor ground cause this problem as well?

as for the 3g, is this the one wire alt? and what would I look for when I buy one? IE off of what year and make of vehicle.

thanks.

Have the alt bench tested - you might have dropped a diode.

The stock gauges isnt accurate enough to use for diagnostics but the general trend of it can be useful. If you were truely at 9 volts, your battery light should have come on however. Set up a DMM in the car so you can know what's going on for sure.

There's a sticky thread about the 3G conversion. It's not usually done as a 1 wire (it can be done as a two wire however). A 94-95 Mustang comes with a 130 amp 3G alt.

Good luck.
Good luck.

hungrymonkey · Sep 15, 2007

HISSIN50 said:
Have the alt bench tested - you might have dropped a diode.

The stock gauges isnt accurate enough to use for diagnostics but the general trend of it can be useful. If you were truely at 9 volts, your battery light should have come on however. Set up a DMM in the car so you can know what's going on for sure.

There's a sticky thread about the 3G conversion. It's not usually done as a 1 wire (it can be done as a two wire however). A 94-95 Mustang comes with a 130 amp 3G alt.

Good luck.
Good luck.

hey thanks.

I havent had time to test the alt output, I just noticed it this morning when I was getting off of work. especially the popping and sputtering in the engine.

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