Mustang_GT_91

I am having a problem with charging on my 91 Mustang GT. The problem started after I took it to an electromechanic because my old stock alternator died, I decided to buy the 1995 Mustang GT 5.0 alternator and have the mechanic install it. The new alternator worked fine for about a week but now the voltage drops when the car is at about 900 RPM my RPM is steady and the charging problem only happens when the car is at idle.

I talked to the same mechanic about the problem and he said my RPM might be to low but I don't think that's the problem because from what I have read a normal RPM at idle should be at about 900RPM for a stock 91 GT like mine.

The mechanic did use the pulley that came with the new alternator. I am thinking he should have used the pulley from the stock alternator.

Any suggestions?

mustangjon

was this with any accessories on or not? the 3g alts have to be energized to produce charge, and with wrong speed, at dead idle it wont charge.. could try changing the pullies out.. does you car have underdrives? mine at idle... with all accessories (being a lincoln has more draw) lights, wipers, stereo, amp, fan, ect all on will discharge but at 1000-1100 rpm it works fine.. but idle energized 3g only puts out 65amp usually, could try bumping your idle screw up a bit and see what rpm it does actually start charging at.

silver91gt

did you change the wiring when you made the switch? if you used the stock wiring then you probably blew the fusable links in the end of the wires.

HACKGT350

the whole point to the 3g swap is so that you have a better charge at idle and while driving. something is obviously wrong. start by switching the pullies back

mjr46

make sure the pullies are of the same diameter, compare the two to see if that may be the issue

aode08

I HAVE THE SAME ISSUE

you have to use the pullie from the OLD ALTERNATOR

Usually, its bigger.


Underdrive pullies are notorious for causing charging issues on a fox.

Mustang_GT_91

My voltage drops when the car is at idle even with no accessories on. I compared the stock pullie to the one that came with the new alternator and they looked the same, that's why I decided to use the new one. My car has no underdrive pullies. The mechanic did change the wiring to a better one.

I no longer have the old stock pullie for the alternator, but I will try to find one at a junk yard to se if that fixes the problem.

Mustang_GT_91

You have or you had the same issue? Did you solve the problem by using the stock alternator pullie?

Thank you for your help.