Charging gremlins - 1994 GT

[Mustang5L5]

This is for a friend of mine. We are trying to figure out a weird electrical prob on his car.

1994 GT vert 5-spd.

We've put 3 alts on this car and 2 batteries trying to figure it out. latest alt is a brand new Ford OEM.

Basically, the voltage fluctuates as if the regulator isn't working. With the first batch of alts, the voltage would stay consistant 12.5 v as if the alt just wasn't working. As you drove, it would go down. Rev the engine and it does not jump at all. It's almost like it was running off the battery.

Fast forward to latest alt. Voltage will hang at 12.4v and then occasionally jump to 14V and hold for a min or two, then would drop back down to 12v. The gauge on the dash reflects this as well going to the center when the voltage rises to 14 and then dropping to N when the voltage drops. This is on its own with no RPM changes.


We've replaced both battery terminals, new batteries, new alternators, replaced a questionalable stator plug wire and cleaned up some connections. No change.

We are going to start ohming out the wiring next, but wondering if anyone had any input of expeirence. It's like the votage regulator only works when it wants to. I checked the power to the large 3-wire plug on the alt and one of the wires does have constant 12V+ from the battery (with the car off), so perhaps something is being lost on the other wire?

 

[Chythar]

Did you test the output voltage from the voltage regulator while the engine is running? Sounds to me like you're on the right track, it's likely the wiring between the alternator and the battery.

 

[urban96]

check all the grounds.

make sure where the alt bolts to the bracket is free of paint/powdercoat (how the alt grounds)

I used to have charging issues untill I added another ground from the block to the frame

 

[Mustang5L5]

Did you test the output voltage from the voltage regulator while the engine is running? Sounds to me like you're on the right track, it's likely the wiring between the alternator and the battery.

How would we check that with the engine running? Where would we test at? Pin through the output wire to ground??


I forgot to add the reason this is a prob is because that battery will sometimes just die while driving as it drains as if the alt just was not charging.

I suspect voltage regulator. I have good 12V to the alt on one side, and since we put 3 different alts on i suspect they should have good voltage regulators internally.

I guess we have to chase the wiring back. Does anyone have some good wiring diagrams? I lose where it goes after it gets to the connector under the coil.

 

[HISSIN50]

The reg wire with battery voltage is the sense wire. Get in the underhood fusebox and be sure the "alt" fuse is in good shape. If the terminals are loose or dirty, the sense circuit goes nuts (thus your regulator bounces around). Sometimes the females that receive the fuse blades get loose or corroded.

Underhood fuses

Also be sure the power connection on the PDC is in good shape. This connection being corroded (an issue on these cars) will cause tons of issues. The PDC bolt is under a sliding cover on the side of the underhood fusebox. This is where your alt's power cable connects on the battery side (it's like the battery lug on the fender-solenoid on a fox III).

The fusible links on the charge cable are between the PS pump and the underhood fusebox.

On your reg, be sure lt grn/red shows some voltage (2-5 volts is normal, KOER).
Be sure white/yel shows battery voltage (within 200 mV or so of actual battery voltage).
With the car on, the stator circuit should show about 1/2 battery voltage. If it doesn't, disconnect the reg plug and the lone stator plug and ohm out between both ends of the stator wire. Look for about 1 ohm of resistance.

For a general check: Peel back the wire loom leaving the regulator plug. The wire vulcanizes and can cause issues.

Alternator wiring schematic

Good luck Mike.