Alternator or Regulator

How do you tell if it is the alternator or voltage regulator is bad?

Symptoms - Car won't start because the battery is not being charged. Jump started the car, it ran. Shut the car off, it wouldn't restart. Battery read about 12.2 volts on my $4 digital multimeter. Jump started the car again, removed the jumper cables, and voltage between the thick wire on the alternator and the alternator case was about 11.9 volts. I left the jumper cables connected for several minutes, and the battery got charged enough to start the car on its own after the cables were disconnected.

Previous owner says that he recently bought the alternator from Pep Boys with a lifetime warranty. It doesn't look new, though. The voltage regulator looks like its been on the car since before the car (or even the plant in Lorain) was even built.

Are there any tests I can run to determine whether it is the alternator or the voltage regulator without running to the autoparts store with both?

Thanks.

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Mike Walsted - NATO member
1986 5-speed TurboCoupe and 1985 5-speed XR7

If you can determine the field terminal on the voltage regulator, run a wire from there to the positive side of the battery. This bypasses the voltage regulator and allows the alternator to put out full charge. If it starts charging then yuou know the alternator is good.

You GROUND the field terminal on the alt to full field it, you dont hook it to 12 V!!!!!

Take tha alt to a parts store that tests alts, and they should be able to test it for you.

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Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 20 psi, forced air intercooler, water injection, bypass valve, Ranger roller cam, subframes, etc., etc.. // 86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP nitrous, ignition, gears, suspension, etc., etc.... // 91 Escort: Bone stock winter car // 00 Windstar (wifes vehicle)

It was the voltage regulator. I figured out the polarity at the field terminal on the voltage regulator by tracing the circuit path in the EVTM (but still don't know how it works), hooked it up, and the alternator charged the battery. So, I got a new one.

Thanks for the help.

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Mike Walsted - NATO member
1986 5-speed TurboCoupe and 1985 5-speed XR7

this will also happen when the battery goes south. i'm having the same problem with my mustang.

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69 mustang coupe (next project)
86 turbo coupe (it's fun beating a honda...with something twice it's size!)
86 SVO (it's amazing what people can screw up)
90 mustang GT (project scratch and dent)

...i'd rather be trolling...

Is it the external regulator? If it is, unplug the reg. & jump 1st & 3rd terminal. This will full field the alt. You'll be able to hear it. If that doesn't make a difference, it's the alt. or maybe no power to it, like a fusible link.

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Dave
'83TC
'85TC
'87TC
'90SC