How do you test an alternator? - Ford Mustang Forum
 
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post #1 of 10 (permalink) Old 07-22-2002 Thread Starter
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How do you test an alternator?

Can anyone tell me how to test an alternator? I have a multimeter but don't know what to check for or where.

Any help will be appreciated.

`90 Mustang LX 5.0 hatch - 331 w/ Australian ported heads - Cobra Intake - 65mm TB - MAC Headers and High flow Cats, Dynomax muffelers - ATI Procharger P1SC @ 11#'s intercooled - Aeromotive fuel system - 130amp alternator - Centerforce clutch - Full Griggs GR40 suspension - MSD 6BTM - 60# injectors - AEM EMS w/ wideband O2's converted to MAP - Torsen diff w/3.73 - Baer Brakes (13") w/ 5lug conversion, 17X9 Bullitt wheels with 275-40-17's on all 4 (Nitto 555RII)
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post #2 of 10 (permalink) Old 07-22-2002
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You are better taking it to a specialty shop, they can perform all the necesary tests for around $15-25

Heres a quick test:
With a d. c. voltmeter, you can do a quick test to give you a clue. When idling, with accessories off, most cars will charge the battery from 13.2 to 15 volts. You can turn on some accessories, (or headlights) and the battery voltage should stay in this range. If the car has many accessories, you may need to rev up the engine a bit for the alternator to keep up with the load. If the output is low, it could be a bad alternator, bad regulator (often part of the alternator) or bad connections. If the output is high, it is probably the result of a bad regulator, or connection.

Battery voltage and charging voltage:
While you are at it, if you are having trouble starting your car, you can measure your battery voltage (remember to set the range up on manual meters to around 20 volts). The - terminal on the meter goes to the negative battery terminal, and + to positive. A fully charged battery will measure around 12.6 - 13.2 volts with the car off. With the car running, you can measure the battery voltage at idle with lights, and accessories off. Most cars will charge from around 13.2 to 15 volts. If it is lower, check your connections, belt tension on the alternator, and the alternator. If the voltage is high, you may have a defective regulator. Alternators can fail gradually, with a bad diode, for example. In this case the alternator may still work, however not be able to keep up with accessories such as lights and the heater / air conditioner. When this happens, the car battery may slowly run down

Rob Hernandez, AFM co-Founder.
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post #3 of 10 (permalink) Old 07-22-2002
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Sears, Pep Boys and most shops will sometimes check the alternator and batteries for free. Many will test both on and off the car. Call around and check it out. I'd watch their tests as they perform them and ask questions.

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post #4 of 10 (permalink) Old 07-22-2002
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Auto Zone checks it off the car, as well as on the car. Starter and battery as well either on or off. Free too!:green
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post #5 of 10 (permalink) Old 07-22-2002
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how to test an alternator

Just start the car up and then disconnect the positive terminal from the battery if the motor dies then you have got problems.
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post #6 of 10 (permalink) Old 07-22-2002
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I heard that this could potentially fry the computer and some other electronics on the newer stangs. I could be wrong though.

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post #7 of 10 (permalink) Old 07-22-2002
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How to test an alternator

Whoops sorry I didn't think of that I do not have fuel injection so I was only thinking of carbed cars. I wouldn't want you to cause new problems. I am glad someone corrected me.
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post #8 of 10 (permalink) Old 07-22-2002
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Yes, you can take out a computer on FI vehicles by disconnecting a terminal. Plus, the sensors need the constant voltage or it really throws things off. The same argument can be said against removing your alternator at the track.
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post #9 of 10 (permalink) Old 07-22-2002 Thread Starter
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Excellent tips! Especially about free tests!

I'll probably do all of the above (minus the test for carb'ed cars)

Thanks All!

`90 Mustang LX 5.0 hatch - 331 w/ Australian ported heads - Cobra Intake - 65mm TB - MAC Headers and High flow Cats, Dynomax muffelers - ATI Procharger P1SC @ 11#'s intercooled - Aeromotive fuel system - 130amp alternator - Centerforce clutch - Full Griggs GR40 suspension - MSD 6BTM - 60# injectors - AEM EMS w/ wideband O2's converted to MAP - Torsen diff w/3.73 - Baer Brakes (13") w/ 5lug conversion, 17X9 Bullitt wheels with 275-40-17's on all 4 (Nitto 555RII)
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Good call.....I'd pass on the "whoops" test too

95 GT Convertible
BBK shorties, X-pipe, Magnaflow CB
CAI, 70mm TB, GT40 intake, 1.7 RR, UD pulleys
SFC/braces/JR, AUC Arms
1966 C Coupe - New restoration project
2004 Chev Silverado 4X4
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