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How do I test the alternator to determine if it's good?

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  • Wayne Gordon
    Wayne Gordon Apr 21, 2017

    OK,new to fixya. Should have said, "How do I test the alternator on my Craftsman 18.5hp riding mower? Thanks!

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With the engine running,check for correct voltage at the output connection on the alternator.(14.5 volts on a 12volt system).If no voltage or low voltage and if you have a field connection apply battery voltage to the field connection with engine running and check your output voltage,if it has improved you may need only a regulator.
No voltage still or low voltage,you may need an alt.
If the alternator is on the bench and you have a field connection on the alternator,apply battery voltage to the field connection and ground the alt case.At this time the alt pulley should become magnetic if there is a good strong field circuit and the alt should work.No magnetic pulley? Most likely needs brushes.

Posted on Apr 21, 2017

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  • Cars & Trucks Master
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You need a multi meter so you can measure output -----if dont have a meter its easier and free service at most auto stores
How to Check an Alternator How to Test an Alternator

Posted on Apr 21, 2017

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Take the vehicle to a auto parts store , they check them for free .

Posted on Apr 21, 2017

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Posted on Jan 02, 2017

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1 Answer

I have a 2001 jeep grand cherokee,the alternator is not charging,I have replace three and same result what is the problem?


Who replaced the alternator 3 times?

Was any testing done, to determine any of the 4
alternators had any problems & how was that testing
done?

Unless you got a couple bad rebuilt parts,would you
not conclude further testing was needed, as the problem
may be outside the alternator & in the wiring or other circuits
feeding wires to it !

You went thru 4 alternators
Did you not test all of them at the parts store?????

Sep 24, 2014 | 2001 Jeep Grand Cherokee

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What does it mean when your battery light is on then cuts off when you start diving then when you make a stop cut back on


The battery light is telling the driver the alternator is not charging the battery. The alternator output has to be above 12 volts to make the light go off.

Jan 25, 2014 | 1995 Nissan 200SX

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99 mustang battery not charging


First off, disconnecting athe battery while the engine is running is the quickest way to damage an alternator. Unlike the old DC generators, the alternator needs the battery in circuit to protect it against voltage runaway.

If the alternator has been bench-tested and seems to be functional, I would start by verifying the related vehicle circuits. Witch a DC voltmeter, start out by measuring the voltage at the alternator's output terminal, the heavy black/orange wire. It should be full battery voltage. If it is anything less than battery voltage, you must determine why. Some things to look at are the gray 12-gauge fusible links A & B, and the various splices and connections in the circuit (see diagram below).

Next, disconnect the voltage regultor harness plug from the alternator and check the voltage at the wiring harness regulator plug's "A" terminal connector - the yellow/white wire. It too should be full battery voltage. If it is anything less than full battery voltage, you must determine why. In this case, look at the 20-amp ALT fuse in the battery junction box, and at the various splices and connectors in that circuit.

Then, turn the key switch to its "RUN" position and measure the voltage
at the wiring harness regulator plug's "I" terminal connector - the light green/red wire. It should be somewhere between 8 and 12 volts. If there is no voltage present, check Fuse 5 (15 amps) in the main fuse panel and the various connections in that circuit.

The final test is to measure the voltage between the batterty positive terminal and the alternator housing or frame. If it is anything less than full battery voltage, check the alternator mounting.


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Jul 19, 2012 | 2000 Ford Mustang

1 Answer

Changing the alternator on my truck do u hav detailed instructions


First disconnect battery, next remove serpentine belt, then unhook wiring to alternator, unbolt and replace with new/rebuilt alternator. Reverse steps and you should be good to go. You can have your old alternator bench tested (tested off vehicle) at almost any autoparts store for free to determine that alternator is bad/not charging. I test everything I drive and work on and advise others not to speculate.

Jan 07, 2011 | 1996 Chevrolet S-10 Pickup

1 Answer

1994 XJ6 battery wont charge alternator ok battery ok help?


The battery needs to be load tested first. once this is determined to be good or not, have the charging system checked just be sure. Electrical components can fail quicker than you can blink. Just because it tested good once does not mean it still is. Most autoparts stores will do this for free. Let me know what they say.

Oct 26, 2010 | 1994 Jaguar XJ6

3 Answers

Battery wont charge changed alternator and battery still wont charge


fit a new reconditioned alternator and not one from a junkyard

Mar 30, 2010 | 1992 Ford F150 SuperCab

1 Answer

Altenator good battery good starts then 3minute


CHECK ALT.OUTPUT AT BAT. WHEN THE LIGHT IS ON..SHOULD BE 14.5V...IF NOT YOU MOST LIKELY NEED AN ALTERNATOR.. VOLTAGE REG. IS IN ALT.

Mar 20, 2010 | Dodge Stratus Cars & Trucks

2 Answers

How do i test my alternator to make sure it still works good


Start the engine and turn on all your lights and your a/c blower on high. You should have 13 to 14.5 volts at your battery, if you have less then 13 volts. Then you have a weak alternator or faulty voltage regulator, to test the regulator, turn everything off and check the voltage again with the engine running. You should have 13 to 14.5 volts again, if anything different as to being too high or low, then you have a faulty regulator which is built in to the alternator so your still going to have to replace it. To test the battery, you should have 11.5 to 12 volts with everything off and engine not running.
Good luck and hope this help. A battery older then 4 years may no long be able to hold a charge and will need to be replaced. A bad battery will cause the alternator to fail much quicker then a good battery.
Thank you for using Fixya

Mar 16, 2010 | 1995 Chevrolet S-10 CA Edition

1 Answer

Battery wont hold charge, trying to determine if anything else should be assessed before replacing the alternator.


Get a volt meter and check alternator voltage at battery while engine running, should have no less then 13.5 volts, if you have good voltage at battery, disconnect battery terminal neg. hook test light at cable (neg) and at battery, make sure every thing is off and door's are close if test light lights up you have a battery draw you'll need to fine short.

Jan 04, 2010 | 2002 Mitsubishi Montero Sport

2 Answers

The alternator is still good but everytime i jump start the car it makes it half a block and dies.


Your alternator is charging however it is not still good if it does what you are stating. Also what is the condition of your battery if you have to keep jumping your car? Car batteries don't like to be completely discharge and may not hold there charge again once run dead and may need replaced... Alternator has to have be bad as it needs to be load tested to best determine it's true output. Most auto parts stores do a run test and no load test. Load test will tell amperage rate and true quality of an alternator.

Aug 28, 2009 | 1993 Mitsubishi Mirage

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