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NEVER EVER disconnect a battery cable while the engine is running. The surge will fry your computer.Here's a link to the proper way to test:

http://www.ricksfreeautorepairadvice.com/repair-topics/alternator-generator/testing-an-alternator-without-blowing-it-sky-high

Check with your local auto parts store. Most can check the voltage on the car.

You can also start the motor, while it's running remove neg battery terminal if motor stays runnung your alternator is good.

This technique is best done at night/in the dark. Turn headlights on whilst engine is running. If idle is set correctly, when you press gas pedal you should notice the headlights/dash lights become brighter. If they become brighter your alternator is working.

Another check you can do at night is to observe the headlights with the engine running, then shut off the engine. The headlights should get slightly dimmer. If not then the alternator ( and connections) should be checked further

While the engine is running you can take the positive off of the battery terminal. If the car stalls, it's a good bet your alternator is shot.

re-distribution of electrons is toughMost of these answers don't distinguish between a bad alternator and a bad voltage regulator, the symptoms are often the same. Voltage regulators are cheap and usually easy to replace, depending upon how deep the alternator is buried. Then again a voltmeter only costs a few bucks from harbor freight or radio shack.

Like the drunk searching for his keys under the light pole, (the light is better here) diagnosing electrical problems is best done with the easiest/cheapest jobs done first: Checking voltage drop through connectors and harnesses, Cleaning terminals (especially those on the alternator and main fuse panel), Replacing the voltage regulator, Having the battery and alternator inspected (usually free) by an auto parts chain store, etc..

Your alternator might also be ok but but marginally underpowered for the equipment in the car. This'll happen a lot to cars that are loaded down with aftermarket audio and lighting gear. I'm looking at a weird case like this right now, whenever the AC is on, and I'm just puttering around town, the battery starts sagging down under 12 volts. Since I've little aftermarket stuff in the car, I suspect that either the AC clutch or the aux fan is pulling way more current than it should.

My choices seem to be to put a bigger alternator in, or to hook up amp meters to the suspect components to see if they're pulling more than their rated power.

First buy a multimetre from a an auto store or hardware store, $15-25.

Start the car. As the user above pointed out, pull a battery cable if it continue running, you at least have running alternator. However, the alternator could be not charging enough, or over charging.

Set you multimetre to show voltage flow with at least the accuracy of one decimal place. Place the probes on the correct battery poles, red probe on the positive post and the black on the negative post.

If the multimeter reads below ~13.5 volts, it is not charging your battery, or charging it so slowly it would have to run for quite some time to fully charge; may need to replace the alternator.

A reading of ~13.5 to ~14.5 is somewhere in the norm.

And a reading above ~14.5 is probably a little high and you alternator is overcharging the battery; most likely a cheap-to-replace voltage regulator is the culprit. However, having being overcharged for an extensive amount of time may have damaged the battery.

Finally if you are still not sure, go to a trusted auto store and have them test your alternator; if you don't have a trusted auto store, don't be afraid to get a second opinion.

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โˆ™ 2015-07-16 18:04:50
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Q: How do you test an alternator while still mounted to the engine without using a meter?
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You put new alternator and volt regulator in 1987 f600 370 engine and volt meter says 12.8 with engine off and on?

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How do you know whether its your battery or your alternator?

Start the engine and using a digital volt meter check the voltage at the battery with the engine running. If the alternator is good you will read from 13.5 to 15.5 volts DC.


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Check the voltage with a volt meter at the battery with the engine running at idle. You should read 13.5 to 15.5 volts. Any less and the alternator is not charging properly.


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How do you check if the alternator is working?

Get yourself a mulitmeter, start the car then attach the meter to the battery.If the alternator is working then the meter should say that there is 13.2 to 14.8 Volts. Under NO CIRCUMSTANCES should you remove a battery cable while the engine is running. This could cause you to DESTROY THE COMPUTOR due to a voltage spike!

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