The Ford Barn

1929tudor · 09-14-2010, 08:53 PM

I was wondering if there is a way to test an alternator to see if it is charging right or is it best to take it to shop. Thanks

MikeK · 09-14-2010, 09:04 PM

To get the best picture of performance and health, an oscilloscope is needed to see the evenness of the diode performance during a load test over varying speed. The best equipment will give you a graphed printout.

Likewise with a generator, an oscilloscope will "see" the armature while running like a static segment growler bench test would. An open or shorted segment will show up, even though the genny is still working sufficiently to not be noticed with an ammeter and a voltmeter.

PC/SR · 09-14-2010, 09:14 PM

A cheap alternative is a multimeter voltage check while running. Should be around 13+ volts. Some auto stores have little gizmos that measure alternator/generator output, battery voltage, etc. Some alternators have to be revved up before they start charging.

Mike V. Florida · 09-14-2010, 11:43 PM

Simple Alternator Test:

1) Start Car and run at a normal idle.

2) Connect a Voltmeter Directly Across the battery Terminals.

3) Turn on Headlights to High Beam.

4) check the battery Voltage.
*** The Battery voltage Should read between about 13.8 to 14.2 volts. (Possibly up to 14.5 Volts.)

5) Increase motor speed to about 1500 RPM. The Alternator should now produce a battery voltage between 14.2 and 14.6 Volts.
*** If it is Lower than 14.2 volts, the Alternator probably has one or more Bad Diodes.

6) Now, Maintain the 1500 RPM and Turn off the Headlights. Run this test for One Full Minute.
"The Battery Voltage should NEVER Exceed about 14.8 Volts".
***If it Does, the Alternators Voltage Regulator is probably Faulty, OVER CHARGING the Battery.

BRENT in 10-uh-C · 09-15-2010, 02:18 AM

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The easiest way to initially check to know if it is charging is to stick a a screwdriver to the back of the case on the metal plug where the backside of the armature is when the engine is running. If there is magnatism, then it is charging.

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09-14-2010, 08:53 PM	#1
1929tudor Senior Member Join Date: May 2010 Location: indiana Posts: 135	how to test alternator I was wondering if there is a way to test an alternator to see if it is charging right or is it best to take it to shop. Thanks

09-14-2010, 09:04 PM	#2
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: how to test alternator To get the best picture of performance and health, an oscilloscope is needed to see the evenness of the diode performance during a load test over varying speed. The best equipment will give you a graphed printout. Likewise with a generator, an oscilloscope will "see" the armature while running like a static segment growler bench test would. An open or shorted segment will show up, even though the genny is still working sufficiently to not be noticed with an ammeter and a voltmeter.

09-14-2010, 09:14 PM	#3
PC/SR Senior Member Join Date: May 2010 Location: Santa Rosa, CA Posts: 1,280	Re: how to test alternator A cheap alternative is a multimeter voltage check while running. Should be around 13+ volts. Some auto stores have little gizmos that measure alternator/generator output, battery voltage, etc. Some alternators have to be revved up before they start charging.

09-14-2010, 11:43 PM	#4
Mike V. Florida Senior Member Join Date: May 2010 Location: South Florida Posts: 14,054	Re: how to test alternator Simple Alternator Test: 1) Start Car and run at a normal idle. 2) Connect a Voltmeter Directly Across the battery Terminals. 3) Turn on Headlights to High Beam. 4) check the battery Voltage. * The Battery voltage Should read between about 13.8 to 14.2 volts. (Possibly up to 14.5 Volts.) 5) Increase motor speed to about 1500 RPM. The Alternator should now produce a battery voltage between 14.2 and 14.6 Volts. * If it is Lower than 14.2 volts, the Alternator probably has one or more Bad Diodes. 6) Now, Maintain the 1500 RPM and Turn off the Headlights. Run this test for One Full Minute. "The Battery Voltage should NEVER Exceed about 14.8 Volts". ***If it Does, the Alternators Voltage Regulator is probably Faulty, OVER CHARGING the Battery. __________________ What's right about America is that although we have a mess of problems, we have great capacity - intellect and resources - to do some thing about them. - Henry Ford II

09-15-2010, 02:18 AM	#5
BRENT in 10-uh-C Senior Member Join Date: May 2010 Location: Eastern Tennessee Posts: 11,537	Re: how to test alternator Sponsored Links (Register now to hide all advertisements) The easiest way to initially check to know if it is charging is to stick a a screwdriver to the back of the case on the metal plug where the backside of the armature is when the engine is running. If there is magnatism, then it is charging. . __________________ . BRENT in 10-uh-C . www.model-a-ford.com ...(...Finally Updated!! ) .