How to test your alternator
Let the car sit for a while to allow the charge in battery to settle down.
1. set your voltmeter to the 20vdc setting.
2. Hook up the voltmeter to the battery poles. black lead to negative and red lead to positive. with the engine off the voltage should be around 12vdc.
3. Turn on the engine (alternator must be running) and the reading should go up to at least 13vdc but no higher than 15vdc. If the voltage does not go past 13vdc with engine on, the alternator is bad. The alternator output must be constant regardless of engine rpms.
4. turn the engine off and you will see the voltage in the ba
tery start to settle down to 12.0 vdc.
If you alternator is putting out between 13vdc to 15vdc with the ignition on but your car is not starting without a jump start then you battery is bad (not holding charge anymore) and it needs to be replaced. You alternator could also be bad if it puts out more than 15vdc.
Check your car's manual for the low and high voltage limits for your alternator.
Also, check the actual battery posts connectors, they may be loose broken or dirty. Check the cables also.
EZ to make alternator tester
We all need a voltmeter gauge if we don't have one.
How to install a voltmeter gauge in the 500SL.(mbvolm)
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Possible alternator failures
Posted by: Zirvana on July 07, 2005 at 10:09 AM
1. Failed diode(s).
2. Failed voltage regulator.
3. Worn or damaged brushes.
4. Broken winding (armature or field windings).
5. Thrown segment on armature (shorted armature).
6. Connection problem (internal wiring problem to connectors/post).
7. Damaged bearings (front or rear).
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