How to test my alternator: SOLUTION

Voltage while running S/B in the 14.X volt range. If it's in the mid 13's or below then there's something wrong with it and that's why the light came on.

bluebarry1964b wrote:
I have a 2003 Holiday Rambler Neptune (Diesel pusher). I just took it for a spin and the "alternator" dash light was coming on intermittently. Not sure if it's a short in the light, or if the alternator is failing. I was told that if I start the engine then shut off the chassis battery cut-off switch, then if the engine dies the alternator is shot. Does that sound right?

NO,,under no circumstances should you disconnect the battery with the engine running.I own a starter/alternator rebuild shop and have heard this many times.I'll not go into details but it is a definite NO.

As was mentioned check voltsage with a meter,but if light is out voltage will be correct.if you have a local shop available remove unit and have them look at it.

We work in automotive as well as industrial/commercial equipment.I have experienced your problem many times.something insdie alternator is either worn out or breaking down under a load.

This is how I do it, I disconnect the positive cable from the battery and if the engine dies that means your alternator is no good...:W

If you have a volt meter, then check voltage with engine running with the red lead on alt output. Should be close to 13+ volts. Now have someone run the engine up and voltage should jump up to close to 15. As long as the voltage does not stay stable then the alt is good.