engine and drivetrain Voltage regulator problem?

[MAD02]

I bought a 80 amp alternator from Ebay years ago and its been great. Recently the alternator doesnt seem to be working at low RPM, as the lights are dim at night. If I rev the engine to 3500 to 4000 RPM the lights instantly get bright and stay that way. I remember some one here saying it was a voltage regulator issue.

Is that the problem or could it be something else. Ive taken the alternator to a Pep Boys and Autozone and either their tester is broken or they cannot test my alternator for some reason.

Thanks!

[c.d.iesel]

take VOLTage readings

key off ______ V?

@idle speed _____V?

@3,500 rpm ____ V?

the regulator comes off with 2-screws,

remove it and look at the self contained brushes,

replace the regulator with a NEW one.

testing the battery condition

VOLTs at battery

key off _____ V?

during starter motor cranking _____ V?

[srnoncom]

Could also be as simple as a belt adjustment