M20 '87 Alternator Testing

[cmwiley2]

It appears as tho I'm not generating enuf volts! My battery is wearing down and the voltage across the posts is 11.5v when engine is idling. Can anyone tell me or point me in the right direction regarding diagnosing the problem? Could this just need a replacement Voltage Regulator???

Chris

[Kershaw]

you should be holding the rpms at 2000 (or is it 2500?) and then testing the voltage. not idle.

[cmwiley2]

thx will do...any idea where the voltage should be at 2-2.5k rpms? I see the alternators are rated at 14.1v but not sure if that is what I'll see...seems high.

[jlevie]

If the alternator is working properly with a fully charged battery you should see ~13.5v at idle and ~14v at 2500rpm.

The two most common charging problems on an E30 are a burned out alternator warning bulb or worn brushes in the regulator assembly. If the warning light does not come on when the the ignition is switched on the alternator won't get exciter power and thus won't charge. If the bearings in the alternator are good, replacement of the regulator may be all that is needed.