Alternator or Battery Problems?

[dj888]

Greetings! I have a 1989 320i. my car battery had been discharged twice. i have had my alternator checked and the mechanic told me that the culprit is the battery for my alternator is working fine and hits 12 on the meter (pretty low) still, it's charging. however, once i hit on the lights and aircon, it drops to 10-11, hence, it stops charging.


what could be causing this? bad alternator or battery? if it's the alternator, what could be the fix for this?

thanks in advance!

[Brianmoooore]

voltage produced by the alternator should be more like 14 volts plus, so it sounds like a dud alternator to me.
Before condemning it, check for any voltage between the alternator body and battery negative.
Any appreciable voltage here indicates a poor earth return path.

[dj888]

thank you, brianmooore! which part of the alternator body should be tested? i understand there are three parts of the body which could be used for testing. thanks!

oh, by the way i couldn't get the alternator to budge, im having a hard time with the bolts underneath

[daimlerman]

There should be an earth wire from the back of the alternator to the engine block.Try soaking the stuck fastener in WD40,then work it forward and back until the swine gives in.

[Brianmoooore]

There should be an earth wire from the back of the alternator to the engine block

Doubt it, on a 320. Not all E30 alternators are rubber mounted.
Test for voltage between any part of the alternator body and battery negative, but scrape the chosen spot clean first. This checks for a poor connection: between the alternator and its mounting bracket, the mounting bracket and the engine block, at either end of the engine to body earth strap, and at the battery earth strap.

[daimlerman]

My 320iSE has an earth lead,I know because I failed to refit it and fryed the alternator as a result....it attaches via a phillips head screw to the rear.