Alternator / Charging System 1988 325ic - BMW Forum

mto386 · 11-17-2009, 09:51 PM

I thought i was having a charging/battery problem so i took it to a shop. It was basically a brand new battery so i thought it was the alternator. Well after about 2 hours the mechanic tells me that it is indeed my alternator. Unfortunately the one they had in stock was defective and it would be another 30 mins before they could get another one. The new one arrives and is installed. A few hours later he tells me it might have to stay overnight. They said the second new one wasn't charging the system either. They said it passed the bench inspection but when they put it in, it still wasn't charging the system properly. Any thoughts to what it may be? I thought it was the alternator because i was driving at night and my headlights and stereo went out. The car kept driving with a dead battery though. I pulled over, turned the car off, and tried to start the car but the battery was completely dead. I brought my battery to the auto parts store and they charged it for me. Car started right up with headlights and radio. Could 2 new alternators with built in voltage regulators really be defective? He said that they would have to do some sort of complete car diagnostic test. They would search the car for any sort of electrical drain on the system. This doesn't make sense to me because even if the car has some sort of short or drain the alternator should still be pushing out the correct power, but its just not reaching the battery. The car can sit for days and the battery holds enough charge to start perfectly. Is this guy an idiot? Should i just give in and bring it to a specialist?

maxfoutz · 11-19-2009, 02:40 AM

Think of it as unused capacity. It's not pumping more power into your electrical system all the time -- it is an "on demand" system that gives you only what is required, up to the maximum amperage, and then you start running down the battery. That's one reason people with systems sometimes run two batteries.

maxfoutz · 11-24-2009, 03:04 AM

Think of it as unused capacity. It's not pumping more power into your electrical system all the time it is an "on demand" system that gives you only what is required, up to the maximum amperage, and then you start running down the battery. That's one reason people with systems sometimes run two batteries.

ebrisse · 03-11-2013, 11:17 PM

I'm having a similar problem with my car right now. '91 318i. I've got a serious grounding issue or something else. I took out my alternator and had it tested. It passed all the tests. But something is keeping it from charging my battery. My car ends up running the battery down to nothing and eventually dying. I'm guessing you got it fixed? What ended up being the problem?