I have a 1998 BMW 318i 5 speed that has recently been draining batteries. I had the alternator checked and it showed

Got it, give me a few min here.

Hi again,

To start, no, you cannot use a used ecu in your car. Once paired to a vehicle, they cannot be transplanted. It is very rare for an ECU to fail, by the way.

I hate to not have better news for you, but this sounds almost certainly to be a bad alternator.

There are other problems that could cause a complete cut of the power to your starter, but since your starter does click, we can rule those out. Also, since you have been disconnecting the battery when you park, we can rule out a current leak that drains the battery while it is parked.

With those possibilities eliminated, I would say it is about a 99% chance your alternator is bad.

The part number for a replacement for your car is 12311247310 or 12311247288, you can use these numbers to search online for an inexpensive replacement.

Please let me know if you have further questions.

I hope that helps. If so, please press the green ACCEPT button below so that I am paid for my time and effort. I will continue to answer any follow up questions should they arise. I do not receive any credit for my work until you press accept.

Thanks and good luck.

Steve

It's not 100% necessary if it is in good shape and is not damaged when the alternator is removed. It might be a good time to do it though, if it hasn't been replaced in a while.

Steve

Hey,

I can't believe it wasn't the alternator. If you've got a new battery and new alternator and the problem is still occurring, you're probably going to need a serious diagnosis job by an automotive electrician. BMW questions that get posted to this site get answered within minutes - except when they are really complex like your situation now appears to be. I would suggest to revise your new post to detail exactly the situation (you unhook the battery when you park, etc) and exactly what has been replaced and the exact remaining symptoms. You might get a bite then, but problems like yours are difficult to diagnose remotely when they aren't simple/obvious fixes (like I thought the alternator would be).

Sorry you're still having troubles.

Steve

If the battery is full, it can power the car yes, but not for long. It is the alternator that charges it, yes.

I thought about the final stage blower coming on (or something else) for your original question, but eliminated it based on the fact that you unhook the battery when you park.