Something is draining the battery on our 2002 bmw z 3. The battery and alternator checks out good. What could be the problem??

If the alternator is charging the battery it will not be the fuse. The alternator would simply stop working if the fuse has blown.
If the battery isn't holding charge you might have a power drain somewhere in the vehicle. Something in the electrics may not be turning off correctly and draining the battery. Do you have any warning lights on the dash? Have you access to an OBD2 scanner to check for any Trouble Codes?
If it is a power drain the simplest fix is to get a good Auto Electrician to check the vehicle. Finding a power drain or an electrical short can be a real pain in the tail bone job because there is so much electronics on a car. Paying an expert to do it is a lot quicker and easier as they have the right tools to do it.

Most likely you have either a bad relay, or a short circuit. My 1998 had a relay someone had put in that was the wrong relay such that it caused a current drain. Use amp meter between battery ground cable (disconnected from battery) and battery ground post to see how much drain exists, then pull relays one at a time to see if one stops the drain. If not, do the same with fuses to find the bad circuit and fix drain.

HELP 2000 Z3 Battery Drain - Bimmerfest - BMW Forums

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Feb 18, 2007 - 7 posts - ‎6 authors
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Just because parts are new does not mean they are good. Have alternator output tested.

Check the current draw when the car is switched off. if it is not obvious whet is staying on, remove fuses one buy one and see when the current draw goes down. Note that the car does not go to "sleep" until something like 16 minutes after you turn off the engine.

How low is the alternator? 13.5 v with the motor running is about as low as I would want, but if you hear something running after you turn it off, it could be draining your battery. It is not uncommon for cooling fans to run after you turn the car off but shouldn't run for long. Can you look at yout cooling fans and see if that is what is running? How long do you hear the noise after you turn the car off?

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Did you have the battery tested? It should hold 13+ volts at any time. Wal- mart will test it for you. If the battery is good, get the car fixed before you kill the battery, car batteries can only go dead so many times before they are shot. If the alternator is not charging, get that thing to the shop too and have it tested, if it is OK, then you have to have something running like a clock, radio, alarm system, something is draining the battery, you have to find it and get her fixed or carry a spare battery. I have heard of fuel pumps running on, and draining the battery, see if you can hear that thing going when the car is off. Check it out. Hope this helps.

i had the same problem on one of my cars it was the fan control module. replace that and it will fix the problem.

it seems like something is draining your battery while the car sits overnight. causing the battery to get weaker and weaker putting stress on the alternator making it fail prematurely. the dealership will do a paracidic drain test where they'll find out what is draining your battery while your car sits overnight. it could be a bad module somewhere that's staying on when it shouldn't be.

CHECK ALTERNATOR OUT PUT IT SHOULD BE 13.5 TO 14.5 VOLTS WITH CAR RUNNING.CHECK ALTERNATOR WIRES AND FUSE.ALSO HAVE THE BMW DEALERSHIP SCAN YOUR CAR BECAUSE ALTERNATOR IS PCM CONTROLLED.