How to diagnose what's wrong with a battery that goes dead once a month?

The FSU must have been the culprit because changing it out solved the battery drain problem. Same battery - never dead anymore.

Cany anyone list the most common things to drain a battery in 39s? Cd changer, fsu, _____,____?

This may be helpful to some, infcluding my self

Five month old battery died yesterday. Had AAA test it, and they found the battery was draining even with everything shut off.

E39s seem to have the same common problems. So far, I've had a non-opening trunk, inconsistent A/C fan speed, and a broken CD changer - all of which are potential battery drainers.

I've fixed the trunk problem (broken wires leading into trunklid), and the CD changer hasn't worked the past 4 years or so, so I'm guessing it's the FSU. Guess I'll bring the car to the indy mechanic tomorrow again....

Originally Posted by emPoWaH

they found the battery was draining even with everything shut off.

You seem to have the same problems I had ... (except the CD changer which I don't have since I only have the single Business CD) ... but I do agree ... E39 problems seem to be relatively common ...

The FSU isn't all that bad to replace ... it's 50:50. Either you will replace it so quickly you'll wonder what all the fuss is about; or, you'll be cursing the evil ears on the FSU connector like some of the rest of us (me included).

Of course, I broke off the evil ears ... and then my FSU came right out ... but I don't recommend such draconian satisfactions ...

Bluebee

Originally Posted by bluebee

You seem to have the same problems I had ... (except the CD changer which I don't have since I only have the single Business CD) ... but I do agree ... E39 problems seem to be relatively common ...

The FSU isn't all that bad to replace ... it's 50:50. Either you will replace it so quickly you'll wonder what all the fuss is about; or, you'll be cursing the evil ears on the FSU connector like some of the rest of us (me included).

Of course, I broke off the evil ears ... and then my FSU came right out ... but I don't recommend such draconian satisfactions ...

Bluebee

Since it's my mother's car, and she doesn't want to be stranded again, she ended taking it to a local indy shop (South Bay Independent BMW) today. The FSU unit was $110+tax and labor was $90.

Pelican Parts has it online for $60 shipped; the local dealer wants $158 for the same part. I suppose I could have DIY'ed and saved $150, but that's the price you pay for convenience and assurance. It's still way less than a monthly lease payment...

Originally Posted by emPoWaH

I suppose I could have DIY'ed and saved $150, but that's the price you pay for convenience and assurance.

Always a battle between getting it done and doing it yourself (with limited time, limited tools, and limited experience).

Without these boys on Bimmerfest, I'd never have attempted half of what I've done so far ... and certainly not what I must contemplate moving forward.

So, kudos to you for getting it done. Far better to pay the $150 and not have a battery go dead on you than to never do it at all!

Bluebee

Long term progress report:

It has been two years since the FSU was replaced and the battery hasn't died since!

Originally Posted by bluebee

Long term progress report ... It has been two years since the FSU was replaced and the battery hasn't died since!

In the past two years, we've learned to stay away from the GKR FSUs (which my original FSU was) in favor of the Sitronic/Bosch for the E39.

If you're interested in the details, xref is here.
FSU (aks FSR) fan blower speed resistor autopsy and wiring diagrams (1) (2) (3) (4) ... (1) (2) (3) (4)

Just to update this thread, since I often refer others with dead battery overnight symptoms to it, this seems to be a reasonable diagnosis:

- If your battery goes dead overnight (intermittently)
--> Then test your battery drain (1) (2) & test your alternator output (1)

- If the drain and alternator are good but the battery still dies overnight
- Then feel the FSU to see if it's hot a few hours after shutting the car down
--> If the FSU is hot, then unplug the FSU & order a new FSU

- If the FSU wasn't hot yet the battery is still dying, then unclip the FSU
--> If the problem goes away while unclipped, then order a new FSU.
Note: Only order the FSU brand recommended in this thread!

Note: You may also note oddities in the fan blower operation; but then again, you might not as it seems to often be intermittent.

As a cross reference, here's a UK car whose battery kept draining and replacing the FSU solved the problem even though there were no symptoms of erratic blower operation or no-blower operation.

Also, as an xref, here's also a new battery test thread.

From the VERY best of E39 Links:
- DIY how to test a BMW E39 battery & alternator

Just for reference, I'm getting a parasitic drain again. Dunno why yet. Details here:
> E39 (1997 - 2003) > It's my turn again: Dead battery ... two days in a row ... (yeah, I know what to do)

Originally Posted by bluebee

Here's a video, which looks like that very same FSU ...

Originally Posted by n0_odles

what the FSU looks like ...for an e39

I had serious battery drain. First I thought my old battery died so I replaced it. After 4 days, that battery was dying, I had to charge it every other day. Finally after 2 weeks that battery crapped out and they replaced it under warrenty - dead cell. Tried all the FSU tests, FSU seemed ok. Began testing for parasitic drain, when I found the 'hood open' limit switch unplugged. I had done brake work last month, checked the brake fluid, and forgot to plug the limit switch back in. That was the source of the drain.

Sometimes I am my own worst enemy.

Jerry