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

For the third time this month, the car is completely dead after leaving it for 8 hours. Twice before I needed a jump, and, then it worked just fine.

I'm, I guess, panicking, so, if you have good references, please shoot 'em my way as I'm not sure what the problem is. I think it could be the FSU but I have no evidence that it is ... but I'm gonna call AutoHaus tomorrow to get a new FSU anyway.

The battery itself is only about 2 years old. Is there an indicator that goes on if the alternator isn't working? This only happens after a long park (overnight or during the work day).

The lights were NOT on, nor was anything plugged into the cigarette lighter. I'll debug if it's the battery by picking up a density gauge (I have a voltmeter but I don't know if that will tell me much).

Is there a good troubleshooting DIY for a periodic dead battery? Bear in mind, it's perfectly good 9/10ths of the time; but, lately, about once or twice a month, it's totally dead (the horn won't even blow ... sometimes it clicks when I try to start it; sometimes it's totally dead).

But, as soon as I connect the donor jumper cables, the car springs back to life and acts normally. Until the next time.

Please advise, (Sorry I don't have time to research but I gotta' get it jumped now!).

Ill try and help out here...for one how old is the battery? An old batt will do lots of crazy things including die after a few hours. Also, lock the car at night and wait 20 minutes. Then check if the small orange light next to the shifter is off, if not...your car isnt going into "sleep" mode. There are many possiblities, just throwing a quick couple out.

I actually just got done fixing a similar problem on my W124 benz.
Turned out to be the voltage regulator. Swapped it out for a new one... prob solved.

Originally Posted by bluebee

I guess, panicking

OK. Now I'm less panicked as the E39 is now working fine again.

- I didn't see your help until now so I'll check that little orange light by the automatic shifter (I always wondered what that was ... what is that light there for anyway?)
- I have no idea how to check the voltage regulator (but now that I'm running, I can look it up)
- The 2-year old battery looks pristine (no crud or anything on the terminals)
- I can never read the color of the sight glass (what kind of perverse test is that?)
- I don't see anywhere to open the battery to get to the sulfuric acid density so I didn't buy the density gauge yet)
- I don't see any light that indicates charging (or no charging for that matter)
The electrical problems I know of is the FSU has never been replaced (to my knowledge) and the interior blower was exhibiting some minor wierdness so I ordered a new $72 Final Stage Unit (FSU) from AutoHaus.

In addition, my recent DICE Silverline 2nd-generation iPod touch radio mount won't charge any iPod so perhaps the iPod is drawing the battery down. Tom, over at EAS, has been wonderful and is replacing the Dice Silverline, so, until it arrives, I unplug the iPod whenever I leave the car.

Now that I'm running again, I can dig for some good diagnostic procedures to find out WHAT is draining the battery?

Here is how to check your system
This is a battery maintenance DIY (I am still on the OEM battery, and it cranks great even at -30°C - no problems)
This is a battery voltage check
And here a DIY to replace the OEM battery - attention at the vent.
Now you have to find the leak. If you did not changed the FSU, it might be that one. But, if the A/C does not behave strange, maybe something else?

The orange light is just indicating that the car is still "awake". If it does not shut off then many computers/moduals are still communicating and wasting power. The light should shut off 15 mins after you lock the doors.

I had the same problem, then one day I noticed my trunk light had melted and burnt. The actuator in my trunk locking mechanisim had died in the open position triggering the trunk light to stay on, unknown to me. I had that replaced and all was good.

I found out the actuator was dead when I tried to use my remote to open the trunk and nothing happened.

Good Luck!

Originally Posted by dannyc9997

The orange light is just indicating that the car is still "awake". If it does not shut off then many computers/moduals are still communicating and wasting power. The light should shut off 15 mins after you lock the doors.

Actually 16 minutes.

Until my FSU arrives, what I'm doing now is to listen attentively to the sounds inside the cabin after I remove the key in my E39.

If I hear the faint sound of the fan blowing from deeply inside the dash, I start the car back up and fiddle with the A/C and heater settings, punching random buttons, until when I shut down the E39, I no longer hear the faint fan.

So far, this takes two or three attempts but the good news is my battery hasn't died overnight after doing this procedure. Lesson learned.

I'm not sure if it's working, but, at least, I feel like I'm doing something about it.

Glad you sorted it out.
That pic of the FSU, it's the old one you pulled right?
I think I spied somwhere a thread with pics of the new style FSU, and it looks smaller, but with more cooling "fins", compared to the old one that has fewer but bigger "fins".

Also, there was a thread on this forum, and I cannot find it (I tried) - a member from France I believe, had the curiosity to open up a "damaged" FSU to find out the failure mode.
His point was that the electronics are pulled out by the silicone type material they are imbedded. The silicone, after years of hot/cold cycles seem to shrink or pull out of that "mold" where it sits together with the electronics. Some nice pics were posted as well.

Originally Posted by doru

That pic of the FSU, it's the old one you pulled right?

Nope. That was a reference pic. My replacement FSU hasn't arrived from UPS yet so mine is still safely ensconced next to my glovebox.

Also, there was a thread on this forum, and I cannot find it (I tried) - a member from France I believe, had the curiosity to open up a "damaged" FSU to find out the failure mode.

I would LOVE to find that thread! I searched and didn't find it. I also googled and didn't find it. When mine arrives, maybe you guys can help me disassemble it so we can determine the failure mode in the investigative autopsy.

Found it here.

If you want to check if the FSU is your problem, why not just unplug it everytime you leave the car for more than a few hours? Have you measured the drain on the battery?

Originally Posted by dannyc9997

If you want to check if the FSU is your problem, why not just unplug it everytime you leave the car for more than a few hours? Have you measured the drain on the battery?

Hmmmm..... unplug the FSU? Most people said it was a bear to unplug the FSU. Sure, it's only one bolt, but, that bolt is apparently way way up there and very hard to get to unless you're really lucky and you get it on the first shot. In essence, my assumption is that it's way too inconvenient to unplug the FSU nightly.

As for the drain on the battery ... I don' t know how to measure it. I generally don't mind buying useful diagnostic tools (if I can reuse them for other cars), so, if you have a suggestion as to what to check and what tool to use, let me know. (It might be on the previously referenced threads.)

Originally Posted by bluebee

Hmmmm..... unplug the FSU? Most people said it was a bear to unplug the FSU. Sure, it's only one bolt, but, that bolt is apparently way way up there and very hard to get to unless you're really lucky and you get it on the first shot. In essence, my assumption is that it's way too inconvenient to unplug the FSU nightly.

As for the drain on the battery ... I don' t know how to measure it. I generally don't mind buying useful diagnostic tools (if I can reuse them for other cars), so, if you have a suggestion as to what to check and what tool to use, let me know. (It might be on the previously referenced threads.)

Im having a total mind blank on the name of the tool, ill figure it out in a min though...as for unplugging it there are no bolts. Simply remove the trim peice, get on your back and unclip the plug, push down on the lever at the bottom, and pull er right out. Youll probably cut your hand in the process, however you can just leave it hanging and fit the trim peice over it if you want, untill you get the new one. Since you will eventually have to take it out to replace it its no extra work. However since you already ordered the part I guess you might as well just wait for it

Since you need to remove the FSU anyway to install the new FSU, go ahead and remove the old FSU and leave it out. Then see if the problem goes away. If it does, you can sleep better knowing you have diagnosed the right fix! If the problem recurs, back to troubleshooting. You should replace the FSU when the new one comes in, even if it is not the right fix, since these FSUs seem to have a limited lifespan (at least the original ones did). Good luck. I am betting it is the FSU as mine caused battery drain issues before it caused noticeable fan issues.

amp meter

Originally Posted by dannyc9997

Since you will eventually have to take it out to replace it its no extra work.

This is a very interesting concept. Are you saying that if we remove the FSU, that the car will still work?

I hadn't thought of that. I suspect all that will fail is the interior blower (and maybe not even that because the high setting isn't regulated by the FSU according to what I read). So, maybe without an FSU, the fan would just blow 100% (or 0%?) all the time?

What happens to an E39 that has no FSU connected to it?

Originally Posted by bluebee

This is a very interesting concept. Are you saying that if we remove the FSU, that the car will still work?

I hadn't thought of that. I suspect all that will fail is the interior blower (and maybe not even that because the high setting isn't regulated by the FSU according to what I read). So, maybe without an FSU, the fan would just blow 100% (or 0%?) all the time?

What happens to an E39 that has no FSU connected to it?

The blower simply will not work. No power gets to the motor I suspect. Mine was unplugged for awhile with no issues, you even still have a small amount of heat while driving, plenty while on the highway due to the airflow though the system.

Originally Posted by dannyc9997

amp meter

I know about the basics of electricity. I do have access to a Radio Shack voltmeter ampmeter. But there is no way that meter is going to take the amps of current if I put it in line with the battery.

So we need those clipon things that go around the wires, don't we?

I realize the ampmeter needs to be in the circuit (not across the circuit) so that's why it's not so simple as inserting my $15 radio shack voltmeter/ampmeter into the battery circuit.

How much current are we looking for and what two wires do we break to insert the ampmeter?

Originally Posted by dannyc9997

The blower simply will not work.

Until now, I never thought of this as a diagnostic procedure, but, it seems pretty simple.

1. Car dies periodically (and blower has wierdities at the same time)
2. Unplug FSU ...
3. If intermittent battery death stops, then you've pinpointed the problem
4. Replace FSU

I should have done this looooooong ago!

Originally Posted by bluebee

Until now, I never thought of this as a diagnostic procedure, but, it seems pretty simple.

1. Car dies periodically (and blower has wierdities at the same time)
2. Unplug FSU ...
3. If intermittent battery death stops, then you've pinpointed the problem
4. Replace FSU

I should have done this looooooong ago!

Give it a try, hopefully thats your issue.

The info for how to measure drain is in post #2 in this link, also other good info in the thread here...

http://forums.bimmerforums.com/forum....php?t=1079602

Originally Posted by bluebee

This is a very interesting concept. Are you saying that if we remove the FSU, that the car will still work?

I hadn't thought of that. I suspect all that will fail is the interior blower (and maybe not even that because the high setting isn't regulated by the FSU according to what I read). So, maybe without an FSU, the fan would just blow 100% (or 0%?) all the time?

What happens to an E39 that has no FSU connected to it?

Effectively nothing except no interior fan function, which shouldn't be too much of an issue in CA. On the other hand, not having a heater/defroster fan here in NE in Jan can be a major issue! Others have done this and their blowers become non-functional but their battery issues disappear. This indicates the cause of the drain is the FSU. If your drain continues after FSU removal, the cause is something else. A pretty simple approach to trouble shooting and diagnosing battery drain issues with no ill side effects.

Originally Posted by Fudman

Others have (removed the FSU) and their blowers become non-functional but their battery issues disappear. This indicates the cause of the drain is the FSU. .

OMG. It was not easy to remove that FSU from my 2002 E39! It was very difficult to get the two clips to release and I ended up breaking one of the clips while you guys were watching the Superbowl today.

The good news is I have the new FSU in and I don't hear anything running when I shut off the E39.

Time will tell if that was the reason for the monthly dead battery after sitting overnight or during the work day.

For diagnostic purposes, I took old/new resistor measurements which I'll post to the FSU thread.
If everyone with the dead-battery problem does this, maybe we'll notice a diagnostic pattern to the measurements?
And, it will be interesting to take the old FSU apart to see what's inside.

I'll let you know if the battery still dies but I hope it was the FSU.

Originally Posted by bluebee

Time will tell if that was the reason for the monthly dead battery after sitting overnight or during the work day.

So far, so good.