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03 E55. Desperate and I'll try to keep it short. Out of the blue, after 3 days of sitting, battery completely dead (2 years old and tested fine). After many hours with a meter and isolating everything I can think of, I have 2 scenarios.
1) Significant current draw on wire or circuit F81 which goes from F33 in the trunk which I'm assuming is the rear SAM, and supplies power to F34 in the engine compartment. Completely isolated F34 and the draw was still there.
2) Same current draw on the main positive battery cable supplying power to the front pre-fuse box (F32). Removed all of the other feeds inside the car from F32 and got the same result. I'm assuming this leaves F32 only supplying power to the alternator and starter from the terminal below the floor. Disconnected the alternator with no change. I did not get under the car to check for any current draw at the starter. I also disconnected the auxiliary battery with same result.
Can anybody tell me if there are any other components that are fed from the main terminal on the F32 besides the alternator and starter?
What’s baffling is I have the same current draw on the wire connecting F33 to F34 but according to the Starfinder diagrams, this shouldn’t be possible.
Is there anything at all in common with F34 and F32 that could be creating this draw?
This car is fun until it breaks. Thank you for any assistance!
24.5 milliamperes is not an exceptionally large amount of current to be drawn in the car's idle/parked state. If your battery is only 6 months old, its charge state might be low from many short trips. Have you put a charger on it for a day to bring it up?
F32 = Front Pre-fuse block
F33 = Rear Pre-fuse block
F34 = Interior fuse block on side of dash
See attached for a better breakdown of the circuits. Be advised the E55 electrical harness has changed at least three times over production.
You can also look at or download the WIS for your car here: https://charm.li/Mercedes%20Benz
I agree with streborx, 24.5 mA is not an issue. Amperage draw of a couple of amps or higher is a cause for concern. Could you have left parking lights on or the key in the ignition with it in position 1 or 2? Don't laugh, I did that a few weeks ago and 24 hours later my battery was flat, flat, flat. I went out and purchased another because it was so bad.
FWIW, I use a Solar BA-5 battery tester (now out of production but newer equivalents are available) to verify battery health. The new battery is rated 900 CCA and tested at 1010 CCA. When the battery tests at 600 CCA or below, I replace it.
Last edited by bbirdwell; Sep 28, 2025 at 03:57 PM.
One further note -- the handheld battery testers need to be attached directly to the battery terminals, not to the jumper terminals, in order to measure correctly.
Thanks for your reply. The date on the battery is 02/23. Ever since it went completely dead, I first brought it back with a good charger, then if I'm not going to drive it for 2 days, I put a tender on it. Otherwise it will die again.
I'm not good with the whole meter thing but here is the meter with F32 and F33 isolated.
Thank you for all the feedback. I'm going to dive into the attachment a little later.
I may not have the meter selected correctly but I just uploaded another picture with everything isolated. Leaving something on was the first thing I thought of and was hoping for but not the case and I don't ever leave the key in the car. And I didn't laugh! Years ago, I had an 85 Porsche 944, and for some reason, the left side parking lights stayed illuminated. After many hours under the dash and scratching my head, I discovered that I didn't know how to operate the headlight switch and left it in that position that they use overseas to leave lights on when parking close to the road. At least I got to laugh then.
I'll have to check out that battery tester. Thanks!
Not quite sure what you mean by "F32 and F33 isolated". You pulled all of the fuses? I'm looking at page 14 of the document.
I can easily be persuaded the 2.3mA draw is the Battery Control Module N82. My rule of thumb is if the parasitic draw is ~50mA or below, I don't worry about it. For myself, if I'm not going to drive the car for a week or so, I'll put a small 1/4 amp battery tender on the battery. On my W211 just opening a door or the trunk activates all of the modules and those pull a lot of current. Once doors are closed the modules will begin to de-energize and all should be asleep after about 30 minutes.
The battery I just replaced lasted ~5 years, 8 months by my records. The previous battery lasted ~3 years, 7 months (that one I killed by leaving the parking lights on in the garage for a day or so).
Try a battery tester for peace of mind. Not that expensive and you can use it to monitor the health of any battery in any vehicle you own. I literally put it on the battery/batteries every oil change. I have found the batteries seem to degrade slowly and then suddenly; hence why I replace them before I get stranded at a rest area 200 miles from home. It is unlikely but possible you ended up with a battery that is failing early.
In the link I provided, this section has good stuff like this:
Yes. I disconnected everything from F32 (Front Pre-fuse block) except the wires on the underside of the block that go to the starter and alternator. On F33 (Rear Pre-fuse block) I disconnected everything. When I I reattach the wire (f81) supplying F34 (Interior fuse block on side of dash) is when the milliamps go back up to 24.5.
This won't be the first time I've gone down a rabbit trail when troubleshooting electrical issues. It was the fact that I was out of town on business for 3 days and the battery was completely dead. Nothing was left on and I immediately went to parasitic draw. I guess the battery could have an internal defect that killed it the first time? I had a 97 E320 years ago that was killing the battery and it took me forever, but it turned out to be a bad blower resistor.
NAPA still sells the Solar BA-5 battery tester and I'm picking one up on Tuesday. I really hope that I can update this post on Tuesday night to say "It was the battery, stupid (me)! Thank you to you and streborx for hopefully getting me on the right track.
Thank you again for sending those links. They're a lot easier to understand then automotive wiring diagrams and have a wealth of good information. I'll keep you posted for sure!
@mcanix PM me your email address and I'll send you about 12 emails with multiple attachments containing technical information on your car similar to what I attached above.
No longer stock '06 E55, A3 3.2 Quattro, GLE 400d, R107 280SL, Golf Polo
Check your alarm / alarm battery. Located in front right fender behind arch lining. I chased a drain on my car for ages and it was this unit that was faulty - can't recall what fuse block it was from. Also check rear SAM for any water ingress/ corrosion - also auto boot closer and the whole cluster of electronics in the rear. Any water in there and you will get battery drains as units end up not turning off.
If you not aware, you can test the battery drain by the cross fuse voltage drain / drop - but you will need a sensitive voltmeter. Measure across the two terminals OF THE SAME FUSE. Sounds crazy but this does work. If you get a voltage drop across a fuse that fuse is feeding current.
"To measure current draw across a fuse for parasitic drain testing, set your digital multimeter to a millivolt DC range and place the probes on opposite sides of the fuse terminals to get a voltage drop reading. You then use a parasitic draw chart or resistance calculation to convert the millivolt reading to an amperage (current) reading to determine if the draw is within normal limits".
The biggest pain for this is to measure all this with the car closed and sleeping and you have to manually latch the boot and door latches that are open as well *** the boot & door light sensor so car thinks it is closed and can begin to go to sleep.
I just tested the battery with the Solar BA-5 and got the following results. Open circuit voltage 12.40, Determined rating 927 CCA with an amber light indicating that the battery needs charged (I drove it 40 miles yesterday and 15 miles this morning). The battery is rated at 900 CCA, reserve capacity 160, and amp hours 95. The battery itself (and I know there's 1000 different opinions on it) is an Everstart (Walmart) Platinum which was installed on 03/14/2023. The aux battery was also replaced at the same time.
Voltage is good. CCA rating is good.
At this point I would check all connections on the battery and BCM. Then I would follow the diagnostic procedures listed in the Workshop Information System listed in post #9. There is also a "BCM reset" procedure listed in the technical bulletins.
Please let us know how you resolve this. (Pushing your car off of a cliff, although entertaining, does not qualify as "resolved").
Last edited by bbirdwell; Sep 30, 2025 at 06:45 PM.
So I just started going through task AR54.10-P-1135A Check battery control unit 23.10.07
MODEL 211 up.
The fault code for the BCU was 9040 which is undervoltage. Terminal 30 voltage was 12.2. Battery charging current was -13.5, and the actual voltmeter battery voltage was 12.28.
It then directs me to run a "jumper" from the negative post to ground then disconnect the negative cable from the BCU. At this point my very thin wire jumper started to smoke so I figured it was too undersized. I quit at that point.
Check your alarm / alarm battery. Located in front right fender behind arch lining. I chased a drain on my car for ages and it was this unit that was faulty - can't recall what fuse block it was from. Also check rear SAM for any water ingress/ corrosion - also auto boot closer and the whole cluster of electronics in the rear. Any water in there and you will get battery drains as units end up not turning off.
If you not aware, you can test the battery drain by the cross fuse voltage drain / drop - but you will need a sensitive voltmeter. Measure across the two terminals OF THE SAME FUSE. Sounds crazy but this does work. If you get a voltage drop across a fuse that fuse is feeding current.
"To measure current draw across a fuse for parasitic drain testing, set your digital multimeter to a millivolt DC range and place the probes on opposite sides of the fuse terminals to get a voltage drop reading. You then use a parasitic draw chart or resistance calculation to convert the millivolt reading to an amperage (current) reading to determine if the draw is within normal limits".
The biggest pain for this is to measure all this with the car closed and sleeping and you have to manually latch the boot and door latches that are open as well *** the boot & door light sensor so car thinks it is closed and can begin to go to sleep.
I checked the alarm area and everything looked good. Also, there's no evidence of any water intrusion in any of those areas.
I watched a couple of videos using that cross fuse voltage drain / drop method but wasn't able to verify if I had to disconnect the ground from the battery and use a jumper instead. I'm not versed in any type of electronics except a hot and a ground.
I've been able to keep all of the wake up switches in the closed position so far.
When performing the cross fuse voltage drain / drop check, do you keep the battery corrected as normal or use a jumper between the negative terminal and post?
I checked the alarm area and everything looked good. Also, there's no evidence of any water intrusion in any of those areas.
I watched a couple of videos using that cross fuse voltage drain / drop method but wasn't able to verify if I had to disconnect the ground from the battery and use a jumper instead. I'm not versed in any type of electronics except a hot and a ground.
I've been able to keep all of the wake up switches in the closed position so far.
Thanks!
you can’t visually check the alarm for water intrusion. There are rarely any outer signs that this has been compromised,.
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