Hi Everyone,

I've been trying to diagnose my wife's 2001 C240's dead battery problem and am completely stumped. The car cannot sit for more then 1 day without completely draining the battery. This problem started out of the blue one day about a month ago. I measured the current draw from the battery at 1700 milliamps when the car is off and the alarm is armed, so I figured that something must be shorted to be drawing that much current with the car off. I then proceeded to pull fuses and measure current across each fuse. I made a chart attached to the end of this post which contains the results. It seems that the big winners in terms of current draw are the Tele-Aid system at 190ma and the COMAND system at a whopping 670ma.

Some other strange things happening is that the cooling fan for the COMAND never turns off no matter how long the car has been sitting, the fan for the ambient temperature sensor in the dome light area is always on no matter how long the car has been sitting, and the power light on the COMAND unit never turns off. These items will stay on all night.

From looking at the current draw across the fuses, it looks like all the current draw is coming from components connected to the body CANBUS system. If anyone could explain to me how the body CANBUS system works, I would greatly appreciate it. Is it possible that the components do not know the car is off so don't shut themselves down?

The car is out of warranty and the dealer has said that they cannot give me an estimate to fix the problem. I have the feeling that the dealers method of swapping out electrical parts until they find the culprit could be very expensive for me. Since the car is out of warranty I would like to do as much of the diagnosis as I can myself before sending the car off to the dealer.

If anyone has any ideas or comments, I would really appreciate it.

Thanks in advance,

Lawrence

----------------------------------------
FUSE CURRENT TABLE
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Fuse #8 ??? ma ( Could not be measured because alarm goes off when pulled)
Alarm
Convenience Locking
Remote Trunk Release

Fuse #9 100 ma
Alarm
Convenience Locking
Dome Lights
Garage Door Opener (SE)
Make Up Mirror (SE)
Mirror Automotic Dimming (SE)
Rain Sensor (SE)
Switch Illumination
Sunroof

Fuse #21 80 ma
Central Locking
Convienience Locking
Courtesy Lights
Mirror Adjustment
Mirror Automatic Dimming (SE)
Mirror Fold-in Feature (SE)
Mirror Heater
Power Window, Front
Power Window, Rear
Remote Trunk Release
Seat Adjustment, Driver
Switch Illumination

Fuse #22 90 ma
Central Locking
Convenience Locking
Courtesy Lights
Mirror Adjustment
Mirror Fold-in Feature (SE)
Mirror Heater
Power Window, Front
Seat Adjustment, Passenger
Switch Illumination

Fuse #27 60 ma
Seat Adjustment, Driver
Steering Wheel Adjustment (SE)

Fuse #32 80 ma
Central Locking
Convenience Locking
Power Window, Rear
Switch Illumination

Fuse #33 80 ma
Central Locking
Convenience Locking
Power Window, Rear
Switch Illumination

Fuse #38 60 ma
Seat Adjustment, Passenger

Fuse #40 190 ma
Tele-Aid

Fuse #41 50 ma
Air Bag Warning Lights
Air Conditioner
Alarm
Automatic Heater System
Blower Motor
Central Locking Interior Switch
Engine Residual Heat System (SE)
Hazard Warning Flashers
Heated Rear Window
Parktronic System (SE)
Rear Head Release
Rear Windscreen Wash System
Seat Heater (SE)
Sun Blind for Rear Window (SE)
Switch Illumination

Fuse #42 80 ma
Air Bag Warning Lights
Instrument Cluster

Fuse #57 80 ma
Central Locking
Electronic Stability Program
Start Enable

Fuse #64 670 ma
COMAND

 

Sounds like the can is active for sure. Having just witnessed a co-worker spend 2 weeks on a similar issue, and having been there myself, I will tell you that fixing this is by no means simple or easy. Current draws are incredibly time consuming and complicated. There is a lot of waiting around for the bus to go to sleep every time you open a door or something that wakes up the bus again.
A dealer may get lucky and immediately see a problem with a control unit or CAN that is keeping the bus active. He might end up spending days tracking this thing down. You really need the mercedes laptop to run a short test, remove a unit from the CAN, then see if the draw goes away, you repeat until the draw is gone, then you do a short test and see which control unit you unplugged, it won't show up on the short test anymore.
There could also be a wiring problem causing this, a shorted can bus will never go to sleep.
I think you are on the right track with CAN being the problem.

 

Lawrence, the CANBUS system works as a bus network, in promiscuous mode, meaning all devices can listen and talk at any time, and accept all messages on the CANBUS and it is up to each device whether to accept and act on a message or reject it. If you have more than one device consuming power, it is likely that something on the bus is outputting messages that keep these devices awake.

Your testing of current draw drop is a common method, but it does leave out several diagnostic abilities. For one, you cannot measure the actual current draw of a single device, just the drop of all devices when one is disconnected. Because of that, you don't really know, for instance, if COMAND actually draws 670ma, but only that 670ma less is used when COMAND is disconnected.

To solve that issue, I suggest making a device to test the actual draw of each fuse. You can do this by getting a blown fuse, then wrapping leads around the blades and have those lead to your ammeter.

After testing COMAND and TeleAid again, you can see how much they actually draw and can compare that with drop when disconnected.

You may also want to try disconnecting the CD changer separately. This is a known power draw which causes problems. There are other known issues that won't be so easy for you to diagnose.

Did you check three fuseboxes?

A shorted CANBUS can also cause the problem. You can try to disconnect wires one at a time at the CANBUS termination points and see if one of those wires causes the problem, then follow that wire back to the device and determine whether the device or the wiring is faulty. Star Diagnose does help in this regard because most devices can detect and report a shorted CANBUS, and this will be reported in the error log.

-s-

 

I had a similar issue with the fan on the comand not shutting off and it was a CAN short out. There are 2 CANBUS B termination points. One is in the drivers foot well area. It's on the black plastic piece. The termination strip is a long 5-6 inch row with a bunch plugs on it. Keep removing one at a time and see if that solves your problem. The other termination strip is on the right of the passanger foot well under the carpet. Remove the plastic door sill plate and dig there. All CANBUS wires are twisted brown and brown/red wires. make sure they stay twisted also. There is a CANBUS C strip also but I don't think that is your problem.

 

Thank you saturnstyl, scorchie, and sunman for helping me out with this problem. I was feeling very frustrated about the car before posting because of the lack of a purchasable service manual for the car and I felt I would never be able to diagnosis the battery drain without sending it into the dealer. But with the knowledge that you have provided, I feel that there is hope in at least finding out what the problem is before sending the car in if I cannot fix it myself.

The car did not come with a CD changer, so that possibility can be ruled out. I did manage to find all 3 fuse boxes after opening the fuse box in the engine compartment and cursing when I saw there was no legend for the fuses stenciled on the underside of the fuse cover.

I've done some more reading on the CANBUS and I think I understand more about how the network operates. From all of the information so far, it looks like the devices on the CANBUS which runs through the passenger compartment (CANBUS B?) are not going to sleep when the car is turned off, so the question becomes why they are not turning off. After reading parts of the CANBUS specification, I've come up with some theories, which may or may not be correct. Here goes...

The CANBUS network consists of 2 wires which are normally at a voltage higher then ground. Devices on the network send messages on the network by shorting the wires to ground. The network is fault tolerant so that if 1 of the 2 wires is shorted to battery or ground through a wiring fault, the devices on the network will stil listen on the faulty wire but will use the other good wire to transmit messages. So possibility #1 is that one, but not both, of my CANBUS wires has shorted to ground. If both wires had shorted, then the CANBUS would not work and 90% in the interior cabin would not be working. Since one of the wires is always grounded, all devices on the network stay awake because they think some other device on the network is trying to send a message.

Possibility #2 is that the CANBUS network does not have any wiring faults, but does have a faulty device attached which doesn't know it is supposed to go to sleep when the car is turned off. By this device staying awake and transmitting messages on the network, this device keeps all the other devices on the network awake because they are required to listen to the messages being transmitted. I'm assuming that the devices put themselves to sleep if they don't hear any messages on the network for a set amount of time. I'm not sure if that is true or not though.

Possibility #3 is that the devices on the network do not put themselves to sleep if they don't hear any messages on the network for a set amount of time, but must be explicitly sent a shutdown message and the component ( ignition key switch?) which is supposed to send the shutdown message is faulty. This possibility doesn't seem likely since I read all 3 replies above as saying that if the faulty device is disconnected from the network, the other devices will go to sleep correctly, implying that there is no special device which sends shutdown commands.

I am going to dig into the termination points and start unplugging connectors tonite to see what happens and hopefully narrow down the problem further. Since I don't have the mercedes diagnostic laptop to do this correctly, I'm wondering if I need to reboot the CANBUS by disconnecting / reconnecting the battery after pulling a plug from the CANBUS termination block. Also, I'm not sure how long I need to wait for the CANBUS to go to sleep after the doors are closed and the alarm is armed. I'd feel pretty stupid staring at the COMAND hoping it will go to sleep for 30 minutes when it's really supposed to be asleep at 5 minutes.

Thanks again everybody!

Lawrence

 

Although it is in the spec to work with only one wire... and it should work in theory... in practice, I've never seen a Mercedes-Benz CANBUS system still operational if even one wire is shorted.

This sounds likely.

There is no implicit "shutdown" message. Usually the ignition on message keeps items awake that should be awake, or causes them to be "active".

Whoa, do NOT disconnect the battery on the 203 if you don't have to! That causes way more problems that it will solve. Everything should shut down in about 90 seconds if it is going to shut down.

-s-

 

I had can low and can high wires reversed once and it kept CAN awake forever. The battery died in 1 day. But since you already said you have not done anything with the car, it puzzles me. check www.startekinfo.com also for more info

 

i have seen several 203s with faulty seat control modules drawing excessive current with key off. it would be fairly easy to reach under both seats and unplug the seat control modules too see if that helps.

 

I didn't have much time to tear apart the car this weekend, but I did have quite an adventure with it.

On Friday night, I dug around and found the CANBUS termination blocks. I was feeling pretty stupid about the drivers side termination block because I ended up removing the drivers side kick panel, steering wheel underpanel, and hood release cable before I realized that the termination block could have been reached by popping off a 1" x 4" access cover on the steering wheel underpanel. The passenger side termination block was much easier to get to since it just took the removal of the passenger door sill and a little digging in the rats nest of wires that live underneath there. After finding the termination blocks and rereading the info from sunman, i realized that he had told me exactly where to look for the drivers side termination block in his post, I don't know how I missed that.

Figuring out how to disconnect the connectors from the termination block was fun. 30 minutes, a few choice curse words and 2 broken plugs later my wife pointed out that maybe I need to push the 2 tabs on the side of the termination block. I pushed the 2 tabs, the termination block separated into 2 pieces, and the plugs came out easily. Hopefully I can order 2 plugs from the dealer to replace the 2 I broke. The 2 plugs still connect, just are kind of deformed from the pliers I was using. It was getting kind of late, so I put the car back together since me and the wife had an early flight the next morning and we needed the car to get to the airport.

Thinking that I was smarter then the stupid car, I had a plan in case the car drained the battery while it was sitting in the airport parking lot. It was a good plan too. A plan to be proud of! I brought along a spare fully charged battery in the back seat figuring I could jump the car from the spare battery if the car wouldn't start. We left the car in the airport parking lot for 2 days, got back from our trip tired, sleepy, and grumpy and the car wouldn't start. I told the wife no problem, pulled the spare battery out of the back seat and said, "Stand back! I know what I'm doing because I have a plan!"

20 minutes later my plan has failed miserably and my wife is on the phone calling Roadside Assistance. We get a jump from roadside assistance, the car starts, and we are off. We make it about 200 feet when I realize the the transmission refuses to shift out of 1st gear. The gear shift must be working because it let me get into Park, Reverse, and Drive, but it's acting like I have the manual shifting on and left it in 1st gear. At this point, I just want to get home so I'm not thinking that clearly and decide to drive the car to my friends house about 10 miles away from the airport so we can leave the car there and get a ride back to our house from my friend. We manage to limp the car there going 20mph in 1st gear all the while thinking nasty thoughts about Mercedes Electrical Engineers and their super cool gear shifts which don't seem to want to work after a jump start. About 2 miles into the trip, I started to think that maybe this isn't such a good idea and that I should have listened to my wife and called AAA for a tow truck. By now, we were in the middle of nowhere so I figured the best thing to do is just continue and finish the trip. We get to my friends house, turn off the car, restart the car, and now the car decides to shift correctly. Go figure. So we drive the car the rest of the way home.

This morning, we rent a nice shiny Chevy Cavalier from the neighborhood rental car place so my wife can have some piece of mind driving a reliable car for a week while I try to investigate more into what's wrong with the car.

I'm going to try some of the suggestions about the reversed CANBUS wires and the seat controller tonite. If that doesn't work, I'm going to drag an oscilloscope out to the car to spy on the CANBUS to see what I can see. I ordered a W203 Technical Information Manual for the car from europe which should get here in a couple of weeks. I don't know how much info will really be in the manual since it's not a factory service manual but any information should help.

I checked out the link for the www.startekinfo.com and it looks like it has all the info I will need to diagnose the car. I think I am going to hold off using it until I run out of other options because I feel like I really need to figure out what questions I need answers to before I give them my $18 for 24 hours to look for answers.

Lawrence

 

Lawerence,

If you have not modified the car in anyway, then the wires would not be reversed at all. The 2 connectors are eaisly available at the dealer. The Part number is on the connector in teh following format. xxx-xxx-xx-xx Try unplugging the seat control modules first and see. Not the plugs behind the module. Those go to the motors themselves. The long connector when you look under the seat is the one. It should have power, ground, and CANBUS wires so you know

 

Woohoo! I unplugged the long connectors from both the passenger seat control unit and the drivers seat control unit and my current draw dropped down to 170milliamps which looks way way better then the 1700milliamps when I started. So this is good news. I lost the power seats and power steering wheel, but at least the car won't be draining the battery anymore.

The long connector on the passenger seat control unit only had the 2 CANBUS wires going into it. I hope that is normal. The long connector on the drivers side had the 2 CANBUS wires with some other wires going into it. If either the drivers side or passenger side control units are plugged into the CANBUS, then the car doesn't shut down correctly. Only if I unplug both units does the car shut down correctly. I don't think both units could be bad, so something else must be causing the power seat units to behave badly. The CANBUS wires don't look like they have been tampered with in the connectors. I have brown with red stripe on top, and just brown on the bottom for both connectors.

Something that I think may be really important that I have neglected to mention is that the car has an SRS malfunction. The car has always had an intermittent SRS malfunction since we bought it and it's been to the dealer many many times to try to get it fixed under warranty. The dealer tried their best to fix it, but since it was intermittent and never came on when the car was in the dealer's possession, there really wasn't much that they could do. You can't fix something if it's not broken when you have it. The SRS light is on all the time now. I can't be certain, but it may have come on all the time about the same time as the battery drain problem started.

Is the SRS system somehow connected to the power seat control units? I can think of a couple reasons why they might be connected, but not sure if any are valid on a W203. Maybe the power seat control unit controls the seatbelt switch which the SRS system needs to know about? Maybe the power seats need to brace for impact if the SRS system detects a collision?

I'm hoping that the power seat controller problems stem from the root problem of the SRS malfunction which should be pinpointable since it is a malfunction which the car's ECU can see. Thanks again everyone for all the info you have provided, it's helped an amazing amount in my understanding of how the car's interior systems work.

Lawrence

 

" You can't fix something if it's not broken when you have it." Wrong thought process. Stop thinking a piece of metal with an obvious break in it. The same thought process that conjured up the complexity can cure it. You make it sound like it's the owner's fault for having an intermittent problem. If something is built in such a manner that it cannot be repaired then the job wasn't completed. If you have a record of a failure under warranty, it wasn't fixed, and the warranty expired....take it back and they will honor the original warranty. Intermittent problems are not unsolveable...they just may take longer.

 

I hope your battery drain is cured with the connectors unplugged. SRS has nothing to do with the seat control modiles. Check to see if you have a bad connection. the yellow plugs are all SRS related under the seat.

 

It's been about 3 weeks since I unplugged the power seat controllers from both the drivers side seat and passenger side seat and the car's battery drain has stopped. I've even left the car sitting at the airport for 2 of those 3 weeks and the battery survived.

I'm thinking that I may have only stopped a symptom of the battery drain problem by unplugging the power seat controllers and have not actually found the cause yet. It seems improbable to me that both the drivers side and passenger side controllers would fail at the same time. If one or the other had failed, I could see replacing the controller, but to have both controllers suddenly exhibit the same problem leads me to think that there is a root problem somewhere else on the CANBUS network causing the seat controllers to misbehave.

Lawrence

 

For the past 2 weeks, my 2001 C320 brand new battery keeps dying after 24 hours and the SRS malfunction indicator is on.

I took it to the Benz service dept in San Fran and they told me the battery drain was due to a short in my seat control module and the SRS wiring needed to be repaired. They have to order a new control unit and the total estimated cost came out to $1,100.

I'm because the warranty expired in December, and this sounds like a common problem for many W203s.

I'm going to have them repair the SRS wiring and uplug the power seats which comes out to $450.

 

Hackman,

I have had the exact same battery drain/ SRS problem on my 2001 C320 (see reply to your post). I want to unplug my power seat unit(s). Could you tell me where is the to start?

Thanks

 

Hi, I have 2002 C240, I am having same problem with battery drain. It started few weeks after my last major tune up. One thing I noticed is that the radio stays on even the key is off, and the battery will be totally drained just after 1 day. If this is happens so often, maybe MB should be fixing all of our cars for free?

 

most common culprit on 203s is the seat control module, BUT that doesnt sound like it in this case, like the guy above said, this problem is very time consuming, and can be expensive, best way I have found is forget the fuses, find the canbus and start unpluging can connectors, watch the draw till it goes down, then run a short test and see what control module or modules have no communication on the can bus, then you have narrowed it down considerably, and usually throwing different parts at the car is a last resort, due to the fact that most control modules must be married to the vehicle and cannot be reused......I personally avoid that unless its neccesary

 

Most modules are NOT married. Instrument Cluster, EIS and ME, airbag/teleaid are the only married modules. All other electronics are not such as door control, seat control, audio related, phone related, etc. The seat control modules ARE the most common problems on these cars causing battery problems. That's probably why the part numbers have been superseeded so many times. I believe they even changed manufacturers of these modules. The modules are totally different for 2005 and later cars including connectors. I presume this problem does not exist on these cars.

 

Hello guys. I am doing some research on battery drainage. My car has the same symptons that you all experience. I've already replace the battery which looks like it only last about 3-4 years anyway. Both of my seat modules are unplugged. I've already took the car to 2 independent MB specialist as well as the stealership (at least they give me a MB loaner). After a bunch of money in only Diagnosis & Inpections, my problem still persists. Some agree on the seat modules, others in an ESP ring on the steering wheel? (which I already replace), my rear SAM is blown and the rarest is the COMAND unit. As Hackman stated a while back ago, the Comand unit is NOT going to sleep when the car is turn off... I even noticed that the "power button" light is on all the time (kind of when you have the lights on and the interior iluminates). I can turn off and on the radio, play cds, do everything without the key. I explain this to my SA and he told me: "oh, we are going to have to replace the whole thing"... since I'm not under warranty, it is around $2,800... in other words, not an option. The frustrating part is the way he "solves" things... it's like there are no mechanics anymore, just people that replace stuff for new ones and hope that it gets the job done. Do you call that a technician!? I explained this to my SA which he is pretty cool btw... and he looked at me in a shady way and gave me a phone number of a indepedent radio shop... he told me not to send him emails or phone calls about this since it's not stealership related. I'm going to give it a try... Is there anything in particular I need to check before taking the whole unit out?

I've master to jump start the car in 2 minutes every morning with my other car. Our cars are great machines with lots of high tech goodies but it can get really frustrating.

I enjoy this forum everyday and I'll appreciate if you guys have any feedback on this.

 

First unplug the command unit to see if that stops the battery drain problem. If not, I remember seeing a very detailed post on this forum (?) on how to diagnose the problem by measuring the drain with a multimeter and unplugging plugs/systems until the drain goes away and narrowing it down from there.

 

I think I need a special set of tools to remove the comand unit... do you know of any other way? Thanks.

 

I've been having the same battery drain problem with my 2002
C240. I'm out of the manufacturers warranty, but I have an extended, which covers electrical problems such as this. Problem is they need my car for possibly days in order to duplicate this intermittant problem. Can't afford to pay for a rental while they trouble shoot, order parts, etc...
Will the dealership give me a loaner after the original warranty expires? I have a current extended warranty with another company?

 

My 2002 C240 driver side seat control module went last year. The usual battery drain that everyone on this site talks about. Guess what? Battery drain again last week and now MB says its the passenger side seat control module. Plus they say the overhead control module is defective too.

Is anyone aware if there is a class action suit on the seat control module issue? I think MB should be held accountable for this very common problem in their year 2000's cars.

MB is making a killing ($1000+) per seat control module replacement. What a ripoff.

 

I have a 2001 C240, replaced both modules, and now dealer claims it is my instrument cluster. When disconnected, it does not drain the battery. For now, looking for a replacement C240 cluster. I am with anyone suing MB for these issues.