An Interesting Mercedes Parasitic Draw

Keith Diagnostician Oklahoma Posted   Latest   Edited  
Case Study
Trained By Techs
Network Communications
2007 Mercedes-Benz C230 Sport 2.5L (272.920 Flex) 7-spd (722.9) — WDBRF52H57F933542
Parasitic Draw
Dead Battery
Dead Overnight

The vehicle in question is a: 2007 Mercedes Benz C 230 (W203 Chassis)


The battery is dead if the vehicle sets overnight.


After a full network sweep yielded nothing to write home to mom about, the trunk, hood, and driver’s door latches were closed to simulate a secured vehicle for accurate testing to allow access to the vehicles interior, trunk, and under-hood SAMs (Signal Acquisition Module) (fuse box) (N10/1) for testing. 

A total system current draw of approximately 1324 milliamps was measured with a current clamp around the negative cable at the battery after more than 35 minutes with doors “shut”, car locked with the fobik, and vehicle left undisturbed.

Although I did not look for the exact specification for acceptable draw, we can all assume that this is excessive, and would be the cause of the dead battery concern.


The Mercedes in question has been to quite a few local shops before I was called to look at it. Unfortunately, the other shops had it for months and gave no documentation to the current shop about previous repairs, yet the customer has expressed they have paid for many repairs that did not fix the problem.


I voltage drop tested all fuses, I found multiple fuses, approximately nine, in all three SAM’s to have current readings adding up to approximately the total system draw. I read the current on all the battery cables coming out of the junction box (F32) by the battery, cables 1, 2, and 4 have current. Those three cables go to interior, trunk, and under-hood SAM units that house the suspect circuits with the measured draw on the fuses.


I started with searching wiring diagrams and service information to determine if there was a common component or module that was associated with all the affected circuits. 

With all three SAMs having current, my thought was it must be the network staying alive as it seems the most likely to have a multitude of devices drawing current, as the architecture of the circuits with draw are housed across multiple networks in the vehicle along with the devices that are drawing current are in-fact across multiple networks.


I removed all modules from the two CAN bus shunts(usually in the lower floor runner) , adding them back in one by one, I found that the WSS module, the Radio module, the SCM module, and the OCP module could all be added back to the network, and any one of them would bring the network back to life, regardless of the order they were reconnected, even if the others were disconnected from them. I had zero faith that all four modules had failed obviously.

During my research, I had planned on finding a bad seat module, just based on the network architecture. That was proven false obviously, I had to revisit my test plan and re-evaluate a direction. In the service information, I found what I was looking for.

The EIS (Electronic Ignition Switch) was the gateway for both networks, one of its functions is to be coded for which modules that are available for each chassis are actually equipped on the particular vehicle its installed in. The module uses this coding to send a message on each network to each module for it to "go to sleep". If the module is coded incorrectly, or lost its coding, it may not be aware of some module on the network, and never tell them to "go to sleep". 


The car was coded to only have a left front door module, not a right front or rear doors, it was not coded for a OCP, but it has one, it was coded for TELE-AID, which it does have, and so on. 

This was the issue, I used the Autoland Scientech iScan WT II to change the BUS coding. A short 14 minutes later, current draw was measured at the negative battery cable, with 41 ma....I was happy, again I did not check the specification, but its fixed in my book!

Chris Diagnostician
New York

awesome Keith. What do you think caused the original issue? Dead battery? Shoddy EIS?

Ð Awarded
Keith Instructor

I think it the EIS swapped form another car, and VIN wrote in to be correct, and then a failed attempt at coding was done...... 100% a guess based on what I found considering at least 3 shops were involved with it before I got to it, that I am aware of.

Ð Awarded
Michael Owner/Technician

Nice write up Keith! I didn't realize you were able to SCN code with a WT. Are the files on the tool or do you have to go online?

Ð Awarded
Keith Instructor

Only the bus coding was available. It was a simple "equipped" or "not equipped" option to assign to each available module to be adjusted in the Bus settings of the EIS.

Ð Awarded
Brian Diagnostician

Nice! Definitely an interesting find. Good stuff Keith

Ð Awarded
Raymond Diagnostician

excellent write up! on a side note, when you suspect a CAN component is the cause I find it more advantageous to measure voltage directly at the CAN X connector using MB#… Using this eliminates the need for disconnecting the battery and fuse etc. When can is dormant "asleep" you will measure 12v at this connector, awake you will likely see around .5v or so. Disconnecting your

Ð Awarded
Mario Diagnostician

Excellent write up Keith. This is one of those things that must be engrained in the back of our minds. Not everything is in plain sight. Well done.

Ð Awarded