MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
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In this guide (5 sections)
Recently, we have been working on a Mercedes-Benz with some major and unexplainable electrical issues. The problem started with the Keyless Go feature. The key would unlock the car via the key fob and start by inserting it in the ignition. But Keyless Go stopped working. Pressing the door handle to unlock the car stopped working, and so did the push button start. Pressing unlock on the key fob still opened the doors, but the instrument cluster then displayed “Key not recognized.”
What the car was actually doing
Battery drain overnight
The next problem this car developed was the battery dying when the car was parked for an extended period or parked overnight. It was confirmed that no lights were left on. After further troubleshooting, we noticed a problem with the COMMAND system. Even though it would turn on, it wouldn’t go to sleep. This was confirmed after paying close attention to the head unit. You could still hear the fan working in the back, even though the radio/navigation was turned off.
A door open warning with every door shut
The next problem soon occurred: an unexplained door-open warning message on the instrument cluster. As you drive the car, the red warning light comes on, stating that the door is open. But all the doors were closed, and the car symbol did not show any door open.
Windshield wipers turning on in dry weather
The windshield wipers turned on a few times despite no rain. Mercedes has the rain sensor mounted up by the rearview mirror. The sensor can technically fail, but something else is happening here, as you already know.
Climate control cutting out at random
Another issue this particular car developed was that the climate control would randomly stop working. Other times, the climate control would operate completely normally.
A battery warning light on a car that still starts
Next, the battery warning light came on. Yet the car still started without issues. When the battery warning light came on, several other problems were also experienced. This was not the same issue as with “electronics consumers being turned off.”
Park assist beeping at nothing
This particular Mercedes is equipped with park assist sensors. The system would start to beep as if you were approaching an object, yet nothing was near the car.
The engine would not shut off with the key removed
Last, and strangest of all, the engine would not turn off when the key was removed from the ignition. The instrument cluster lights went out as normal, but the engine kept running and still responded to the throttle. The video below shows it happening.
Removing the fuel pump relay turned off the car. It’s not something you want to deal with every time you park your car.
Read that list again and notice what it is not. It is not one fault. It is eight unrelated systems misbehaving on one car inside a few weeks, and that combination is actually good news, because several unrelated faults appearing together almost never means several faults.
When systems that share nothing functionally start failing together, they are sharing something physically, and on a Mercedes there are only three candidates worth your time:
- The power supply. A weak battery or a corroded ground strap starves everything at once. This goes first because it mimics every other cause on the list and costs nothing to rule out.
- The data bus. The control units talk to each other over CAN. One faulty module can flood that bus with nonsense, and every other module on it then misses the messages it needs. Park assist beeping at nothing and wipers sweeping in dry weather are not two faults, they are two modules acting on corrupted data.
- Water at one connector. Water does not need to reach eight modules. It needs to reach one, somewhere the drainage has failed, and the bus spreads the consequences for it.
So the useful question is never what is wrong with the wipers. It is what the wipers, the park assist and the climate control have in common on this particular car, and the answer is almost always a location or a wire.
Common Cause: Control Unit or SAM Module
Every time we start seeing random electrical issues, the first guess is to inspect the control units and SAM modules. You need a good OBD2 scanner that can read Mercedes-specific codes to perform this step. Something else to investigate is if there is a chance that any parts of the car have been exposed to water recently.
In this case, the first problem was with the Keyless Go system. First, we investigated the Keyless Go module in the trunk.
Surprisingly, we noticed water in the spare tire beside the Keyless control unit. Even though there wasn’t much water, it splashed on the Keyless Go module every time we accelerated.
Removing the plugs from the module showed that the contacts had started to corrode, and water was present at the connection.
This was all caused by the trunk drain plug being clogged. The plug is designed to allow any water to drain. The rubber plug can easily get clogged by dirt and trash.
One clogged rubber drain plug in the trunk floor caused every symptom on this car. That is worth generalizing, because the drains are the weak point on these, not the modules.
Water collects at the lowest point it can reach, and Mercedes put control units in several of those points:
- The spare wheel well and trunk floor. On many models this is where the KEYLESS-GO control unit and the rear SAM live. The floor has rubber drain plugs that silt up with leaf litter and road dirt, and once they are blocked the well simply holds water.
- The front passenger footwell. Fed by blocked cowl drains, a blocked air conditioning condensate drain, or a torn door membrane. Fuse and relay panels sit low here on several models.
- Under the rear seat. Another common module location, and another place water reaches once the drains above it stop working.
- Sunroof drain hoses. They run down inside the A pillars and discharge low on the car. When one blocks or splits, the water goes into the headliner and down inside the cabin rather than onto the road.
Lift the trunk floor and the spare wheel, press a hand into the carpet in both footwells, and smell for damp. A corroded connector is often bone dry by the time you find it, so look for green or white powder on the pins rather than for standing water.
Removing the Keyless Go module resolved all the mystery issues we were experiencing, except the Keyless Go. That outcome is itself a test worth remembering. If unplugging one control unit makes a list of unrelated faults disappear, that unit was the thing corrupting the bus, and you have just found your fault without a single part being bought. Only do it with a module whose loss you can live with for a few miles, keep the car off public roads if anything safety related goes down with it, and do not leave a module unplugged as a permanent fix, because the car will log faults for the missing unit and some systems will refuse to work at all.
You can try to salvage the old module by drying it, but there is no guarantee it will work.
What this case actually teaches you
Every time you experience malfunctions simultaneously, there is a good chance that a SAM module or control unit is defective. These units don’t fail for no reason. They fail if water or condensation reaches the module or the electrical connections.
Another item you want to check out is the car battery. If the battery is not charged, it can cause several problems, such as the airbag light on, the car being stuck in limp mode, and random warning lights.
What a scan tool is worth on a fault like this
This is the one job where a cheap code reader is genuinely useless, and it is worth being clear about why. A basic OBD2 reader talks to the engine control unit and nothing else. Every symptom on the list above lives somewhere else: the SAM modules, the KEYLESS-GO control unit, COMAND, the park assist module. Plug a generic reader into a car behaving like this and you will very often get a clean result, which is the worst possible answer because it is wrong.
What you need is a tool that scans ALL control units and lists which ones report faults. The pattern of that list is the diagnosis. Faults scattered across modules that happen to sit near each other points at water or a shared ground. Faults across modules that share a bus but not a location points at one module corrupting the bus. Faults in one module alone, with everything else clean, is just that module.
The YOUCANIC UCAN-II Pro runs a full system scan across the Mercedes control units rather than only the engine, so you get that list instead of a single generic code. It covers nearly everything Xentry does apart from online VIN coding, which on a fault like this is the difference between seeing the pattern and guessing at it.
This is where the money goes on these faults. A water damaged SAM, KEYLESS-GO, COMAND or ignition switch bought new from a dealer has to be coded and matched to your car, and on the key and immobiliser side that means the keys too. The part is expensive and the programming is on top.
Module Medics repair the original unit at board level, clean up corroded tracks and connector pins, replace what has failed, and send your own module back. Because it is your module, the car keeps its existing coding and the keys keep working, so there is no programming appointment at the end of it. They handle SAM modules, EIS and ignition switches, key copying and FBS3 work, COMAND, instrument clusters, amplifiers and engine control units.
Fix the water first, though. A repaired module put back under a leak will corrode again.
Related reading
Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.
Hi How do I register?
Also I have a NZ new 1998 E430 (wdb210070) intermittently the front electric windows,door mirrors and electric seat adjusters would stop working and yesterday remote for unlocking and locking,indicators and
windscreen washers/wipers stopped working.Also front interior light not working (rear light works) and cruise control stopped working.
I have checked fuses,computer ans SAM connections – seem to be OK.
Has anyone experienced this problem?
I have scanned with a Launch x41 scanner – confused so many fault codes unsure where to start.
2007 R 350 4matic , windshield replaced by mercedes repair shop. coincidence Im not sure?. the navigation has not worked since. intermittent, and rare to tap navi or map with the enter address screen, 9 out of ten times the screen says ( CAN NOT READ NAVIGATION DVD ). please help. , F Y I< I have lightly dusted the inner compartment by using a can of air duster . tried this 4 times over two months, no improvement… removed command centre from dash and reconnected the harnesses ensuring a snug and complete connection. command reset using the sequence from steering wheel and instrument console ( R ) three second … no improvement.
https://www.mercedesmedic.com/
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