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 keyfob and start it by inserting the key in the ignition. But Keyless Go stopped working. Push Start or unlocking the car by pressing driver’s door handle stopped working. If you pressed the unlock button on the door handle the instrument cluster would state “Key not recognized”.
The next problem that this car developed was the battery dying when the car was parked for an extended period of time or parked overnight. It was confirmed that no lights were left on. After further troubleshooting, we were able to notice 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. In the back, you could still hear the fan working even though the radio/navigation were turned off.
Door Open and Brake Light
The next problem that soon occurred was an unexplained door open warning message on the instrument cluster. As you would drive the car the red warning light will come on stating door open. But all the doors were closed and the car symbol would not show any door actually open.
Windshield Wipers Turn On Randomly
A few times the windshield sippers turned on even though there was no rain at all. Mercedes have the rain sensor mounted up by the rearview mirror. The sensor can technically fail but as you are already noticing there is something else major going on here.
Climate Control Stopped Working
Another issue that this particular car developed was the climate control would randomly stop working. Other times the climate control would operate completely normal.
Battery Warning Light
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 experienced as well. This was not the same issue as with “electronics consumers being turned off”
Park Assist Random Warning
This particular Mercedes is equipped with park assist sensors. The system would start to beep as if you were approaching an object yet there was nothing near the car.
Engine Would not turn off
Last but not least, the issue with this Mercedes was the engine not turning off when the key was removed from the ignition. While the instrument cluster lights would turn off once you removed the key, the engine would still run. The car would even still respond to the gas pedal input. You may find it hard to believe but here is a video showing Mercedes that wouldn’t shut off.
Removing the fuel pump relay turned off the car. Not something you want to deal with every time you park your car.
Common Cause: Control Unit or SAM Module
Every time we start seeing random electrical issues the first guess is to inspect control units and SAM modules. To perform this step you need a good OBD2 scanner that can read Mercedes specific codes. 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 that was experienced was with the Keyless Go system. First, we investigate the Keyless Go module which is in the trunk.
To our surprise, we notice water in the spare tire well, next to the Keyless Go control unit. Even though there wasn’t that much water, every time you accelerate the water was splashing on the Keyless Go module.
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 because the trunk drain plug was clogged. The plug is designed to allow any water to drain. The rubber plug can easily get clogged by dirt and trash.
In this case removing the Keyless Go module resolved all the mystery issues we were experiencing with the exception of the Keyless Go.
You can try to salvage the old module by drying it but there is no guarantee it will work.
Every time you start to experience many malfunctions at the same time, there is a good chance a SAM module or control unit is defective. These units don’t fail for no reason. Typically they fail if water or condensation reaches the module or the electrical connects.
Another item you want to check out is the car battery. If the battery is not holding a charge it can cause several problems such as airbag light on, car stuck in limp mode and random warning lights.