W204 is the third generation C-Class, 2007-2014. This page documents specifically the US 2010 C300 4Matic (2009-2011).
X204 is a crossover SUV, 2008-1014, equivalent to the C-Class, sharing the W204 platform. We document the US 2010 GLK350 4Matic (2008-2012) V6-3.5L.
Fuse Diagrams
The fuse allocation diagram applicable to your car should be located in the repair tool compartment on top of the spare tire in the trunk.
W204 Fuse Allocation
Note that the US model does not have the in-cabin fuse box.
X204 Fuse Allocation
Troubleshooting
Engine cranks but won’t start (GLK)
After leaving a 2010 GLK350 car unused for about two months, the car would crank but not start, even after fully charging the battery. The fuel pump under the rear seat was audible (after removing the seat.) Only after removing the engine air filter the engine finally started. The ODB showed multiple/random misfire error code P0300 and while running the car, the live data SPARKADV degree display showed a wide range of values over time between 0 and 20 degrees. After replacing the oil and filter that improved significantly to only a few degrees. Suspecting a somehow clogged oil filter and air filters making things worse. Planning to replace the engine air filter and spark plugs as a precaution.
Low Fuel Pressure and Replacement of fuel pump and/or filter
Fuel pressure can be measured under the hood using a tool. When the filter is clogged you might need to replace the fuel filter. When the fuel pump is not running or not running properly you might need to replace it.
In some states such as California some models can have the fuel filter and pump integrated in the tank, requiring replacement of the entire tank. In standard models, however, the fuel filter and pump are accessible through two lids beneath the rear passenger seat.
Ensure the fuel level is below 1/2 at a minimum (probably) and below 1/4 even better, to avoid spills, because you will be opening the tank from the top. The filter is on the driver’s side of the vehicle, the pump on the passenger’s. The fuel pump is PZEV or Delphi FE0727. Generic units can be significantly cheaper than OEM.
To remove the rear seat, pull it up in the front, then lifted slightly push towards the back, and pull it out up forwards. With seat removed you should hear a working pump running when trying to start the vehicle. Detach the battery before opening the lids and also review some videos online.
Front wiper and washer pump not running (GLK)
None of the relays ware bad in my case. So as a final attempt I tried to remove and reinsert all relays in the engine fuse box, resetting the SAM control unit in the process. All that somehow solved the problem.
updated: 20240416, 20240420