Hi I have a 04 S320CDI LWB. About three weeks ago the car got stuck in park, the rear brake lights stayed on and the key would not come out of the ignition. The car was then towed to my nearest Mercedes Dealer. They now have the car for 3 weeks and cannot fix the fault. Any suggestions would be very welcome. Thanks

 

If th dealer reads the On Board Diagnostic codes (with appropriate equipment) it should provide more information re cause of problem.

Likely the Brake Light Switch defective since this inputs to the Brake (lights), the safety catch on the Auto gear lever as well as ABS, ASR, EPS, Cruise etc. The switch has more than one circuit. It appears that one of these remains permanently closed when it should normally be open.

If this isn't the cause then you are into CAN issues which are complex to diagnose and explain. However the rear SAM (Signal Acquisition Module) MIGHT also be contributing to the issue.

3 Weeks! What are they doing. Sure you can trust them? If they haven't yet read the codes I assume your "garage" is NOT an MB / BMW / Audi etc. dealer

Stuart

 

Hi , I called into themm this morning and they say that they have found an issue where there is no earth going to the rear lights and that is why they are on they reckon a cable is broken and they are now tracing back the wiring.?Rgds Enda

 

Hi,

The absence of an earth wire on the rear cluster can cause all sorts of issues as the circuit tries to operate.

In the "old days" (pre digital) the usual symptom is indicators coming on when brake lights operated BUT bulbs will illuminate at only half power since they have to go to earth through another bulb circuit due to absence of the proper earth circuit

However, I can't understand how the absence of an earth could cause the brake lights to be on at full power. It is more likely that you have a CAN problem

The rear clusters are controlled through a unit known as the PSE. These give trouble.

Do you have any other symptoms...even if you consider them unrelated. e.g. Interior light operation faulty. Bulb out warning light ON etc.

The rear brake lights are controlled digitally thru the PSE via the CAN (digital network) and unless your garage understand digital circuits they are unlikely to solve the problem..but could cause more problems!

Sounds strange to me but I'm most concerned for you because of the 3 weeks!!

Keep us posted

Stuart

 

Hi Stuart, Really appreciate the response, got a call from them yesterday saying that they found the fault and it was a short in the fuse box? Collecting the car today, probably with a large bill! Thanks again Enda