[kwikdave]

I need help identifying a new noise. Occasionally, when I start my car in the cold, an intermittent "clunking" sound comes from under the dash. It sounds like a gear or motor is trying to turn and is sticking and then suddenly slipping or releasing. I have tried pressing the defroster and recirculate buttons and have turned the air diverter knobs to multiple positions. The noise usually stops after a few seconds but I would like to know if there is something I should be concerned about. Thanks, kwikdave

 

[Nico Nicolas]

I need help identifying a new noise. Occasionally, when I start my car in the cold, an intermittent "clunking" sound comes from under the dash. It sounds like a gear or motor is trying to turn and is sticking and then suddenly slipping or releasing. I have tried pressing the defroster and recirculate buttons and have turned the air diverter knobs to multiple positions. The noise usually stops after a few seconds but I would like to know if there is something I should be concerned about. Thanks, kwikdave

I think is the step motor of your A/C. I have the same problem with my benz. There is a DIY guide somewere in forum how to fix it.

 

[rudeney]

Yes, it's likely a bad servo or broken linkage in the duct. There is a test/reset procedure for the ACC system that might help. With the engine running, simultaneously press and hold the RECIRC and FRONT DEFOG buttons until their indicator lights begin the blink. Release the buttons and the lights will continue to blink for 20-30 seconds while the system tests all the duct actuators. If the lights stop, then it was able to re-sync and things may be fine. If not, then you have a problem that needs proper diagnosis.

 

[kwikdave]

Rodney, once again you are spot-on. I tried the reset procedure and the system clunked and clanked for about 3-4 minutes and then settled down. I tried it again ten minutes later and no noise at all. I'm keeping my fingers crossed that this solved the problem.

On a different topic, my high mounted third stop light in the trunk lid has stopped working and I get the error message in my instrument cluster. I checked the connection at the light bar itself and it seemed in order. Are there any other places I should check for loose wiring or any tests I can do to determine whether the LED bar is bad? Thanks for your expert advice. Kwikdave

 

[rudeney]

Is the brake light cracked? That's a common failure.

 

[kwikdave]

No, the original light did break but was replaced by the dealer under warranty. The new design seems to be designed better to absorb the shock of closing the trunk lid. Strangely enough, after I removed the trunk lid liner and checked the wiring, the darn thing started working again. Maybe it was just a loose wire. Hope so! Dave

 

[kwikdave]

Rodney, the "clunking" noise stopped for a while but came back the next time I pressed the front defrost button. It seems to be coming from the servo actuating the driver's side defrost vent. Is there some way to get at this to check it out? Thanks, Dave

 

[rudeney]

You really need to know which actuator or servo is bad. To do this, you need to get codes pulled from the ACC system. You can't do that with a generic OBDII scanner - you'll need SDS or a 3rd party scanner that goes beyond engine codes and can read MBZ-specific systems. Most of these servos and actuators are a major ordeal to replace - you have to remove the entire dash, and sometimes even the evaporator and heater core.

 

[kwikdave]

Thanks Rodney, but that is way above my pay grade. The defroster seems to be working ok so I guess I'll just have to put up with an occasional "clunk". Dave