- 11/19/2014
- 1 Min Read
- By: Bryan McPhail
Mercedes CL/S-Class Gas Pedal Replacement (00-02)
Cold weather can lead to strange problems with cars - in my case the gas pedal simply stopped working. In the late 90's many BMW and Mercedes models moved to electronic drive-by-wire gas pedals and throttles. Worthy of an honorable mention is the E32 BMW 750iL, which used drive-by-wire way back in 1988! Rather than the gas pedal pulling on a cable that mechanically opens the throttle, pedal movement is detected electronically via a potentiometer or sometimes a "Hall effect" setup. In turn the ECU instructs the electronic throttle motor to close and open as required. Usually there is another potentiometer on the throttle end to confirm the movement.
Pressing the pedal does nothing
Internet lore suggests cold weather can bring out failures in many Mercedes pedals as well as simply age and use. A common complaint is that pressing the pedal does nothing - but if you keep it held then after 3 or 4 seconds the engine will start to rev. This is a sign the ECU has detected a problem and gone into a safety mode. Rather than the pedal simply not putting out a signal, it's putting out a signal that the ECU detects as bad. For safety most drive-by-wire setups use two potentiometers - when both signals match all is well - if one does not then there is likely a problem based on the theory it's unlikely for both to fail at once and in the same way.
There are two electrical connectors
In my case an OBD reader reported code P0120 which is TPS A Circuit Malfunction, where TPS stands for throttle position sensor. Removing the pedal is straightforward - remove the plastic cap and then a single 10mm bolt. The pedal should then just pull out with some encouragement. There are two electrical connectors - the lower one for the kickdown switch, and the upper one for the position sensor. The sensor itself is not available separately as far as I know - instead you have to buy the whole pedal assembly. The part number will be printed on the side so just make sure you order the correct one as there some subtle differences across model years.
Fire up the engine
Reassembly is reverse of removal! Once installed, turn the ignition to position 1 and then make sure you have smooth and full pedal travel. On some models the ECU will 're-learn' the sensor range here, so hit the kickdown a few times as well. Then fire up the engine and hopefully you are good to go again!
This is an easy interior DIY even for non-mechanics that can save you a few hundred $ versus a dealer trip.
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About the Author: Bryan McPhail
Bryan is a longtime BMW enthusiast in Florida.
