How to adjust idle - 1994 Mercedes-Benz C-Class

You cannot the ecu does it i have a 1994 c180 if it ticking over to fast or slow check spark plug gap and air intake valve if this is faulty it will idel iradically

Yes generally you cannot and the ecu controls it! BUT it is still possible and you can trick the computer. First the simplest; why is it there are 2 cable adjuster, 1 located at the throttle and the other 1 on the gas pedal? You can adjust the one located at the throttle to open up a bit the butterfly valve and introduce more air, thus higher idling speed (courtesy of MAF) but it'll affect the fuel economy. Second is complicated; by disabling the wire of the idling switch, disabling the wire on the secondary throttle position sensor and also disabling the wire to the motor of actuator valve. Now you can adjust the idling and the fuel trim/flow to your liking, manually high or low with a adjusting screw inside the throttle body. But I must advice you that you understand about tuning do this at your own risk. Your system can go to open loop always. This method is prior to replacing a defective throttle and extending its service life while waiting for a new replacement.