I have a 1991 500 SL some of you may remember a few weeks ago that had a very loud engine noise, Vid. link on youtube. It was discovered to be a collapsed lifter. Drove the car for a week or so with no problems till I went out and it would not start, No spark at the coils. Checking things over I found the insulation deteriorating off the knock sensors I separated the wires and it started. I realize this was a fluke as it has not started again, and the car should run with them unpluged as they have been since.. our local benz guy offeded up a crank sensor to try, it apeared used as I could see marking from the mounting screw, He assured me that he put it on a car recently and that was not the problem and the sensor was otherwise NEW . What I am asking is there a procedure for testing the crank sensor as well as the Ignition module and I really would like to be sure before purchasing either, crank sensor not a big deal but the ignition module..... IY, YI,IY!