Which light are you suggesting is the crankshaft sensor light? I have just replaced the sensor on my 1993 500sl. The main symptoms are won't start when the sensor gets hot, runs well when driving (as the sensor get ventilated and kept cool) when the problem kicks the car doesn't run smooth and sometimes will cut out altogether if you stop driving. If you have these syptoms then it's defo the crank sensor and it's not easy to change yourself. If you only have a problem with the idle then you may have a problem with the idle valve or perhaps the o2 sensor.