Crank Position Sensor Function?
 

Hello,
What is the function of the CPS? Is it used only during cranking, or during driving, or both, and is it the same as a Reference Sensor? Thanks.

It tells the engine computer where crank is in it's rotation. There is usually a gap, or an additional position, for cyl 1 TDC.

On older cars, such as my W126, it tells the EZK when to fire.

I believe it is the same as the reference sensor. I am also questioning the proper name for this sensor. The parts store has them labeled as reference but the benz mechanic also explained to me it has the same functions even though its commonly called a reference sensor 86 300 E

The correct name is the reference sensor.

crankshaft position sensor
 

I have a 1991 sel 420. would a bad C.P.S. even allow the car to start when hot? I have no problems when cold but when hot, It will start for a couple seconds and then stall. when cooled down I can start no problem. I'm trying to find out if it is the CPS or something totally different.

The CPS will cause problems hot or cold I would think.
Your problem sounds like a coolant temperature sensor problem or fuel pressure issue.

The CPS can cause this hot-start/hard-no start problem, but a faulty fuel pump will do the same. I just went thru this with a 126 gas car. After 15-20 min or miles of driving, car would start bucking and shutdown. After an hr. or so of cooling off, it would restart and drive for 4-5 miles, then do the same again. 15 yr. old fuel pump had given up the ghost. Just another possibility.

C.p.s ?
 

I get up in the morning and I start my car no problems at all, I can drive for hours and have no problems but as soon as I turn it off and go in a store and come back the car hesitates to start. I crank it on it starts for 5 seconds then it shuts down, after about the third or fourth try the car stays on. Sometimes I get it started and take off 10 seconds later my car shuts down with lights all on the instrument panel. My car never idles different through out the day. Its so weired!?!. I was thinking that it may be a fuel check valve and fuel accumalator but replaced both check valves and the accumalator and nothing different. Also I can here the fuel pumps working when hot. Thanks for reading. I'm thinking it may be the cps?

lintaro : what vehicle/engine do you have?

420
 

I have a 420 SEL. 1991 126