Where is engine speed sensor - m104 engine?

dordal · #1 07-22-2002, 08:17 PM

I'm trying to diagnose an idle speed problem (engine intermittently revs to about 1000rpm at idle), and the tech manual says one of the inputs to the computer is the 'engine speed sensor'. I've tested or replaced almost every other input, checked for vaccum leaks, etc. and my suspicion now falls on this sensor. However, I haven't a clue as to where I can find it, how it works, or how to test it.

Any help? The car is a '93 300e 2.8.

Mike Murrell · #2 07-22-2002, 11:57 PM

Check out what Benzmac had to say in this thread:

1993 300E 2.8 still ill !

dordal · #3 07-23-2002, 02:25 AM

Thanks. I saw that thread earlier, along with quite a few others on the idle problem. I haven't replaced the air mass sensor (since its a $450 part), but I don't believe it could be bad, because the car runs perfectly at everything but idle. Seems like there would be other problems... maybe I'm wrong?

I've already replaced the 02 sensor, OVP relay, wiring harness. I've checked for vaccum leaks and the coolant temp sensor.

Haven't replaced the throttle actuator ($900) but I don't suspect it because cruise control works fine - drove from LA to SF yesterday on cruise the whole way, without a hitch.

Right now, I'm suspecting the engine speed sensor, but I can't find much info about it in the tech manual or on the board.

#4 07-23-2002, 08:07 AM

I had the same problem on my 93 300E2.8 about a year and a half ago. At idle, it would vary between maybe 450 and 1200 rpm in cycles of maybe 10 to 15 seconds. Otherwise, the car drove great, so good you had a hard time believing something was wrong. The gas mileage was normal as well. In my case, the check engine light come on and the code said it was the air mass sensor. I replaced the air mass sensor--immediate cure. I bought it at Fast Lane and I think it was about $230 but Fast Lane says it is unavailable right now. If you could find another car and swap to troubleshoot, it is less than five minutes to change. As I understand, if you have the equipment, you should make some adjustment or change some setting after changing but it you do not, the engine's computer will finally adjust itself.