Tach not working
 

I have a 1995 300E diesel and the tach quit working. Just wondering how do I check and see if it is in the wiring harness or the sensor itself. Sometimes it will work when the car is reved up and then it suddenly
goes back to zero. It usually does this when everything is cold. (First started)
Thanks!
Larry L Larkin

does the 606 use a sensor on the flywheel, or the crank?

I think the '95 has an Over-voltage Protection relay like the rest of the W124s so I would check that first. If the tach is not working, the air conditioning and cruise control may also not work. I realize it's the wrong time of year for a/c but you might see if the cruise works. I should know from my own 606 where the tach sensor is but I don't and the car is still at the body shop so I can't go look.

Jeremy5848
Thanks for the information. You are correct and I have found the probllem.
It seems to be the connection between the sensor wire and the over-voltage
relay. I bought a new sensor wire but it did not help matters, but when I started to "jiggle" the wires to the over-voltage relay it started to work. Looks as if the over-voltage relay has been replaced at some point because it was not fastened down. I secured it with wire ties and now I think my tach will work. Thanks again!!
Larry Larkin

My intermittent tach on my '95 was the OVP relay. If yours comes and goes, consider replacing it. Very common......

Jimmy is right. Why OVP fails as much as we seem to read about I do not understand. There's hardly anything in there -- a relay, a Zener diode, and a fuse. If anyone would like to send me their dead OVPs, I will find out what failed. It would be an interesting project and if I can make a repair, the OVP goes back to the donor at no charge. It would be worth it to develop an understanding of the failure modes.

Jeremy