W140 Tachometer/ A/c Connection?
 

I have posted previously regarding my tachometer occasional dropping to zero. Seems like an electrical glitch because it does not effect the operation of the engine, other than the a/c. I have noticed the air conditioning only works when the tach is functioning. Is their a relay or switch that might be bad? ANy help is appreciated as I may have to go the dealer to fix it soon. (Need the A/C!)
Thanks

Hope I'm wrong but it maybe a wiring harness going bad.

Wiring Harness
 

Thta was the dealers first suggestion. The previous owner of this car had two wiring harnesses replaced to try to correct this problem with no improvement. I am hoping there is a relay or sensor that effects both the tach and a/c function. Thanks

It's probably a bad ground. I have the same problem as mine flashes on and off. I am pulling it this wekend to check it out.

I'm not sure if i am on the right track becuase what model is your car? A similar problem on a 124 would or could be the Kilma relay(just behind the battery) becuase that relay reads speeds from the engine(tach meter) and then from the ac and checks if they are the same, if not the ac compressor cuts out. So either the speed signal from the engine is not 100%(not sure why) or the Kilma could be faulty or loose. This isn't concrete advise but it might help.

Kilma Relay
 

My car is a 1992 300SD. I check for a kilma relay. Thanks

Engine speed sensor
 

My garage suggested replacing the engine speed sensor, as this effects both the a/c and tachometer. I'm having this done on Monday. I'll post the results.
Thanks

I had the same problem on my 1992 300SE and it was the instrument cluster. I was told that this was a problem and that MB even issued replacement printed circuit boards to solve it. The dealer was not going to change mine so I sent it to National Technical Director George Murphy who has a business repairing MB electrical items. His contact info is in the Star. Haven't had a reoccurence since then and it has been a couple of years since the repair.

Tim

Tim, who did the work on the cluster. I have the same problem. :confused: