A/C and tacho problems

[Redfar]

I have a problem with aircondition on my 190 2.5 diesel,when I start the engine the a/c starts normally but then it cuts down and never engage again.I took the car to a repair shop and he told me that he have to do an other relay with the klima relay.He done the relay and the a/c semms that it's working fine now.
Do some one here knows about it or is someone having the same problem?

The a/c problem looked solved with the relay that he done for me but now I have a problem with the revcounter(tacho),the tacho seems worked when ever he likes and some times goes up to 2 or 3 and down again,when I rev the engine.Is the relay not letting the revs signal passsing to the tacho,or there is an other thing that I might look at.

Thanks