95 Wagon A/C goes on randomly and erratically
 

For the last two months, the A/C on my 1995 Mercedes E 320 Wagon often does not come on until I have been driving anywhere from 15 minutes to 1 1/2 hours. I've taken it to the shop and they say that it's probably in need of a new key switch, but that makes NO sense to me at all and I don't want to take it back just so they can charge me to try this theory out. There is nothing I can do--no amount of button pushing, key jiggling, etc. that will make the AC come on when it's off, and no logical reason as to why it does eventually switch on when it does come on. And, along with this, tremendously hot heat constantly pumps into the car through all of the vents if I have any of the A/C buttons pushed in when it is not working (the worst being the defrost vents, which I can't shut). It has been in the 90's here for the last few days, and life is miserable behind the wheel. Any suggestions??

Well, it depends. How mechanically inclined are you? There is mucho information on how to debug the 124 climate control system in the archives here. Use the search function, a few good keywords, and settle in with a cup of java...

Frankly, it sounds like you have multiple failures. The hot air problem sounds like the climate control unit shutting down to protect itself. My first suggestion is to unplug the auxillary hot water pump (hint: it's under the washer fluid reservoir) and see what happens.

- JimY