AC system evacuated, compressor won't kick in
 

1998 E320
###
I installed a new drier and evacuated the system with a 2.4 cfm pump, it went down to 24 cfm and held a vacuum for one hour. I started the car, cranked the AC, and released one can of 134a 12.oz and the compressor didn't turn on.
The 'low side' on the gauge is reading 110 and the high side (closed ) is reading zero. Shouldn't the compressor kick on?

usaims

I don't know about your specific car, but typically there is a low pressure and high pressure sensor. If the pressure is too low or too high, it will not allow the compressor to kick on.

Those sensors on many cars are on the high side.

I am thinking that with compressor off pressure should be same on hi and Lo side.

I think one can should give enough pressure to turn the compressor on. Both high side and low side will be the same pressure. Sounds like you did not open the high side gauge. Have you checked to be sure the EC button on your display is NOT lit up.

In my experience, one can is sometimes not enough to get the compressor to kick on. Also, Mercedes tend to fill very slowly for some reason. I'd try another can and if it doesn't kick on then, I'd manually kick the compressor on.

It should be noted that if weather is mild, that will make the car fill slower. You can also put the can in a bucket of hot or warm water to speed things up.

edit: 110 sounds high for low side pressure. This sounds stupid but are you sure that the fill valve is not stuck? I have had that happen before.

After a while when the engine is off the low and high sides equal each other.

On my W201, after evacuating the system I short the high pressure switch at the drier to turn the compressor on to charge the ac.

I don't get your meaning. "short the high pressure switch at the drier"?

I'm finding that while this is a simple thing to do (for us experienced folks), is not so easy to explain!

I'll give you several links so you can read up on what you may not know about this...

Drier...
Three important functions of the receiver/drier in your car’s A/C system | Mobile Air Conditioning Society (MACS) Worldwide

Your Car's Air Conditioning Components

https://www.youtube.com/watch?v=KmpybbtxQmM

How A/C systems work...
https://www.youtube.com/watch?v=w17DpGCcRj8

Pressure switches...
https://www.youtube.com/watch?v=13HcuJ3030g

A/C Pressures...
https://www.youtube.com/watch?v=i9yv0euT7xA

How electrical switches work...
Basic Electricity Tutorial - Switches

Then "short" means to bypass the electrical pressure switch. This would be done by making an electrical connection between the wires going to the switch. BUT BE DARN SURE YOU KNOW THE CORRECT WIRES AND KNOW THEY GO TO A SWITCH AND IT IS THE CORRECT SWITCH!

I took the car to a MB tech. He hooked it up to the DAS system. It indicated a faulty clutch on the compressor -- basically I need a new compressor. He gave me a print out of my car which was nice, tested all the electrical connections, and lifted the car to check for power at the compressor. I bought a brand new compressor for $232.00 which I will install Sat.