I've had it at an Independent shop, they don't know what to do except to start replacing parts. Here’s what I know: The system has refrigerant pressure, that it might be as simple as to add some refrigerant, I know where the pressure switch, condenser, evaporator, dryer, and expansion valve are located, I know where the two fuses are located. I know that taking it to the stealer etc…..

What I don't know is how you trick the pressure switch into thinking that it's not low on r134 so that the clutch will engage and suck some r134 into the low end of the compressor.

Are there some other tricks that I might try before I start replacing expensive parts.

Are you sure you need to have the compressor engage to add refrigerant. I know when I used the over the counter cans on my last car it pressurized the system enough to kick it on.

In other model cars you somethimes have to jump the switch in order for it to engage the clutch.....that's what I'm asking ... if someone knows how to do this.

What rjs1222 is saying that the pressure supplied by the refrigerant container will give the pressure to make the compressor engage.

Thanks to everyone thus far..........I have hooked up my gauges to the system and the pressure from the can is not enough for the clutch to engage.... yes I rev'd the engine to overcome the low idle cutout thing.

pm Zulu95 or The Cleaner. They'll be able to give you more direction.

Here is what the manual says, kind of technical, but I'm not sure you can just jumper out the switch. It says the compressor will cutout if pressure is too high or too low. Good Luck.

Evaporator controlis activated by pressing the A/C button in the operating unit. A/C standby mode is indicated by the LED in the A/C button. If controlled compressors are installed, the cutout threshold is dependent on the outside temperature and may be between 1 °C and 3 °C. If a standard compressor is installed, the threshold is reduced by 1 °C at high outside temperatures (> 30°C). Disengagement of the compressor clutch is scarcely achieved by the mechanical control of the compressor output on the controlled compressor . The A/C compressor is controlled directly by the heating/air conditioning control unit and the auxiliary fan by the digital motor or diesel electronics (DME/DDE) . Requests for A/C compressor and auxiliary fan are sent via the CAN/K-bus to the DME/DDE. The pressure sensor also monitors the refrigerant pressure and switches the A/C compressor off if the pressure is too high or too low. A pressostat that checks the refrigerant pressure and, if necessary, interrupts the supply line to the compressor clutch is only installed on the M51 diesel engine.
Switching conditions for the magnetic clutch
ON:A/C button ON and evaporator temperature > 4°C (depending on the outside temperature) and coolant temperature < 117°C and terminal 30 at control unit > 9.7 Volt.
OFF: A/C button OFF or evaporator temperature < 3°C (depending on outside temperature) or blower stage 0 or coolant temperature > 120°C or terminal 15 OFF or terminal 30 < 9 Volt.
DME_AC idle speed increase
When the A/C button is pressed, the heating/air conditioning system requests an idle speed boost via the K-bus in order to provide adequate cooling capacity at idle speed.
DME_KO compressor activation
The DME receives the instruction to switch on the compressor via the K-bus . If no full load cutout is set a feedback signal enables the DMD/DDE to activate the compressor clutch. The load moment of the A/C compressor is derived from the pressure sensor signal and passed on via the K-bus to the DME.
Infinitely variable auxiliary fan control
The refrigerant pressure in the air conditioning is registered by means of a pressure sensor and converted by means of a table into an auxiliary fan stage which is then sent to the DME/DDE via the K-bus/CAN bus . From this, the electronic engine control system then generates the control voltage for the infinitely variable auxiliary fan motor .

You may have a faulty pressure sensor switch. From the discription above, the switch may not be a simple on/off type but a progressive resitance output so jumpering it may not work.

Update, I now believe it's the Aux Fan keeping the DME from allowing the signal to fire up the compressor. I need to order an aux fan does anyone have a part #? I can not find it on oemparts.com.

It's under Heating & AC -> Condenser AC -> Pusher Fan. Part# 64546921381
You might also want to check out the relay for that fan as well as it is prone to failure.