cannot refill freon
 

My 87 300SDL is at the garage getting AC looked at. Mechanic states that freon was empty so he is trying to charge system. So he purges the system and added freon but it would only allow him to fill it to half. Compressor still has not started. And it is blowing out hot air from vents. He is going to put a hot wire from the battery to the compressor to see if he can get it to start. The CCU is another rebuild so I did not even pull the Klima to test to see if there was a signal coming from the CCU. But what is the issue related to the excess pressure where the mechanic cannot fill the sytem completely?
Thanks
A

I'm an automotive student so take this with a grain of salt as I'm new to MBs:

There might be a restriction somewhere that won't allow refrigerant to completely fill the system. The low pressure switch will keep the compressor clutch from engaging.

Thermostatic expansion valves/Fixed orifice tubes can become plugged if the dessicant bag in the receiver/dryer or accumulator ruptures or if your compressor fails and the system becomes contaminated with metal shavings.

When is the last time your reciever/dryer was replaced? Did the tech pull a vacuum of 29 inches/HG for at least 15 min to draw out the moisture from the dessicant and then leak test the system?

The tech should jump the low pressure switch to test your compressor clutch. I bet you have a major restriction somewhere.

Wouldn't a blocked expansion valve cause this?

I don't think its a big issue, when I charge A/C systems sometimes my recycle machine will stop about halfway through the recharge. For some reason the last half of the charge won't enter the car - If I start the car and turn on the A/C system and continue charging vapor through the low side the rest of the charge completes in a short time period. I'm not sure why this happens it seems to happen more than 50% of the time on MB cars, on other brands it happens much less frequently.

If the charge is below the threshhold the low pressure switch won't allow the compressor to start. You have to jump the low presure switch across the terminals at the accum/dryer

Yes.

Mechanic tried to put 12v directly to compressor but it will not start. I am not sure what you mean by jumping the low pressure switch across the terminals. Would yopu elaborate please?
Thanks

Drier/receiver was replaced last year

It doesn't matter if it was replaced last week. If the system was open to the atmosphere (leaked out charge or a hose off for more than 30 minutes) then it is moisture contaminated.

My car did this when I recharged it two years ago. eventually by watering down the condenser it took the rest of the charge, or maybe it was coincidence? Well anyway, try cooling the condenser off.

The low pressure switch is on the TOP? (please check manual) of the accum./drier and prevents the compressor from operating on low charge. Unplug the connectors and put a jumper wire across the leads. This bypasses the pressure switch and activates the compressor clutch. You should see it turn. I would think that if he was able to get half a charge in then it should be enough to operate the compressor. Not sure how he could get half a charge in anyway if the compressor wasn't working (assuming he was filling from the low side).

You might try removing the belt and then jumping out the compressor clutch and see if you can turn the compressor, if not it might be seized.

If you jump across the pressure switch on top of the evac (just unplug 2 leads and jump together), the comp should turn on. If not, you have comp problems. Could be wiring to comp, or comp could be seized. And I also dont think you could get 1/2 charge in with no comp spinning.

He said the mechanic jumped 12v directly to compressor and the clutch did not engage, that kind of rules out any issue with the low pressure switch doesn't it ? bad compressor ?

The compressor on my 300D would also not engage, it turned out that there was excessive clearance between the clutch and compressor...my mechanic used a tool to press the clutch on farther, and now it engages every time.