Mercedes: Air condition, scotch brite pads, aircondition system

Question
QUESTION: I have a 1993 E300 250d with an erratic aircondition system. The freon is OK, but the aircon stops working when it rains or if I have just washed the car. It also stops working when the car has been in the sun a long while. I have checked everything that I can including any dampness getting into the electric panels under behind the battery under the bonnet on the right hand side. I can only conclude that it is the sensor (thermostat??) under the air inlet below the windscreen wiper, since the fault does not appear if I do not wet that area when washing the car.
My questions are:
Is that sensor a thermostat?
Could that stop the a/con when damp (if faulty)?
Could that stop the a/con when in the sun.(if faulty)?
What freon does my car use?
Thank you so much for your help

ANSWER: Patrick,
Sounds like a high resistance short. Some thing is getting wet and ground out the system.
Corroded terminals or bad insulation could by your problem. The change in humidity and or temperature could certainly be at fault.
Terminals may look OK visually, but terminals must be cleaned to remove corrosion. I would pull connections and polish with scotch brite pads. Bright tight connections are what you are looking for.
IF your AC has R134A there will be large charging connections. R12 will not fit. If you have R12, you will require the services of a licensed AC mechanic, as they must recover and clean any refrigerant before servicing. All leaks must be repaired prior to recharging. Placing a charging hose on the worn pressure side, can cause serious injury.

Good Luck
Robert


---------- FOLLOW-UP ----------

QUESTION: Hi Robert

Thanks for your help. I have gauges and can get gas. What is the procedure for charging a car system. I do my fridge systems in my restaurant, so the procedure is probably similar, only I would need to know what pressure the system should run at. I would put a/con on full cold with the fan on maximum and put the gauge on the low pressure side. Am I correct?

Thanks again

Answer
Windows up AC on high COOL. The 134A is added to the low pressure side, as a gas, not a liquid, so keep the can upright. If liquid is introduced then it could ruin the valves in the compressor.
I am not familiar enough with your system to tell you what the gauge pressures should be.
You should have about a 10 to 15 degree drop below ambient at the dash blower ports.
Too much and it will stop cooling because there is not enough room for expansion.
Good Luck
Rob