Mercedes: Intermittent A/C, AC, Freon

Question
QUESTION: The A/C in my 1987 Mercedes 300SDL stops after 10-15 minutes max, regardless of the outside temperature. If I stop the car and wait 2-3 min and restart it, it works again for a while. After I replaced the compressor, I went to a regular shop to have the freon added, and maybe the system has still a mix with air. Is there anything else I can check or replace ?
Thank you so much, Paul, El Dorado Hills,CA

ANSWER: Paul,
By stop... do you mean blower and compressor? Why are you completely stopping the car?
Is the blower continuing to blow air when you loose the cool?
When you replaced the compressor did you completely wash out the system before reconnecting the lines? Did you install an NEW receiver dryer at the time of compressor replacement?
When you had the FREON added( I would think you used R12)Yes? R134 Conversion? Did the shop pull a complete vacuum before recharging? How many pounds of Freon were added?
Is the compressor magnetic clutch engaging and disengaging rapidly when it is cooling?
When cooling is there a 20 degree drop at the outlet over the outside temperature?
Let me know?
Thank
Rob




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

QUESTION: Hi Rob
I'll try to respong in order. I stop the engine completely for 5 min and I start again. I found it's better to have the A/C turned on when I start the engine (I can hear the clutch engaging after 10-20 sec). No wash out was performed on the lines when I replaced the compressor, but a new dryer was replaced at the time too. It is converted to R134 from some years back and I don't believe a complete vacuum was done. I don't think I experienced ever after compressor replacement a 20 degree drop.

Answer
Paul,
How long has this replacement compressor been on the vehicle and how many miles are on the compressor?
A couple of issues may be at work here. You have lost freon, or and there is an obstruction in the evaporator.
I would have the vehicle serviced at an independent AC service center.
They should hook up their recovery machine and check for leaks.

During a component install, the proper amount and type of oil needs to be replaced. If the system was not completely washed of R12 oil and replaced with the proper amount of R134A oil, there can be problems because the oils are not compatible.

Anytime the system is opened a complete vacuum needs to be drawn on the system to boil off all moisture, before adding refrigerant to the system. Moisture in the lines can freeze during the evaporation cycle in the evaporator.

One of the key things that the evaporator does is condense moisture as the air is recirculated over the unit. Water runs off and out the drain to the outside of the passenger compartment. If freon get low, there is an inconsistent cooling over the complete evaporator and the water begins to freeze across the veins. As ice collects, air and moisture are blocked with more freezing until there is hardly any cool air coming through the evaporator and from the ducts.
The capillary tube on the evaporator freezes and tells the freon valve thermostat to close and disengages the compressor. When you stop the engine, it allows the evaporator to warm up and the moisture to melt off and then the compressor is re-engaged and the cooling cycle begins again.
A restriction in the evaporator valve can cause similar conditions.

ON average the exit air from the AC should be about 10 to 20 degrees below outside air. 80 degree day 60 degree air. Thus, when you are in traffic the condenser is not able to move the air from the interior of the vehicle to the outside as efficiently as when the vehicle is moving or when the outside air temperature is in the 60.

So having said all this, I would suggest a visit to your local AC shop and have the freon level checked.
Good Luck
Rob