Climate control '81 300D

Saw a recent post regarding climate control problem on '80 to '85 300Ds. Hope the same experts are still around. Problem: AC has been converted to R134a and works fine at idle and slow rpm,
just as it did with R12 refrigerant. However, anything much past idle and it starts getting warmer to the point no cool air at all exits dash ports. Return to idle, everything fine again. Compressor stays on regardless of rpm. Cross over tube at front of engine remains very cold, again, regardless of rpm. Have replaced the climate control module in center console twice, virtually the same results. This problem was also present with the previous R12 refrigerant. Any suggestions? (Please, no sarcastic comments needed)
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I am not an expert,but I would check the vacum, I had a leak and everytime I accelerated the car the blower kicked out. Jim
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Open the hood and then lock it straight up. Get a straight blade screwdriver. On the pass side next to the battery, is the mono valve. Unplug the small connecter on top. Open and close the radiator cap. Remove the 4 screws on the square top. Grab the round part sticking up and lift up. You will be removing the mono valve insert and magnet. Now turn it over aand look at the black rubber diapraghm. It will be torn. Now go to your local dealer and buy a new insert. MAKE you put it together in the correct order. Look at the washer arrangement with the wave washer in the middle. Also, MAKE sure the wide opening on the top edge of the magnet coil shield is over the wire, not the narrow opening.

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"Have replaced the climate control module in center console twice"
By that do you mean the A/C expansion valve or the electronic control unit?
My experience is that the A/C expansion valve's tiny orifice closes up over the years; the A/C is then OK on cool or overcast days but is nonexistent in hot weather.
Have you felt the A/C coils in front of the engine's radiator? They should be quite hot when the A/C has run for a time, if not, the A/C isn't cooling.
Another possibility is that the A/C is fine but the heater is running full ON. The heater control valve defaults to full heat. Feel the heater hoses in warm weather when the A/C is set to its coldest setting, the hoses should not be hot.
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