AC Compressor does not turn off

[arvsreyes]

Car is 1987 w124 300D. USV

I live in a tropical country and the heater components have been taken out. Is it normal that my compressor does not turn off even at the lowest setting? Or is there something wrong with the AC control module? If I don't have the heater components, will the AC control module not be able to turn off the compressor / will it not be able to regulate and set the temperature using the (blue-white-red) Dial?

The previous owner retrofitted a rotary thermostat in place of the evaporator thermostat. I tried to put it back to original by installing the correct thermostat. AC Compressor engages but it seems not to be able to regulate. It is very cold. Temporarily, I placed back the rotary thermostat to regulate the temperature but it is somehow "spotty". It doesn't regulate Linearly. Most of the setting in the dial is either off or too cold. There's a (small) sweet spot where I can get a good enough comfortable temperature.

The buttons on the AC module still work and the (blue-white-red) Dial still regulates the speed of the blower.

sample of rotary thermostat

[97 SL320]

Where is the rotary thermostat's sensing bulb located? ( the end of the sensing wire )

What is the temperature range of the rotary thermostat?

Controlling interior temperatures by cycling the compressor is very difficult due to lag between system building up enough liquid refrigerant, using it, then getting the call to make more.

It is much better to control temps by moving the blend door in the heat / AC system.

You will still need a way to control the AC compressor, generally there is a low pressure switch in the AC system. It opens stopping the compressor when enough liquid refrigerant is made ( suction side is low ) and closes when more liquid is needed ( suction side rises ).

[Diseasel300]

I assume the USV stands for "US Version"?

If so, controlling the compressor is absolutely the wrong way to do temperature regulation in a W124 300D. They used the automatic climate control system that was also used in the W126 and R107.

There is no blend door. They ran the compressor full time when in automatic mode and cycled the recirculation flaps and monovalve on the heater core to regulate temperature. There is a sensor on the evaporator that shuts off the compressor when it gets too cold to prevent icing. That is the only time you should ever hear the clutch cycling.

You don't say how the rest of the system has been hacked. Your easiest way to return the car to a properly working climate control may be to return it to a stock configuration.

[97 SL320]

Originally Posted by Diseasel300

There is no blend door. They ran the compressor full time when in automatic mode and cycled the recirculation flaps and monovalve on the heater core to regulate temperature.

In this case, move the recirculation / fresh air flap to control temp. A simple cable would do the trick.

Originally Posted by Diseasel300

You don't say how the rest of the system has been hacked. Your easiest way to return the car to a properly working climate control may be to return it to a stock configuration.

Given his location, that could be difficult / $$$ .

[leathermang]

Are you guys saying he has an orifice system ?

Otherwise turning the compressor on and off is the standard way of controlling the compressed refrigerant available to the TXValve for it to control the temperature....

That sensing ' bulb' .. the long extension of the TXValve needs to be in contact with the evaporator body and insulated from ambient air.

[arvsreyes]

Where is the rotary thermostat's sensing bulb located? ( the end of the sensing wire ) - it is placed inside the AC box. It is inserted where the original thermostat used to reside

It is much better to control temps by moving the blend door in the heat / AC system. - I think the flaps and the pods have been disabled and are no longer working

USV = US Version. I am noted this because the Euro Version has a different set of AC controls. I have another car, Euro Version, heater also taken off. The temperature is regulated by turning on and off the compressor. This car one works fine

How it has been hacked / changed from original version
1) thermostat replaced as shown in the figure
2) The flaps / pods no longer work
3) The heater components have been gutted out

[leathermang]

Originally Posted by arvsreyes

Where is the rotary thermostat's sensing bulb located? ( the end of the sensing wire ) - it is placed inside the AC box. It is inserted where the original thermostat used to reside

It is much better to control temps by moving the blend door in the heat / AC system. - I think the flaps and the pods have been disabled and are no longer working

USV = US Version. I am noted this because the Euro Version has a different set of AC controls. I have another car, Euro Version, heater also taken off. The temperature is regulated by turning on and off the compressor. This car one works fine

How it has been hacked / changed from original version
1) thermostat replaced as shown in the figure
2) The flaps / pods no longer work
3) The heater components have been gutted out

IT IS NOT better to control temperature by using the flaps... in Hot and Humid climates and OLD systems often converted to the less efficient 134a one often NEEDS the recycling of the inside air to be maximized... just to drop out a reasonable amount of HUMIDITY.... some even find it advantageous to block off the fresh air flap to less than the usual 20% ( ? ) ratio.
Typically the sensing bulb end is in contact with the top of the evaporator inside the AC box...that is where its purpose.. to AVOID freezing temperatures inside the evaporator functions best... because once freezing temps occur.. then ambient air humidity starts FREEZING on the fins... quickly cutting off air flow completely through the AC...