AC knob direction

[wilson234]

The AC knob has one of those progressively increasing logos on the knob but confused which way is max cool. The fan knob, turning clockwise makes the fan turn on and highest fan speed is full clockwise. Is his the same for the temp knob? The problem is I don't really feel any temp difference when the knob is maxed out in either direction. Prior owner installed a Kuel compressor, but the air doesn't feel too cold. Is there anything I can check to confirm system is charged?

[DWeg1998]

I'm wondering the answer to this too as my thermometer reads 61 degrees with the knob turned either way.

[Richard930]

Fully clockwise for max cold. If its not going colder you'll need to do some investigation. If the system is charged properly then your temp switch and capillary line to the evaporator could be bad. Griffiths website has some troubleshooting tips to get you started.

[Bob Kontak]

I'll speak and more than likely I'll be wrong then someone who knows will shore things up.

The temp knob is connected to a probe inside the evaporator. It sniffs the temp and will quell cooling if you get to a certain "low" temperature. Given you are probably not at optimal operating efficiency (e.g., low on r-12/R-134) you are not reaching that low temperature.

[wwest]

Your A/C system has an over-ride control, TXV, that prevents the system from freezing up the evaporator core. Initial cabin cooldown rate/time is often limited by this "extra" control aspect.

[wwest]

Your system has an "over-ride" control (TXV) that prevents the evaporator core temperature from declining to the point wherein the evaporator freezes up and blocks all system airflow. Until the cabin is cooled down to within the range of your setpoint comfort level the max cooling level will be limited by the TXV control.

[kuehl]

Wilson,
PP has a schematic on system here:Schematic

Assuming stock factory system for example:

When you turn the RH evap blower knob clockwise from off to on you have 3 fan speeds (nothing, nothing and full blast as they say). In either of the 3 fan 'on' speeds the ac system is on and ready to run. The evaporator motor should be blowing air through the AC vents. The compressor clutch and front condenser blower motor will come on and off depending upon whether or not the Thermostat's contacts are open or closed. The thermostat is the knob on the LH side. So with the RH knob in any 3 blower 'on' positions there is power sent to the thermostat and when the contacts close on the thermostat the compressor clutch will engage and the front condenser blower motor (on cars with front condensers) will blow. Blower switches fail over time simply because there is a lot of load on the contacts and you can't expect a 25 year old switch to last forever.

The thermostat can be described as having 2 sections per se ; the knob/thermostat elec function and the aluminum capillary sensing tube section (the later is brazed to the first section and is sealed with a refrigerant gas). The aluminum capillary sensing tube runs through the center console, under the carpet, through the bulkhead (fire wall), into the top of the evaporator box in the 'smugglers box well' in the front trunk,
and into the evaporator coil. When you turn the LH themostat knob CW (to the right) the distance between the contacts in the switch and a bellow in the switch connected to the sensing tube changes in distance. As the evaporator coil gets warmer the gas inside the sensing tube expands, this expands the bellow and makes the contacts in the thermostat 'make' or allow current to pass through and hence the compressor clutch is engaged and the front condenser blower motor operates. As the evaporator coil gets colder the gas inside the sensing tube contracts, the bellow contracts and depending upon where the thermostat knob is set (CW is colder and CCW is warmer) the contacts in the thermostat will open and turn off the compressor and condenser blower motor. Its a very simple and reliable design that has been used for many years in various AC cooling systems, later model vehicles use an electronic sensor such as thermistor or other device.

Occasionally thermostats can fail. If the aluminum capillary tube is broken, say someone is working in or around the center console installing a stereo, alarm or doing repairs and break the aluminum tube, or working in the smuggler's box area the aluminum tube is broken.. .the gas in the tube escapes and the thermostat stops functioning and cannot be economically repaired... a new thermostat. Or, if the thermostat is very old and the contacts wear or are fused, the thermostat is in an always 'on' condition, the compressor never shuts off. If the aluminum sensing tube is not making good contact with the fins in evaporator coil the circuit could remain on constantly and the evaporator will ice up.

Unless your 911 has a cockpit sensor, the cockpit temperature has no affect/effect on whether the thermostat turns the compressor on or off. The driver of the car controls the thermostat and the temperature of the evaporator coil and setting on the thermostat turns the compressor on and off.


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The AC Temperature Switch