Range Rover (L322)–Rules for MAX A/C, Rest Heating & the Defrost Programs

Range Rover - L322

Reading through the various tech docs, I can always find some small bit of info that I never knew before.

Here are the rules for the MAX A/C button with controls both the Maximum A/C function as well as the Rest heating function.  Maximum A/C is just that, it runs the system for the quickest cool down on a hot day.  The Rest heating function uses the residual heat from a warm engine to keep the Range Rover’s interior warm with the engine off for up to ~15 minutes on a cold day, it’s great!

Maximum A/C

Pressing the maximum A/C switch while the engine is running and the ambient temperature is more than 7 °C (45 °F) provides the maximum cooling possible from the system to cool the cabin down as rapidly as possible. When maximum A/C is selected the coolant valves are held closed, the A/C compressor is engaged, the blower speed is set to maximum and the recirculation doors are set to the recirculation position. After 12 minutes the recirculation doors will be set to fresh air for 1 minute then returned to the recirculation position.

The maximum A/C function is cancelled when any of the other A/C system switches are pressed.

Rest Heating

When the engine is not running, pressing the maximum A/C switch activates the rest heating function to heat the cabin with residual heat from the engine. The rest heating function activates provided the following conditions exist:

  • It is less than 15 minutes since the ignition was selected off.
  • Ambient temperature is less than 15 °C (59 °F).
  • On the previous ignition cycle engine temperature exceeded 70 °C (158 °F).
  • Battery voltage is 11.4 V minimum.

To provide the rest heating function, the ATC module activates the auxiliary coolant pump, coolant valves, control door and blower. The ATC module regulates the temperature (to the driver setting selected before the ignition was switched off), opens the face level distribution doors and runs the blower at speed 4.

The rest heating function is cancelled after 15 minutes or when:

  • The maximum A/C switch is pressed again.
  • The ignition is selected on.
  • Battery voltage decreases to less than 11 V.


And the Defrost program, which has been a Range Rover staple since at least the previous generation, does a great job getting the ice melted off the glass and warming up the cabin quickly with the press of a button:

Defrost Program

When the defrost program is selected the ATC module:

  • Opens the windshield distribution doors and closes the face and footwell doors.
  • Sets the recirculation doors to the fresh air position.
  • Runs the blower at speed 7.
  • Activates the A/C compressor.
  • Activates the heated rear window.
  • Activates the heated windshield (where fitted).

When the ambient temperature is 10 °C (50 °F) and below, the ATC module signals the coolant valves to remain open. When the ambient temperature is more than 10 °C (50 °F), the ATC module checks the existing heater core temperature and, if necessary, increases the open time of the coolant valves to produce a minimum heater core temperature of 30 °C (86 °F).

The defrost program is cancelled by pressing the defrost switch again, or pressing the automatic, A/C or maximum A/C switches

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