Mercedes M-Class Model 163 climate control system '98 to '01

The heating, ventilation, and A/C (HVAC) system in the M-Class has semiautomatic temperature regulation and manual control over the air source and distribution. The temperature control is achieved without the use of a heater valve, also called mono valve in other models. The M-Class heater core is thus always hot, at about 87 deg C.

Air Conditioning Switch

The On/Off switch connects to the A/C control module (N19), which, through the All Activity Module (N10) controls the activation of the air conditioning compressor clutch. The A/C compressor operates only when the engine is running and the blower motor is switched on. The AAM (N10) also uses other information from the CAN to choose when to operate the compressor clutch, i.e. engine temperature.

Air Temperature Control

An NTC resistor behind a grille below the low range switch senses the in-cabin temperature. A hose connects it to the main climate box thus drawing air across the sensor.

Air temperature is controlled by a variable resistor rotary control. Signals from the resistor to the amplifier determine the desired interior temperature output of the system. A switch at detent position at the full counter clockwise (maximum cooling) position overrides the air recirculation switch to close the fresh air door for maximum interior cooling (with A/C on).

Air Distribution Control

The switch contains 12 detents for air distribution. A cable operates air distribution doors to direct air flow in the air distribution system to the air flow outlets. The air flow outlets are:

  • Front windshield defroster/side window.

  • Face outlets.

  • Foot outlets.

Air flow from the above outlets depends on the position of this switch and the center air outlet control wheel above the temperature control panel.

Blower Motor Switch

Off and 4 blower motor speeds are available. Switch must be in position 1-2-3 or 4 for temperature system to provide either heated or cooled air. The switch is an input to the amplifier (N19). The logic circuit in the amplifier directs system voltage to the relay/blower motor resistor (R14) for reduced blower speeds (positions 1-2-3). The switch bypasses the resistor for the highest blower speed (position 4).

Fresh/Recirculation (FRS/REC) Switch

The switch selects the air to circulate either interior air or fresh outside air. The On/Off switch controls the air door motor which is linked to the door.

Air Conditioning/Heating Control Module (N19)

The A/C Amplifier (N19) is the control unit for the heating and air conditioning system. The A/C control module senses input signals from the various components and then determines if conditions are met for proper operation. The A/C control module then outputs control signals to energize various actuators directly or signals the AAM (N10).

The A/C control module receives input information from the following devices:

  • Air conditioning switch.

  • Interior air temperature sensor (NTC resistor below low range switch).

  • Temperature selector control.

  • Blower switch control.

  • Evaporator temperature sensor.

  • Air mixture flap door position signal.

The A/C Control Module sends control signals to the following devices:

  • Air mix damper door motor.

  • AAM (N10).

  • Fresh/Recirculation (FRS/REC) door motor.

All Activity Module (AAM) (N10)

The AAM determines if certain conditions are met for the air conditioning/heating operation.

It receives input from the following devices:

  • Hi-Low pressure sensor (via CAN).

  • Engine coolant temperature sensor (via CAN).

Sends control signals to the following devices:

  • Air conditioning compressor clutch.

  • Condensor fans via relay.

  • Relay K21 to power the blower motor.

A/C Compressor

The Denso A/C compressor is a 7 cylinder variable pitch swash plate design similar to the model 210 (E-Class). It is driven by a serpentine belt from the engine crankshaft and activated by an electric clutch controlled by the AAM. If it doesn't come on it might be a bad/lose connection on the underside of the fuse box sometimes at connector M/A pin 2.

Air Mix Damper Door

The mechanical door is controlled by a linkage from the servo electric motor. The door covers/uncovers the air passage through the heater core to mix heated and cool air in correct proportion needed to achieve desired interior temperature. A feed back potentiometer sends a signal to N19 (controller) for door position.

System Operation

Air Flow

Outside or interior air for the HVAC system depends on the position of the FRS/REC (air recirculation) switch.

The blower motor provides air flow through the system. If the blower switch is OFF, there is no air flow through the system. With the switch ON, air flows from the blower motor into an air filter before passing through the evaporator fins into the air distribution housing.

A moveable air mix damper door directs air around and/or through the heater core on its way to the air distribution ducts.

The quantity and proportion of air through each air distribution duct depends on the settings of the air distribution switch on the control panel.

Air Temperature, Cool

If maximum cool air is needed (A/C ON), the air mix damper door directs all cold air around, not through, the heater core.

The air temperature sensor provides a reference temperature midpoint setting of 68 deg F. During system operation, the temperature switch setting, relative to this reference point, will cause the air mix damper door to move and regulate the air temperature accordingly.

When the temperature control is set for maximum cooling, the outside door closes in order to increase the performance of the system.

Air Temperature, Hot

When the A/C switch is OFF and maximum hot air is needed, the air mixture damper door directs all air through the heater core.

Air Temperature, Warm

When warm air is needed, the air mix damper door moves to direct air around and through the heater core to arrive at the desired air temperature setting.