Questions and Answers

Mercedes Benz 300D Climate Control Q. Hi Vincent, I have a 1978 Mercedes Benz 300D which I just bought. Heater/AC worked fine on 500 mile drive home. Now it works intermittently. Sometimes everything is fine, but most of the time when I press the auto, or bi-level button either nothing happens or defrost puts out lots of hot air. Defrost button always gets hot air only even if nothing happens when pressing other buttons.

Cannot get cold air from defrost even though the A/C clutch is engaged. When the system is working it puts out lots of cold air. Could this be a blower motor problem, or control unit, or what?

Thanks for your help.
Jim

A. How about I give you the Description and Operation of the system and you can compare it to how yours is working? Then if there is a problem we can try and troubleshoot it.

PURPOSE:
The Automatic Climate Control (ACC) push-button control unit:

• Controls the air distribution system.
• Activates the A/C compressor, blower motor, auxiliary coolant pump and mono valve.
• Calculates necessary corrections to maintain interior air temperature and blower speed at desired levels.

OPERATION:

Temperature Wheel:
The temperature wheel provides stepless interior temperature control in a temperature range from approx. 60°F (16°C) to 89°F (32°C). If the temperature wheel is set at "MIN", the system operates at full uncontrolled cooling capacity. If the system is set at the "MAX" position, the system operates at full uncontrolled heating capacity.

Rotation of the temperature wheel changes the resistance (potentiometer), transmitting the new resistance value into the electronic system of the temperature control.

Blower Switch:
The blower switch operates in six stages. The top button (f) is maximum setting (6) and delivers full 12 volts. The bottom button (h) is minimum setting (1) and delivers 4 volts. The center button (g) is automatic "AUTOM" (2-5) and delivers between 5 and 10 volts. In "AUTOM" the blower will start in 2nd stage after 10 seconds in functions b, c, and d. The blower will switch up to 5th stage if required.

Function Controls

Defrost "a"
The blower starts immediately in the 6th stage when the ignition is switched on. Regardless of the temperature dial, maximum heated air is routed to the windshield.

The recirculating (auxiliary water) pump runs continuously and the compressor is switched on at outside temperatures above +2°C.

Top/Bottom Air Distribution "b"
The blower starts after approx. 10 seconds, even with coolant temperatures below +35°C in "AUTOM" in stages 3 to 5.

Air is routed to the windshield, side, and legroom vents in heating range; and also the center vent in cooling range. This operation removes fog, if any, from the windshield during damp,cold weather.

Bottom Air Distribution "c"
The blower starts immediately set to "AUTOM" in stages 3 to 5 if the in-car temperature is approx. 4°C higher than the temperature dial setting.

Heated air will flow out the legroom and side nozzles with a leak air share from defroster vents in heating range.

Cooled air flows out of center and side nozzles only in cooling range. The mode changes automatically (heating to cooling) and it is possible for legroom vents and center vents to overlap with heated and cooled air and be open simultaneously.

The compressor switches on at temperatures above +2°C and the evaporator is held to approx. 0°C. If the system switches to "cooling" the fresh air/recirculating air flap is switched to recirculating air.

Economy "d"
In Economy (EC) the system operates with fresh air only.
Heated air flows out of legroom and side nozzles with a leak air share from defroster vents in heating range.

Air is routed to center and side vents only during venting range. The blower operates in stages 3 to 5 in "AUTOM". The blower is not switched on until coolant temperature reaches 35°C.

Off "e"
In Off the air supply is switched off through the main air flap to the inside of vehicle. The electronic system continues to operate while driving and the monovalve remains closed. Blower does not operate.

If yours acts as described, then it is okay. If not, then it is most likely a bad control unit of a loose connection to the control unit.