HEAT, VENTILATION & A/C
1986 Auto. A/C-Heater System Mercedes-Benz
A/C SYSTEM SPECIFICATIONS

A/C SYSTEM SPECIFICATIONS
Application Specification
System Type Cycling Clutch
Compressor Type
300 Series Harrison 4-Cyl.
420 & 560 Series Harrison 6-Cyl.
R-12 Capacity 56.5 oz.
Oil Capacity
300 Series 5.8 oz.
420 & 560 Series 10.1 oz.
Fan Belt Deflection (1) .2-.4" (5-10 mm)
System Pressures (2)
Low Side 22 psi (1.5 kg/cm 2 )
High Side 265 psi (18.6 kg/cm 2 )
(1) With pressure applied at midpoint of belt.
(2) At 80°F (27°C) and 70% humidity.
DESCRIPTION

Automatic climate control system uses variety of sensors, controls and electronic control unit for maintaining selected temperature between 65-85°F (18-28°C). System consists of push button control panel, in-car and heater core temperature sensors, electric (auxiliary) water pump, solenoid controlled heater control valve, electronic control unit, blower speed control unit and basic A/C system conponents (compressor, condenser, evaporator, expansion valve etc.).
OPERATION
CONTROL PANEL

Control panel consists of a temperature selector wheel, 5 mode selecting push buttons and 3 blower switch push buttons.
Temperature Selector Wheel

Selector wheel allows setting of any desired temperature between 65-85°F (18-28°C). Two extreme settings of wheel provide maximum cooling at "MIN" and maximum heating at "MAX". In either of these settings, blower will run at highest speed. These 2 settings will operate only if one of the 3 center push buttons is depressed and blower switch is in "AUTO" position.

Rotating the temperature selector wheel changes resistance value of its potentiometer. This value is then sent to electronic control unit where it is compared with sensor readings.

Fig. 1: Control Panel Switch Locations Temperature can be set to between 65-85 degrees F (18-28 degrees C). Courtesy of Mercedes-Benz of North America.
OFF Mode

When depressed, compressor is off. System operates on only outside air. If heating is selected, air flows mainly from side vents and floor outlets. If cooling temperature is selected, air flows from center panel outlets and side vents. When coolant temperature is below 95°F (35°C), air intake and blower are inoperative.
Normal Mode

When depressed, system operates as described for Economy, but compressor will operate as required. Compressor will engage only if ambient temperature is above 40°F (4°C). If in-car temperature is more than 40°F (4°C) above temperature selector wheel setting, blower will start immediately.

Change in heating and cooling modes is automatic. In full cooling mode, air intake door switches to recirculated air and diverter air door sends most air directly to evaporator with less sent through heater.
Economy Mode

In this mode, the A/C compressor stays off. Air is routed through system to provide ventilation or heating to passenger compartment. This fuel saving setting is recommended when outside temperatures are cool.

In the ventilation mode, air will be channeled to the movable inserts. In the heating mode, warm air will be mainly channeled to the floor outlets, with enough warm air directed to the windshield to prevent fog build-up.
Maximum Mode

Blower will start immediately at any engine coolant temperature. System operates on recirculated air for cooling or outside air for heating. In heating mode, air flows to windshield and floor. In cooling mode, air also flows from panel outlets.
Defrost Mode

With ignition on, blower starts immediately and operates at highest speed. Heated air goes to windshield. Temperature selector wheel position does not affect air temperature. Auxiliary electric water pump runs and compressor is on.

NOTE: If Maximum or Defrost push button is pressed with ignition on, and then switched to other operating position, blower will continue to run, regardless of ambient temperature. To stop blower, Off push button must be used.
BLOWER SWITCH

The 3 push buttons provide varying speeds. When upper button is depressed, highest blower speed only is available. When lower button is depressed, only lowest blower speed is available. If center push button is pressed (automatic selection), system operating mode will use any of the 3 medium blower speeds it determines to be necessary.

In any operating mode other than defrost, blower will operate at second lowest speed after a 10 second delay. It will remain at this speed for approximately 30 seconds, then will move to other speeds as required.
TEMPERATURE SENSORS
In-Car Sensor

Sensor is located under small grille on top of instrument panel. Sensor detects in-car temperature and sends signal to electronic control unit. Sensor has aspirator in blower housing to draw in-car air over sensor when blower is operating.

Fig. 2: In-Car Temperature Sensor Sensor is located on top of instrument panel. Courtesy of Mercedes-Benz of North America.
Heater Temperature Sensor

This sensor is located in heater housing to sense outlet temperature. Any variations in heater temperature are noted by sensor and signal is sent to electronic control unit.

Fig. 3: Heater Temperature Sensor Sensor is located in heater core. Courtesy of Mercedes-Benz of North America.
HEATER VALVE ASSEMBLY

Valve assembly includes heater control valve, auxiliary water pump and cold engine lock-out switch.
Heater Control Valve

Controls flow rate of coolant into heater core. Signal from electronic control unit (in response to temperature sensor signals) activates solenoid of heater valve to open or close valve as necessary. When de-energized, heater valve is open. A 5 second cycle opens valve to fill heater core with coolant. A check valve prevents over-filling of core.
Auxiliary Water Pump

Located near heater valve and connected via a hose, pump helps maintain steady coolant flow through heater at low engine speeds. Water pump is activated by same signal from control unit which opens floor (heater) air doors. Pump, like heater valve, is located on firewall in return line from heater.

Fig. 4: Heater Water Valve and Auxiliary Water Pump
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Cold Engine Lock-Out Switch

Switch is located in coolant passage on engine. It prevents blower operation if heating mode is selected when coolant temperature is below 95°F (35°C).

Fig. 5: Typical Electronic Control Units
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
ELECTRONIC CONTROL UNIT (ECU)

Electronic control unit is located on end of blower housing. Electronic signals from sensor and from temperature dial settings are processed in ECU. It controls coolant flow through heater valve, mode change (heating to cooling), position of fresh/recirculated air door, and signals to blower speed control unit.
BLOWER SPEED CONTROL UNIT

Blower speed control unit is located on side of blower housing near evaporator on 300 Series. On all other models it is located next to ECU above A/C housing. Unit controls speed changes of blower in response to signals from ECU or directly from push button for defrost or maximum setting.

Fig. 6: Blower Speed Control Unit (420 & 560 Series Type Shown)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
AIR DISTRIBUTION CONTROL

Fresh air inlet is located below screen on cowl area. Fresh air is used for all operations except maximum cooling, which uses recirculated air. Air flow through lower ducts to rear passenger compartment is always the same temperature as outside air temperature. Air flow from all other outlets is determined by temperature setting and operating mode.
Switchover Valves

Air flow patterns are operated by response from electronic control unit, which causes vacuum portion of various mode switchover valves to operate their respective doors.

Fig. 7: Switchover Valve Locations
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Vacuum Actuators

Actuators are located on heater-evaporator assembly to open and close various air doors when vacuum signal is received from respective switchover valve.

Fig. 8: A/C-Heater Assembly Showing Air Doors
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
EVAPORATOR TEMPERATURE REGULATOR (ETR)

To protect evaporator against icing, the ETR switch will stop compressor operation if less than 36°F (2°C) is sensed at evaporator. Switch is mounted on evaporator housing with capillary tube extending into evaporator core. On 300, 420 and 560 series, ETR is located below vacuum actuator for diverter air door, near A/C pipe connections. On other models, switch is located beneath electronic control unit.
AUXILIARY FAN

For additional cooling and increased heat exchange across condenser during adverse operating conditions, an auxiliary fan is used. Fan comes on if R-12 temperature is more than 164°F (73°C) is sensed by switch at receiver-drier. Fan will also come on if coolant temperature higher than 212°F (100°C) is sensed by sensor in water jacket.

On 420 and 560SEL, auxiliary fan operates in 2 stages via resistor and receiver-drier switch if low R-12 temperature is sensed, and via coolant temperature switch if high coolant temperature is sensed. Resistor is located near blower motor resistor assembly.
EXPANSION VALVE

A block design expansion valve is used to promote better R-12 vaporization, direct temperature sensing and higher cooling capacity.

NOTE: On V8 models, coolant temperature switch activates fan at 230°F (110°C) coolant temperature.
TESTING
SYSTEM GENERAL TEST

Following test sequence shows proper system functions in all operating conditions. It should be used to attempt to pin-point possible problem area in system. Start engine.
Selector at "2"; OFF Push Button; Blower at "AUTOM"

Heater valve is closed. Compressor is off. Main air door is closed. Blower is off.
Selector at "72"; Economy or Heat Push Button; Blower at "AUTOM"

Slowly rotate dial near "72" until "overlap" occurs (floor outlets and center outlets are both open). Heater valve cycles every 4-5 seconds above 77°F (25°C) coolant temperature. Compressor is on (Heat), after a .7 second delay. Main air door is open. Blower starts after approximately 10 second delay. Auxiliary water pump is on.
Selector at Minimum Cool; Maximum Push Button; Blower at "AUTOM"

Heater valve is closed (energized). Auxiliary water pump is off. Compressor is on if ambient temperature is above 40°F (5°C). Main air door is at recirculation position (80% recirculated air). Blower at next to highest stage.
Selector at Minimum Cool; Economy Push Button; Blower at "AUTOM"

Heater valve is closed. Auxiliary water pump is off. Compressor is off. Main air door is open. Blower is running at next to highest stage.
Selector at Minimum Cool; Heat Push Button; Blower at Low Speed

Outside air door open. Heater valve is closed. Auxiliary water pump is off. Main air door in recirculation position. Blower running at low speed.
Selector at "72"; Heat Push Button; Blower at "AUTOM"

Move dial until both floor door and center outlets are open. Wait for blower to reach next to lowest state. Heater valve cycles 4-5 seconds. Compressor is on. Main air door is open. Auxiliary water pump is on.
Selector at "72"; Economy Push Button; Blower at "AUTOM"

Blower moves up one stage. Heater valve cycles every 4-5 seconds. Compressor is off. Main air door is open. Auxiliary water pump is on.
Selector at "72"; "Economy" Push Button; Blower at High Speed

Blower at top speed. Heater valve cycles every 4-5 seconds. Compressor is off. Main air door is open. Auxiliary water pump is on.
Selector at Maximum Heat; Heat Push Button; Blower at "AUTOM"

Heater valve on. Auxiliary pump on. Compressor on. Blower on at next to highest stage. Center air outlet closed.
Selector at Minimum Cool; Maximum Push Button; Blower at High Speed

Air should come immediately out of center outlet. Outside air door is open.
Selector at "72"; Off Push Button; Blower at "AUTOM"

Heater valve closed. Auxiliary water pump off. Main air door closed. Blower and compressor are off.
Selector at Maximum Heat; Heat Push Button; Blower at "AUTOM"

Detach lead from cold engine lock-out switch. Main air door is closed. Blower is off.
Selector at Minimum Cool; Heat Push Button; Blower at "AUTOM"

Main air door is at recirculation position. Blower comes on after 10 second delay. Reconnect the cold engine lock-out switch.
ETR SWITCH TEST

1. With system operating at maximum cooling for several minutes, place thermometer in center outlet. Depress Low Blower speed button and Heat push button. Temperature dial should be at minimum cooling.
2. Run engine at 2000 RPM. After third cut-out of compressor clutch, outlet temperature should be approximately 40°F (5°C), but not below 37°F (3°C).
3. If temperatures are not as stated, check A/C system for proper R-12 charge and output. If okay, check position of ETR capillary tube. If okay, replace ETR switch.

VACUUM TESTS

Before beginning tests, run engine until it is fully warm (cold engine lock-out switch is off), then shut off engine. Depress Economy push button.
Check Valve

Detach Red/Green vacuum line or Green/Gray (except 300SDL and 420SEL) vacuum line from check valve (in line from manifold vacuum source). Attach vacuum tester to check valve port and apply vacuum. If vacuum leaks down, replace check valve.
Vacuum Reservoir

Detach Red/Gray vacuum line from check valve leading to vacuum reservoir. Connect vacuum tester to vacuum line and apply vacuum. If vacuum reading changes, replace vacuum reservoir gasket or vacuum reservoir.
Switchover Valves

1. On all models, detach Red/Green vacuum line or Green/Gray (except 300SDL and 420SEL) vacuum lines from check valve and attach vacuum tester to vacuum line. Apply vacuum. If vacuum changes, test switchover valves individually.
2. Remove lower right instrument panel trim piece. Remove right side of center console to expose switchover valves. Individually pull vacuum lines from each switchover valve, attach vacuum tester and apply vacuum. If vacuum drops, switchover valve is leaking and should be replaced.
3. To double check, energize switchover valve by turning ignition on and depressing Off push button if testing defrost switchover valve or fresh/recirculation switchover valve (short stroke). If testing other switchover valves, depress Maximum push button instead. Vacuum should drop if okay. Turn off ignition and replace trim pieces.
4. Apply vacuum while watching operation of defrost door. If vacuum drops, replace vacuum actuator or vacuum line. Check mechanical operation of door also. Reattach vacuum line.
5. Detach Red/Light Blue vacuum line from check valve in line to center air outlet. Attach vacuum tester to line and apply vacuum. If vacuum drops, replace line or defrost vacuum actuator.

Center Outlet Vacuum Circuit

1. Remove glove box, lower right instrument panel trim piece and right side console trim piece. Detach Red/Light Blue vacuum line from check valve in line to center outlet vacuum actuator. Plug end of vacuum line.
2. Detach vacuum connection from center outlet switchover valve. Attach vacuum tester to this line. Apply vacuum. If vacuum drops, replace center outlet vacuum actuator, check valve or vacuum lines in this circuit.

Heater Outlet Vacuum Circuit

Pull Red/Yellow vacuum line from floor outlet switchover valve. Attach vacuum tester to line. Apply vacuum, noting operation of floor air doors. If vacuum drops, replace floor outlet vacuum actuator or vacuum line.
Outside Air Door Vacuum Circuit

Detach Green vacuum line from outside air door switchover valve. Attach vacuum tester to line. Apply vacuum. If vacuum drops, replace outside air door vacuum actuator or vacuum line.
Fresh/Recirculated & Main Air Door Vacuum Circuit

1. Detach Red vacuum line from "long stroke" fresh/recirculation switchover valve. Attach vacuum tester and apply vacuum. If vacuum drops, replace vacuum line and check or replace "long stroke" or "short stroke" fresh/recirculation vacuum actuator.
2. Detach Green/Blue vacuum line from "short stroke" fresh/recirculation switchover valve. Attach vacuum tester and apply vacuum. If vacuum drops, replace vacuum line and check or replace "long stroke" or "short stroke" fresh/recirculation vacuum actuator.

IN-CAR OR HEATER TEMPERATURE SENSOR TEST

Remove appropriate sensor as described. Using ohmmeter, check resistance value at electrical terminals of sensor. Compare reading with values shown in following chart.

IN-CAR & HEATER TEMPERATURE SENSOR RESISTANCE
Ambient Temperature °F (°C) Ohms
59 (15) 15.7
77 (24) 10.0
95 (35) 6.5
REMOVAL & INSTALLATION
CONTROL PANEL ASSEMBLY
Removal

1. Disconnect battery. Remove radio. Remove optional switch, if equipped. Remove 2 screws in radio opening and carefully pull out and remove wooden cover panel out from bottom.
2. Remove 2 screws retaining control assembly and pull assembly forward. Remove instrument lamps. Detach 12-pin connector from control unit. Temperature dial and blower switch can be removed independently at this time.
3. To remove push buttons or blower switch from control panel, remove screw near blower switch. Remove plastic cover from control panel assembly. Remove push buttons or blower switch as necessary.

Installation

To install, reverse removal procedure.
ELECTRONIC CONTROL UNIT AND/OR BLOWER SPEED CONTROL UNIT
Removal

Disconnect battery. Remove glove box and lower right instrument panel cover. Detach electrical plug. Remove retaining screws and remove control unit.
Installation

To install, reverse removal procedure.
EVAPORATOR TEMPERATURE REGULATOR (ETR)
Removal

On 420SEL and 560SL, remove blower motor and right air duct under instrument panel. On all other models, remove lower left instrument panel cover. Remove screw in left side console cover near accelerator pedal. On all models, detach electrical connections from ETR switch. Remove retaining screws. Carefully pull out ETR switch, guiding capillary tube out of evaporator housing.
Installation

To install, reverse removal procedure, noting that capillary tube should be inserted up to Red mark. On 420SEL and 560SL, glue an insulation strip onto capillary and evaporator fins.
EVAPORATOR & HEATER CORE
Removal

1. Discharge A/C system, using approved refrigerant recovery/recycling equipment, and drain engine coolant. Disconnect battery. Cover both front seats and slide back. Remove floor mats and carpets.
2. Pull glove box light and disconnect wiring. Pry top half of expanding rivets with a screwdriver. Pry bottom half of expanding rivets out of glove box and remove glove box.
3. Remove screws from left and right panels under dash. Turn plastic clip on center console 90% left and remove panel.
4. Pull interior windshield moldings from left and right side. Release reveal molding from roof frame or remove windshield.
5. Remove speaker covers. Remove screws under speaker cut-outs. On all models, remove steering wheel.
6. Release instrument cluster panel from dash by pulling out with hands only. Disconnect speedometer cable, 2 wire connectors and oil pressure line. Remove instrument cluster from vehicle.
7. Remove in-car temperature sensor from top of dash. Pull light switch knob and remove retaining nut. Pull light switch cover and disconnect wiring.
8. Disconnect parking brake cable. Remove left and right side panel vent ducts. Remove radio and slightly lift control panel from center console.
9. Pull 12-point plug connector from electrical switch gear. Pull 5 and 6-point plugs from temperature dial. Remove 2-point plugs from temperature sensor, air volume control and air distribution switch. Remove control panel.
10. Remove center console-to-dash screws. Remove control cable(s) for fresh air vents. Push plastic defroster nozzle on top of dash and remove. Remove dash mounting screw at top center of dash. Remove screws beneath left and right side of dash.
11. Remove glove box light switch wiring. Slightly raise dash and make sure defroster ducts come out of heater box. Pull remaining cable(s) from A/C-heater housing-to-dash. Disconnect hose for in-car temperature sensor and remove dash.
12. Remove screws from transmission tunnel sides, center tray and near floor shifter area. Remove center console to rear. Remove 3 supporting straps and all heater hoses. Remove air ducts on transmission tunnel.
13. Remove all remaining vacuum hoses, electrical connectors and control cable(s) as necessary. Remove screws from transmission tunnel brace. Pull air ducts for left and right fresh air vents. Remove electronic switching unit.
14. Pull expansion valve housing to remove A/C hoses from fittings. Plug all openings. Pull drain hoses from both sides of A/C-heater housing. Pull electrical plug from switchover valve.
15. Remove remaining screws and nuts from support brackets. Pull A/C-heater unit housing complete with heater hoses from vehicle. Hold heater hoses up so coolant will not spill.
16. Remove clips from top of A/C-heater case. Remove heater core case and core unit. Pry 2 clamps from lateral member. Pull vacuum line to vacuum control motor. Disconnect actuator control rod and vacuum motor mounting screw.
17. Remove lateral member. Pull plastic shaft and shaft bearing rod out of A/C-heater housing. Pull temperature sensor from A/C-heater housing near expansion valve. Disconnect vacuum controls on main air flap.
18. Pry clips holding main flap housing and remove. Pry remaining clips around A/C-heater (evaporator-to-heater core) housing. Remove screws holding vacuum switchover valve to case. Remove left side panel.
19. Remove main air flap shaft. Separate A/C-heater housing and remove evaporator.

Installation

To install, reverse removal procedure. Make sure evaporator housing joining surfaces are well sealed on reassembly.
HEATER TEMPERATURE SENSOR
Removal

Remove radio. Remove ashtray. Detach 2-pin connector from sensor. Pull sensor from heater housing. On 420SEL and 560SL use a screwdriver to remove sensor from guide tube. Make sure the screwdriver is inserted between guide tube and sensor, not between guide tube and center air duct. See Fig. 3 .
Installation

To install, reverse removal procedure.
IN-CAR TEMPERATURE SENSOR
Removal

Disconnect battery. Remove instrument cluster. Detach electrical connector from in-car sensor. Detach air hose. Pull lower half of sensor downward while lifting retaining tongs. Pull sensor out through top of instrument panel.
Installation

To install, reverse removal procedure.
SWITCHOVER VALVES

See VACUUM TEST for removal and installation procedures.
VACUUM DIAGRAMS

Fig. 9: Vacuum Diagram (420/560 Series)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig. 10: Vacuum Diagram (300E - Up To August 1986)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig. 11: Vacuum Diagram (300E - As of September 1986)
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
WIRING DIAGRAMS

For wiring diagrams see appropriate article:

* For 300E see WIRING DIAGRAMS .
* For 420/560 Series see WIRING DIAGRAMS .

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