A/c-Heater System - Automatic (cs) Audi 90 B4

DESCRIPTION

The A/C-heater control panel has buttons to control system. Blower speed is controlled automatically according to difference between selected temperature and interior temperature. Blower speed can also be controlled manually.

The A/C-heater control panel left side display shows selected temperature and automatic functions. (Scheme 1) The right side display indicates manual functions. The A/C-heater system microprocessor, located within the A/C-heater control panel, has a self-diagnostic feature.

The A/C-heater system automatically maintains temperatures from 64°F (18°C) to 85°F (29°C). If a temperature greater than 85°F (29°C) is selected, the word HI appears in temperature display. If a temperature less than 64°F (18°C) is selected, the word LO is displayed. Selection of these temperatures overrides automatic climate control system.

Scheme 1

A/C COMPRESSOR SPEED SENSOR

Sensor is located on compressor and determines A/C compressor speed. A/C-heater control panel then compares compressor speed to engine speed and calculates belt slippage (as a percentage). If slippage is excessive, control panel switches compressor off.

A/C-Heater Control Panel

A/C-heater control panel has a digital microprocessor that compares values from various sensors. Microprocessor then activates appropriate adjustment motor and A/C compressor clutch to maintain desired temperature. A/C clutch, blower speed, temperature/blend air door position, and mode doors are all controlled by A/C-heater control panel.

Air Distribution

Three buttons control air distribution. (Scheme 1) When selected, uppermost air distribution button directs air to windows. When middle air distribution button is selected, air is directed to dashboard outlets and rear of center console. When lowermost air distribution button is selected, air is directed to footwells.

Automatic Mode

In this setting, air temperature, air delivery and air distribution are regulated automatically to achieve and maintain desired interior temperature. All previously selected settings are cancelled.

Blower Speed Settings

Blower speed buttons can be used to raise or lower blower speed in all operating modes. Blower speed plus (+) button is used to raise blower speed. Minus (-) button lowers blower speed. If minus (-) button is pushed after blower speed is set at its lowest setting, climate control system will be deactivated.

Climate control system will also be deactivated if minus (-) and plus (+) buttons are pushed simultaneously. To reactivate system, press AUTO button, defrost button, one of temperature control buttons or blower speed plus (+) button.

Compressor On/Off Button

This button controls compressor operation.

Defrost Mode

In this setting, recirculation door is open. Blower runs at highest speed and temperature is automatically regulated. All air is directed toward windshield.

Scheme 2

Central Air Distribution Flap Motor

This actuator (motor) is located on front of heater box. The central air distribution flap is used to distribute air flow to instrument panel vents or to footwell/defroster outlets. A potentiometer, inside motor, indicates the position of the air distribution flap to the A/C-heater control panel as a feedback value.

Footwell/Defrost Flap Motor

This actuator (motor) is located on front of heater box. (Scheme 2) The footwell/defrost flap distributes air to footwell or defroster outlets depending on mode selected. A potentiometer, inside motor, indicates the position of the air distribution flap to the A/C-heater control panel as a feedback value.

Temperature Regulator Flap Motor

This actuator (motor) is mounted on left side of heater box. (Scheme 4) A potentiometer, inside motor, indicates the position of the air distribution flap to the A/C-heater control panel as a feedback value.

The temperature regulator flap is used to control air temperature in vehicle passenger compartment. Air temperature regulation is accomplished by using two flaps, one flap before and one flap after heater core. The temperature-regulating flap actuating mechanism also operates the turbulence flap.

AMBIENT TEMPERATURE SENSORS

Two sensors measure outside air temperature and send input signals to A/C-heater control panel. A/C-heater control panel measures sensor readings and lowest temperature value to calculate correction factor for interior temperature regulation. One sensor is located in front of vehicle, behind lower air grille. Second sensor is located in evaporator, next to fresh air flap.

FRESH AIR TEMPERATURE SENSOR

Temperature sensor is located on heater box, downstream of fresh air fan. Sensor measures the temperature of the air leaving the evaporator, to provide a quicker response time to changes of interior temperature.

FAN CONTROL UNIT

Air to passenger compartment is supplied and regulated by fan control unit. Fan control unit is mounted to evaporator box, in air plenum, and is cooled by air flow through evaporator housing. (Scheme 3)

Scheme 3

HIGH PRESSURE SWITCH

Switch controls cooling fan high speed operation. Switch is located on high pressure switch refrigerant line. (Scheme 4) Switch is identified by its Green housing. Ensure switch closes at 190.0-254.0 psi (13.36-17.86 kg/cm 2 ). Ensure switch opens at 153.7-217.5 psi (10.81-15.29 kg/cm 2 ). Switch can be removed without discharging system.

HIGH PRESSURE CUT-OUT SWITCH

Switch is identified by Red housing and located on right/left side of condenser. (Scheme 4) Cut-out switch turns off A/C compressor clutch when refrigerant pressure reaches 409.0-449.5 psi (28.76-31.60 kg/cm 2 ). Switch can be replaced without discharging system.

IN-CAR TEMPERATURE SENSORS

In-car temperature sensors measure interior air temperature and send signals to A/C-heater control panel. (Scheme 2) A small fan drives air over instrument panel sensor to ensure accurate measurement. One sensor is mounted on top of instrument panel and a second sensor is located next to front dome light.

LOW PRESSURE SWITCH

Refrigerant low pressure switch disengages A/C compressor clutch if refrigerant pressure drops below specified pressure. On Audi 90, ensure switch opens at 21.0-23.2 psi (1.48-1.63 kg/cm 2 ).

Fresh/Recirculated Air Flap

This two-way vacuum valve is used to control the vacuum applied to fresh/recirculated air flap door vacuum servo. Valve is located on left side of evaporator assembly, in air plenum.

Two-way vacuum valve is electrically controlled by signals from A/C-heater control panel. When vacuum is applied to fresh/recirculated air flap door vacuum servo, the flap door closes and no fresh air enters vehicle.

Heater Control Valve

This vacuum valve is located on right side of heater box, in air plenum. (Scheme 4) Two-way vacuum valve is electrically controlled by signals from A/C-heater control panel and directs or vents vacuum to heater control valve. When vacuum is applied to heater control valve, no coolant flows through heater core.

A/C CONTROL HEAD CODING

Replacement A/C control heads (A/C-heater control panel) are sold with Code 000 and must be properly coded after installation. Contact nearest Audi parts department to determine correct application.

SELF-DIAGNOSTICS

Note. Scan Tester (VAG 1551) must be used to make full use of the system's self-diagnostic capabilities.

The complete self-diagnostics functions and operating instructions of the VAG 1551 scan tester are not covered. Follow VAG 1551 operator's manual and accompanying trouble shooting manual. The following text highlights the functions available when using VAG 1551 scan tester.

DIAGNOSTIC TROUBLE CODE MEMORY

Note. Diagnostic trouble code memory is cleared when ignition is turned off. DO NOT turn ignition off after driving vehicle, as this will erase fault codes.

If a malfunction occurs in a monitored sensor or component, a Diagnostic Trouble Code (DTC) is stored in memory. This function may be used by technician to access and erase DTCs. Codes may be either hard or intermittent failures.

Hard Failures

If A/C-heater system malfunctions are present for more than 5 seconds, they are stored as Diagnostic Trouble Codes (DTCs). The A/C compressor control module distinguishes data between 19 different trouble codes and stores malfunctions until ignition is turned off (volatile memory).

Intermittent Failures

If a malfunction occurs intermittently, they are stored and considered to be "sporadic" (intermittent) failures. When displayed on scan tester, intermittent malfunctions will have "SP" (sporadic) on right side of display.

MEASURING VALUE BLOCK

Ten measuring value blocks, with 4 measuring channels each, are used. Monitored sensors and components include the A/C compressor switch-off conditions, temperature regulator flap motor, central flap motor, footwell/defroster flap motor, air flow flap motor, and motor potentiometers.

Measuring value block function monitoring also includes the display and measuring values of all ambient, fresh air intake duct, and in-car temperature sensors. Voltage at fresh air blower, engine speed, A/C compressor speed, and vehicle speed are monitored. In addition, inputs from the Engine Coolant Temperature (ECT) sensor, kick-down switch, A/C compressor engagement, and A/C high pressure switch are also monitored.

OUTPUT DIAGNOSTIC TEST MODE

Since the VAG 1551 scan tester is a bi-directional tester, it may be used to actuate a number of A/C-heater system components. The output diagnostic test mode may be used to actuate A/C compressor clutch, fresh air blower, temperature sensor blower fan, and cooling fan.

The A/C-heater control panel segment displays, outside temperature indicator (display), and Idle Air Control (IAC) may also be actuated.

In addition, the temperature regulator flap motor, central flap motor, footwell/defroster flap motor, air flow flap motor, and fresh/recirculated air flap two-way valve may also be actuated.

ON-BOARD DIAGNOSTICS (OBD)

Note. While OBD information is being displayed on A/C-heater control panel, A/C system operation does not take place (A/C compressor, radiator fan, etc. are not controlled).

ACCESSING MEMORY DIAGNOSTIC CHANNELS

1. Turn ignition switch on or start engine. Simultaneously press and hold down air recirculation button and air distribution (up arrow) button. (Scheme 1) Release both buttons. Display panel should read "01c", indicating diagnostic channel No. 1.
2. Pressing temperature plus (+) button will advance display of diagnostic channel by one. Display panel should read "02c", indicating diagnostic channel No. 2. Each time plus (+) button is pressed, system will advance to next diagnostic channel until last number is reached; display will then return to channel No. 1.
3. Memory diagnostic channels identify individual circuits, and are not fault codes. See «MEMORY DIAGNOSTIC CHANNELS»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs). To retrieve information about a particular channel, select desired channel then press air recirculation button.
4. If channel No. 52 is selected and an A/C compressor switch-off condition exists, a segment of "88.8" display will illuminate indicating cause of condition. (Scheme 4)
5. Channel No. 53 is used to identify which A/C electrical components (outputs) are activated. When channel No. 53 is selected, a segment of "88.8" display will illuminate. (Scheme 4)
6. On both channels No. 52 and 53, segments 7, 14 and 21 of "88.8" display must illuminate simultaneously to indicate system function is okay. (Scheme 4)

EXITING ON-BOARD DIAGNOSTICS

To exit memory diagnostic channel display, press AUTO button or turn ignition off.

MEMORY DIAGNOSTIC CHANNELS

Scheme 4

RETRIEVING & CLEARING FAULT CODES

1. To retrieve fault codes using A/C-heater control panel, access memory diagnostic channels. See ON-BOARD DIAGNOSTICS (OBD). If a diagnostic fault code exists, fault code will be displayed in channel No. 1. If no fault code exists, "00.0" will be displayed.
2. If a fault code exists, repair malfunction indicated. See «DIAGNOSTIC TROUBLE CODES»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs) . After malfunction is corrected, clear diagnostic trouble codes. To clear codes, VAG 1551 must be used.

DIAGNOSTIC TROUBLE CODES

TESTING

A/C SYSTEM PERFORMANCE

1. Park vehicle out of direct sunlight. Ensure condenser and radiator are free of obstructions. Ensure compressor drive belt is in good condition. Ensure engine is at normal operating temperature. Close engine hood.
2. Start engine and run it at 2000 RPM. Turn A/C system on and press AUTO mode button. (Scheme 1) Press minus (-) button until "LO" temperature setting is displayed.
3. Press air recirculation button until recirculated air symbol is displayed. Press compressor on/off button until ice crystal symbol is displayed. (Scheme 1)
4. Open all instrument panel air outlets. Ensure cooling fan and A/C compressor run. Ensure blower motor runs on high speed and air flows out of instrument panel vents.
5. Using VAG 1551 scan tester, check DTC memory. Close doors, windows, and sun roof. Record ambient temperature and check outlet air temperature at center instrument panel vent after A/C system has run for 5 minutes. See «A/C SYSTEM PERFORMANCE SPECIFICATIONS»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs). NOTE: If A/C compressor clutch disengages during performance test, go to step 11). A/C SYSTEM PERFORMANCE SPECIFICATIONS Ambient Temp °F (°C) Outlet Air Temp °F (°C) 59 (15) 37-43 (3-6) 68 (20) 37-43 (3-6) 77 (25) 37-43 (3-6) 86 (30) 37-43 (3-6) 95 (35) 39-45 (4-7) 104 (40) 41-48 (5-8)
6. If outlet air temperature is not as specified, remove low pressure switch and jumper connector terminals. Remove high pressure switch, leaving its wiring harness connected. Connect manifold gauge set to high and low pressure service valves.
7. Repeat A/C system performance test. High side (discharge) pressure should increase from base pressure (engine off) to a maximum of 290 psi (20.4 kg/cm 2 ). See A/C SYSTEM BASE (HIGH SIDE) PRESSURE SPECIFICATIONS table.
8. The high pressure switch should switch cooling fan to second speed between 190-254 psi (13.4-17.9 kg/cm 2 ). If cooling fan does not switch to second speed, check cooling fan circuit.
9. Low side (suction) pressure should be as specified in «A/C SYSTEM LOW SIDE PRESSURE SPECIFICATIONS»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs). If both high and low side pressures are okay, A/C system cooling performance is okay. Check low pressure and high pressure switches as necessary.
10. If either high and low side pressures are incorrect, check refrigerant and A/C system for malfunctions (low refrigerant charge, faulty A/C compressor, kinked/plugged A/C hose, etc.). A/C SYSTEM LOW SIDE PRESSURE SPECIFICATIONS Ambient Temp. °F (°C) Pressure psi (kg/cm 2 ) 50 (10) 30-32 (2.1-2.2) 59 (15) 29-32 (2.0-2.2) 68 (20) 28-30 (1.9-2.1) 77 (25) 26-29 (1.8-2.0) 86 (30) 25-29 (1.7-2.0) 95 (35) 25-30 (1.7-2.1) 104 (40) 28-33 (1.9-2.3)
11. Check Diagnostic Trouble Code (DTC) memory. See «DIAGNOSTIC TROUBLE CODE MEMORY»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs__diagnostic-trouble-code-memory). Repair A/C system malfunctions (if any). Erase DTC memory. End session using RAPID DATA TRANSFER, leaving VAG 1551 connected. Repeat A/C system performance test.
12. Ensure scan tester is in A/C/HEATING ELECTRONICS function. With RAPID DATA TRANSFER displayed on scan tester, press "0" and "8" button to select READ MEASURING VALUE BLOCK function. Press "Q" button to enter input.
13. With INPUT DISPLAY GROUP NUMBER displayed on scan tester, press "0" and "1" buttons to select COMPRESSOR SWITCH-OFF CONDITIONS. Press "Q" button to enter input.
14. Read display group No. 1, channel No. 1. If Code 2 (high pressure cut-out switch) is displayed, go to next step. If Code 3 (low pressure switch) is displayed, go to step 20). If other codes are displayed, go to «MEASURING VALUE BLOCK»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs__measuring-value-block) under SELF-DIAGNOSTICS.
15. End session using RAPID DATA TRANSFER. Remove high pressure cut-out switch, leaving its wiring harness connected. Connect manifold gauge set to high pressure cut-out switch Schrader valve.
16. Repeat A/C system performance test and check A/C system high side pressure. High side (discharge) pressure should increase from base pressure (engine off) to a maximum of 290 psi (20.4 kg/cm 2 ). See «A/C SYSTEM BASE (HIGH SIDE) PRESSURE SPECIFICATIONS»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs).
17. The high pressure switch should switch cooling fan to second speed between 190-254 psi (13.4-17.9 kg/cm 2 ). If cooling fan does not switch to second speed, check cooling fan circuit.
18. If required cooling performance is attained and no other malfunction is detected, replace high pressure cut-out switch. If high side pressure is exceeded, check refrigerant and A/C system for malfunctions.
19. If A/C compressor clutch still does not engage, check wiring harness between A/C control panel and high pressure cut-out switch. Repair wiring harness as necessary. A/C SYSTEM BASE (HIGH SIDE) PRESSURE SPECIFICATIONS Ambient Temp. °F (°C) (1) psi (kg/cm 2 ) 59 (15) 56.5 (3.97) 68 (20) 68.2 (4.79) 77 (25) 79.8 (5.61) 86 (30) 95.7 (6.73) 95 (35) 108.8 (7.65) 104 (40) 127.6 (8.97) 113 (45) 142.1 (9.99) (1) Pressures listed are with engine off.
20. End session using RAPID DATA TRANSFER. Remove low pressure switch and jumper connector terminals. Connect manifold gauge set to low pressure switch Schrader valve.
21. Repeat A/C system performance test and check A/C system low side pressure. Low side (suction) pressure should be as specified in «A/C SYSTEM LOW SIDE PRESSURE SPECIFICATIONS»(/audi/90/b4-1992-1995/remont/automatic-hvac-system/#ac-heater-system-automatic-cs).
22. If required cooling performance is attained and no other malfunction is detected, replace low pressure switch. If low side pressure is incorrect, check refrigerant and A/C system for malfunctions (low refrigerant charge, faulty A/C compressor, kinked/plugged A/C hose, etc.).
23. If A/C compressor clutch disengages during test, check wiring harness between A/C control panel and low pressure switch. Repair wiring harness as necessary.
24. If A/C compressor clutch doe not disengage during test, check heater box and evaporator assembly for air leaks. If no leaks are found, A/C system may be low on refrigerant. Check refrigerant lines and components for leaks. Repair leaks as necessary.

ACTUATORS, SENSORS & FAN CONTROL MODULE

1. Ensure all fuses are okay. Turn ignition off. Remove A/C-heater control panel. Connect Adapter Harness (VAG 1598/11) and Adapter Harness (VAG 1598/12) to A/C-heater control panel wiring harness.
2. Leave A/C-heater control panel disconnected. While performing tests, DO NOT connect adapter harnesses to A/C-heater control panel. The A/C-heater control panel will be damaged.
3. Adapter harnesses cannot be connected simultaneously to Test Box (VAG 1598). Set measuring range on DVOM before connecting it to test box sockets, as damage to components may result.

Scheme 5

AUTOMATIC A/C-HEATER SYSTEM COMPONENT TESTING (AUDI 90 CS)

AMBIENT, IN-CAR & FRESH AIR TEMPERATURE SENSOR RESISTANCE VALUES

REMOVAL & INSTALLATION

Note. For removal and installation of components not covered in this article, see MANUAL A/C-HEATER SYSTEMS article.

Turn ignition off. Carefully pry off A/C-heater control panel trim. Remove A/C-heater control panel. To install, reverse removal procedure.

1. Remove center console. See CENTER CONSOLE in MANUAL A/C-HEATER SYSTEM article. Remove glove box, driver's side tray, and ashtray. Remove instrument panel center support. Remove screws and footwell air outlets.
2. To remove central air distribution flap motor, disconnect wiring harness Black connector and linkage at motor. (Scheme 6) Remove screws and flap motor.
3. To remove footwell/defroster flap motor, disconnect wiring harness Red connector and linkage at motor. (Scheme 7) Remove screws and flap motor.
4. To remove temperature regulator flap motor, disconnect wiring harness Brown connector and linkage at motor. (Scheme 8) Remove screws and flap motor. To install motors, reverse removal procedure.

Scheme 6

Scheme 7

Scheme 8

Remove right side plenum tray. Remove fresh air intake duct grille. Remove glove box. Twist temperature sensor and remove from fresh air duct. To install sensor, spray sensor seal with silicone. Reverse removal procedure to complete installation.

Remove glove box. Disconnect wiring harness connector from fan control (fresh air blower) control module. Carefully remove fan control module (heat sink may be hot). To install module, reverse removal procedure.

Remove glove box. Remove screws and hose. Disconnect wiring harness connector. Remove temperature sensor and fan. To install, reverse removal procedure. Ensure hose is securely attached.

Automatic A/C-Heater System Wiring Diagram. Scheme 9

See also:
• MEMORY DIAGNOSTIC CHANNELS
• DIAGNOSTIC TROUBLE CODE MEMORY
• MEASURING VALUE BLOCK
• SELF-DIAGNOSTICS