When Audi engineers were designing the

Audi Climate Control Part One When Audi engineers were designing the 5000 sedan for the 1984 model year, they decided to upgrade the climate control system at the same time. The pre-1984 system was based on a General Motors design and had been a source of problems. Audi decided to do away with the vacuum- and cable-operated dash controls used on previous models in favor of a fully electronic central control head. Along with this improvement came a fully automatic system that regulated the heat and air conditioning to control cabin temperature... or so they planned. As with many new designs, the climate control system was plagued with a fresh set of problems. In this article, we will look at the components and functions of the Audi climate control system as it was introduced in 1984, as well as at the updates and changes made through the years leading up to the system used in the 1987 model. These systems do not have self-diagnostic capabilities. In a subsequent article (Part Two), we will examine the newer digital system introduced with the 1988 model year, which Audi continued to use through 1994. These systems have self-diagnostic capabilities. Also in Part Two, we will offer some tips and tricks to help make your life a bit easier when diagnosing and repairing these systems. Where to begin? It s safe to say that the early systems had their share of problems. So many problems, in fact, that the factory initiated a complete update campaign to correct many while the cars were still under warranty. Before we delve into these, let s take a look at the basic system, its components and functions. System Components and Operation Central Processing Unit The system is built around a central processing unit (CPU) located in the dashboard control panel, which receives inputs from the driver and an array of sensors and switches throughout the vehicle. Based on that information, the CPU processes the information and operates the appropriate components. Programmer A programmer is located under the dashboard, behind the glove box. The programmer receives signals from the control head and operates a series of vacuum switches and a DC servo motor. The vacuum switches control air distribution and the heater valve, while the DC servo motor controls interior temperature. It does this by regulating the position of the heater flaps via a Bowden cable. Based on the input it receives from the control head, 16

the programmer will also switch on the A/C compressor clutch relay, which also activates the radiator fan to its first speed as long as the compressor clutch engages (5000 models only). Switching on the compressor clutch is possible in all modes, except the ECON mode, as selected at the control panel. Blower Fan Control Unit The blower fan control unit is located in the evaporator housing assembly. It controls the operation of the blower and regulates the fan speeds via controlled power flow to the blower motor. This control is based on signals received from the control head. The unit is equipped with heat sinks cooled by the air flow provided by the blower fan. Early versions (1984-85) controlled the A/C compressor clutch relay through the blower control. These early units also incorporated a blower fan thermo-switch which prevented the blower from running if the engine was cold. This feature was changed in the 1986 model. Engine Temperature Sensor Based on inputs from the engine temperature sensors, the central control unit calculates when the engine has warmed enough to produce heat and allows the blower fan to run. On all models, this feature may be overridden by selecting the DEFROST mode from the control panel. Overheat protection is also supplied by the engine coolant NTC sensor. If coolant temperature exceeds 247 F, the sensor signals control head, and the control head shuts down the A/C compressor. This reduces the engine load and resulting heat added upstream of the radiator by the condenser. Outside Temperature Sensor Two identical sensors, one in front of the radiator and the other in the evaporator plenum, supply an outside temperature reading to the control head. The sensor near the radiator provides a more accurate reading while the vehicle is in motion, while the evaporatormounted sensor is most accurate when the vehicle is stopped or in slow traffic. The control head compares the readings from both sensors and uses the lower value to make adjustments to the inside cabin temperature. These sensors also provide information for the outside temperature display on the control panel. Cabin Temperature Sensor Inside cabin temperature is measured via an NTC sensor mounted in the dashboard. A small suction fan draws interior air over the sensor to insure accurate measurement. This sensor is often a source of inaccurate cabin temperature readings, due to dirt, dust or tobacco smoke contamination. However, a simple cleaning using an electronicssafe spray cleaner will usually restore its accuracy. Compressor Clutch Relay The compressor clutch relay controls refrigerant flow through the system by cycling the compressor clutch. The control head uses inputs from the lowpressure switch located on the suction line near the accumulator, the high-pressure switch located on the high pressure line in front of the radiator and the evaporator temperature sensor with capillary tube located on the evaporator housing. Low Pressure Switch On early models (1984-85), remove the low pressure switch to access the low side service port. This was changed in 1986, when the compressor was updated. All later models allow access to both service ports located on the compressor. To protect the system from damage, the low pressure switch interrupts the compressor circuit if system pressure falls below 15 psi. The high pressure switch closes when the high side pressure reaches about 200 psi. The closing of this switch also activates the radiator fan s higher speed to help reduce system operating pressure. Evaporator Temperature Sensor The evaporator temperature sensor, via its capillary tube, toggles the power supply to the compressor relay. Cycling the compressor clutch on and off controls the evaporator pressure and temperature. At evaporator temperatures below 32 F, the compressor is switched off to prevent evaporator iceup. Working in conjunction with the outside ambient air temperature sensors, the control head keeps the A/C compressor from engaging when outside ambient air temperature falls below 37 F. High Pressure Temperature Sensor Later models (1986 and up), use an additional temperature sensor in the high-pressure line between the condenser and evaporator. This sensor monitors the temperature of the refrigerant and sends a corresponding NTC signal to the control head. Using this signal, the control head can calculate approximate refrigerant pressure. If the high side refrigerant temperature and thus pressure exceed a predetermined level, the A/C compressor switches off to prevent overpressure damage. Kick-Down Switch On automatic transmission-equipped vehicles, the A/C compressor also switches off during full-throttle load conditions via a kickdown switch under the accelerator pedal. Activation of this switch disengages the compressor clutch for approximately 12 seconds. By Pat Etzwiler 17

Audi Climate Control The central control unit (shown here) collects and processes data from the various sensors and switches located throughout the climate control system. The unit shown incorporates the HIGH-LOW fan speed control on the panel, which was added for the 1986 model year. The programmer unit is in the right corner of the dash, behind the glove box. The programmer contains the vacuum switches, vacuum lines, distribution manifold and a DC servo motor to operate the Bowden cable for heater/air blend door regulation. The programmer s DC servo motor was updated in late 1984. The updated version has a black bellcrank to differentiate it from the early version. In Part Two of this series, we will discuss the voltage adjustments that must be made to this unit after replacement. The heater/blend-door Bowden cable, shown here, was also updated during the 1984 factory campaign. Assist springs were added to some models to aid the servo motor in the return of the cable/door assembly and to keep the blend door from binding. 18

Here is the air plenum location of the blower fan control module on an early climate control system (1984-85). This unit also controls power to the A/C compressor clutch relay. No updates to this unit were included in the campaign. This is an updated version of the blower fan control unit (1986 and later). It s in the same location, but modified circuitry eliminated some causes of premature failures seen on earlier units. Also, this unit does not control the compressor relay. The low-side pressure switch also serves as the low-side service port on early models (pre-1986). To test its function, temporarily bridge the wiring at the low pressure switch during a system recharge or service on all models. An updated harness for this unit was part of the 1984 factory campaign. The evaporator temperature sensor, with its capillary tube, shares a mounting bracket with the plenum-mounted outside ambient-air sensor (outside air temperature sensor #1). Due to its location below the windshield, corrosion of the wiring connectors is a common problem. 19

All models feature a cabin air temperature sensor, located in the dashboard. However, sunlight striking the sensor can make the sensor send accurate but misleading readings to the control unit. This condition existed until the 1988 model year. An additional inside ambient air-temperature sensor was added for 1988. The sensor is in the roof panel near the upper edge of the windshield. The control unit compares the values from both sensors and adjusts the cabin temperature based on the lower of the two readings.