Sensors and Switches
Inside Air Temperature Sensor
The inside air temperature sensor is located in the IP to the left of center. To provide interior air temperature sensing, a small amount of air is drawn into the sensor housing and passed over the thermistor. The air movement is accomplished by using an aspirator mounted on the top of the HVAC module. Airflow from the HVAC module creates a slight vacuum at one end of the aspirator which draws inside air through the sensor housing and over the thermistor.
Ambient Air Temperature Sensor
This sensor is located in front of the radiator. The sensor data is processed by the heater and A/C programmer and displayed on the heater and A/C control.
During periods of city traffic and idle conditions, engine heat can surround the area of the ambient temperature sensor and cause temperature readings that are higher than the actual ambient air temperature. To help eliminate these false readings, programming techniques are used to minimize unwanted temperature variations.
A temperature memory feature is used in the heater and A/C programmer ambient temperature programming to help provide greater accuracy under engine restart conditions. If engine coolant temperature is less than 10°C (18°F) above the ambient temperature sensor reading, or a two-hour ignition off timer has expired, then the actual sensed ambient temperature is displayed. However, if engine coolant is greater than 10°C (18°F) above the sensor reading, memorized ambient temperature is displayed. This is the displayed temperature sensed when the engine was previously operating. At car speeds greater than 32 km (20 mph), the ambient temperature display may be allowed to increase, but only after a built-in 80-second time delay which allows for ambient air to cool the sensor. The 80-second time delay starts when vehicle speed reaches or maintains at least 32 km (20 mph). If the sensor reading is ever less than the displayed value or if the vehicle speed is 72 km (45 mph) or greater, there is no delay and ambient temperature changes are displayed as rapidly as possible.
Coolant Temperature Sensor
The coolant temperature sensor is located in a coolant passage of the intake manifold. Sensor resistance is monitored by the PCM which in turn sends the coolant temperature value to the heater and A/C programmer over the data circuit.
A/C High Temperature Sensor
The high side sensor is located in the high pressure refrigerant liquid line between the condenser and orifice tube. The heater and A/C programmer monitors the refrigerant temperature and transfers that data to the PCM to use in conjunction with the coolant temperature to determine the need for cooling fans, and to prevent the A/C compressor from operating at high discharge pressures.
A/C Low Temperature Sensor
The low side sensor is located in the low pressure refrigerant line between the orifice tube and evaporator. The heater and A/C programmer monitors this sensor to determine the low side pressure based on the pressure/temperature relationships of R-134a.
Sun Load Sensor
The sun load sensor uses a light sensitive photocell to control the signal voltage to the beater and A/C programmer. The heater and A/C programmer will see a low voltage in direct light and a higher voltage in shaded conditions.
A/C LOW PRESSURE SWITCH
The low pressure switch is located in the low pressure refrigerant line. When pressurized, the switch is closed and allows the compressor clutch relay to energize, feeding battery voltage to the compressor clutch coil. If the pressure drops below 69 kPa (10 psi), the switch opens and cuts off voltage to the clutch. There is a circuit to the heater and A/C programmer which monitors the state of the switch for diagnostic purposes.
A/C HIGH PRESSURE SWITCH
The high refrigerant pressure switch is located in the compressor discharge line and monitors the compressor discharge pressure. This switch acts as a safeguard to prevent refrigerant blow-off from the compressor pressure relief valve and disengages the compressor in the case of refrigerant pressure above 2965 kPa (430 psi). In this function it supplements the A/C high side temperature sensor.
Should pressures reach that level, the switch will open, cutting off the power supply to the relay and disengage the compressor clutch. As soon as the compressor shuts OFF, the pressures will begin to drop. When they drop to 1379 kPa (200 psi), the switch will close and allow the compressor to engage. Compressor cycling due to pressure switch cycling can occur under conditions such as high temperature, high humidity, extended idle, slow vehicle speed or tailwind, and is considered normal. Should cycling occur without the above conditions, a blocked line or condenser is possible and appropriate service should be performed.