Sunload Sensor through A/C Cycle
Sunload SensorThe sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are integrated into the sunload sensor assembly along with the ambient light sensor. Low reference and signal circuits enable the sensor to operate. As the sunload increases, the sensor signal decreases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.
The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If sensor is open, no sunload condition occurs. If sensor is shorted, maximum sunload condition occurs with increased cooling mode.
A/C Refrigerant Low Temperature Sensor
The dash integration module (DIM) monitors the sensor to determine low side pressure based on the pressure to temperature relationship of the R134a. This sensor is located on the low side line. The sensor is used to cycle the A/C compressor ON and OFF to prevent the evaporator core from freezing. A thermistor inside the sensor varies its resistance to monitor the temperature of the Freon. The DIM monitors the voltage drop across the thermistor when supplied with a 5-volt reference signal. The DIM will send a class 2 message to the PCM to stop requesting the A/C compressor clutch operation if the temperature drops below 2°C (28°F). Vehicles operating above 32 km/h (20 mph) will be controlled to a minimum ON time of 46 seconds before the DIM will allow the clutch to disengage. The sensor must be above 10°C (50°F) before the DIM will request A/C compressor clutch operation again.
The sensor operates within a temperature range between -40°C (-40°F) to 215°C (-355°F). If the DIM detects an open in the A/C refrigerant low temperature sensor or circuit, the class 2 message sent to the PCM will not allow A/C operation. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module (IPM) will also display A/C OFF on the module as long as the condition is present.
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5 volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.
The A/C refrigerant pressure sensor prevents the A/C system from operating when an excessively high or low pressure condition exists. The PCM disables the compressor clutch under the following conditions:
- A/C pressure is more than 2850 kPa (413 psi). The clutch will be enabled after the pressure decreases to less than 1750 kPa (254 psi).
- A/C pressure is less than 204 kPa (30 psi). The clutch will be enabled after the pressure increases to more than 220 kPa (32 psi).
If the PCM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module (IPM) will be invalid. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module will also display A/C OFF on the module as long as the condition is present.
Heating and A/C Operation
The purpose of the heating and A/C system is to provide the following:
- Heated air
- Cooled air
- Remove humidity from the interior of the vehicle
- Reduce windshield fogging
Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:
- Recirculation actuator setting
- Difference between inside and desired temperature
- Difference between ambient and desired temperature
- Blower motor speed setting
- Mode setting
- Auxiliary HVAC settings
The HVAC control module commands or monitors the following actions when an air temperature setting is selected. WARMEST POSITION - The air temperature actuator door position directs maximum air flow through the heater core. COLDEST POSITION - The air temperature actuator door position directs maximum air flow around the heater core. BETWEEN THE WARMEST AND COLDEST POSITION - The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:
- Sunload
- Ambient temperature
- Inside temperature
- Duct temperatures
The A/C system is engaged by selecting any switch on the HVAC control module (IPM) except the "A/C OFF" switch. The A/C switch will illuminate "A/C OFF" when the A/C switch is selected. The control module (IPM) sends a class 2 A/C request message to the powertrain control module (PCM) for A/C compressor clutch operation. The dash integration module (DIM) must communicate with the PCM in order for the A/C clutch to be engaged. The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 125°C (257°F). The following conditions must be met in order for the PCM to turn on the compressor clutch from the IPM request:
- DIM will allow A/C operation if the following limits are within normal operating range
- A/C line pressure
- A/C refrigerant low temperature
- Engine coolant temperature
- Battery voltage
- HVAC control module (IPM)
- Evaporator Temperature more than 4°C (39°F)
- Control module operating range 9 and 16 volts
- PCM
- Engine coolant temperature (ECT) is less than 128°C (262°F)
- Engine RPM is more than 0 RPM
- A/C Pressure is between 2850 kPa (413 psi) and 204 kPa (30 psi).
Once engaged, the compressor clutch will be disengaged for the following conditions:
- Compressor thermal switch is opened
- Throttle position is 100%
- A/C Pressure is more than 2850 kPa (413 psi)
- A/C Pressure is less than 204 kPa (30 psi)
- Engine coolant temperature (ECT) is more than 128°C (262°F)
If there is a malfunction in the A/C system, the driver information center will read "SERVICE A/C SYSTEM" to alert the driver. When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.
Dual Zone Operation
The HVAC control module has temperature settings for the driver and the passenger. If the passenger's setting is turned off then the driver's setting controls both driver and passenger temperature actuators. The passenger's setting can not be used without the driver's setting also being ON. The passenger's setting can be turned ON or OFF by pressing the temperature switch on the passenger's side of the HVAC control module (IPM). When the passenger's setting is ON, the passenger temperature can be adjusted independently from the driver's setting and the passenger temperature is displayed on the passenger's side of the control module. A different sunload or duct temperature on one side of the vehicle may cause different discharge air temperatures even when the passenger's setting is not turned ON.
Auxiliary Heating and A/C Operation
The auxiliary HVAC control module (IPM) provides additional airflow and temperature offset for the back seat passengers. The auxiliary blower motor will operate as long as the IPM is ON. Passengers can operate the system in both manual or automatic modes. The only time the auxiliary HVAC control module can not be controlled is when the IPM is in FRONT DEFROST. The auxiliary blower motor icon will flash to indicate that the blower is only available in low speed. Auxiliary HVAC temperatures can be set 4°C (7°F) cooler or warmer than the primary setting.
The IPM provides power and ground to the auxiliary air temperature actuator. The IPM receives power from the rear fuse block through the ignition 3 voltage circuit.
The auxiliary air temperature actuator, located in the air temperature actuator assembly, is a reverse polarity motor. Each circuit provides both power and ground to the auxiliary air temperature actuator. When the auxiliary air temperature actuator is being held in position, both of the auxiliary air temperature door control circuits have 12 volts applied to both sides of the actuator motor. This holds the actuator stationary. When a cooler temperature is requested, one of the auxiliary air temperature door control circuits will ground, driving the auxiliary air temperature actuator to the desired temperature. When a warmer temperature is requested, the other auxiliary air temperature door control circuit will ground. This moves the auxiliary air temperature actuator into the desired offset.
Automatic Operation
In automatic operation, the HVAC control module (IPM) will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.
To place the HVAC system in automatic mode, the following is required:
- The blower motor switch must be in the AUTO position.
- The air temperature switch must be in any other position other than 60 or 90 degrees.
- The mode switch must be in the AUTO position.
Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected (except in the extreme temperature positions. The HVAC control module (IPM) performs the following functions to maintain the desired air temperature:
- Monitors the following sensors:
- Inside air temperature sensor
- Ambient air temperature sensor
- Upper duct sensors
- Lower duct sensors
- Sunload sensor
- Regulate blower motor speed
- Position the air temperature actuator
- Position the mode actuator
- Position the recirculation actuator
- Request A/C operation
When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation the blower will stay on high and the air temperature actuators will stay in the full cold position.
In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.
Steering Wheel Controls
The HVAC control module receives class 2 messages that the driver has activated a steering wheel control switch from the radio interface. The HVAC steering wheel control button controls driver set temperature increase and driver set temperature decrease.
- Fan speed increase
- Fan speed decrease
- Driver set temperature increase
- Driver set temperature decrease
The HVAC system interprets the set temperature switch on the steering wheel as if the driver had activated the same switch function on the HVAC control module (IPM).
Engine Coolant
Engine coolant is the key element of the heating system. The engine thermostat controls the normal engine operating coolant temperature. Coolant pumped out of the engine enters the heater core through the inlet heater hose. The air flowing through the HVAC module absorbs the heat of the coolant flowing through the heater core. The coolant then exits the heater core through the heater outlet hose and returns back to the engine block.
A/C Cycle
Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air. The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.
Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.
The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the orifice tube.
The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the orifice tube flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs the moisture and heat from the ambient air. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.
The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.