412-04 Control Components: Description And Operation (page 11 of 21)

When A/C is selected, the ATCM maintains the evaporator at an operating temperature that varies with the in-vehicle 
cooling requirement. The ATCM increases the evaporator operating temperature, by reducing the refrigerant flow, as the 
requirement for air cooling decreases, and vice versa. During an increase of evaporator operating temperature, to avoid 
compromising the dehumidification function, the ATCM controls the rate of temperature increase, which keeps the cabin 
humidity at a comfortable level. 
 
When the economy mode is selected, the PWM signal holds the refrigerant solenoid valve in the minimum flow position, 
effectively switching off the A/C function. 
 
The ATCM incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high 
pressure limit, the duty cycle of the PWM signal is progressively reduced until the system pressure decreases. When the 
system pressure falls below the low pressure limit, the duty cycle of the PWM signal is held at its lowest setting, so that 
the A/C compressor is maintained at the minimum stroke, to avoid depletion of lubricant from the A/C compressor. The 
protection algorithm is calculated at a high rate, to enable early detection of the rapid pressure changes possible if a 
system fault develops. 
 

A/C Compressor Torque 

The ATCM uses refrigerant pressure, evaporator temperature and engine speed to calculate the torque being used to 
drive the A/C compressor. The calculated value is broadcast on the medium speed CAN bus for the Engine Control 
Module (ECM), which uses the calculated value for idle speed control and fueling control. The ATCM also compares the 
calculated value with a maximum A/C compressor torque value received from the ECM over the medium speed CAN bus. 
If the calculated value exceeds the maximum value, the ATCM signals the refrigerant solenoid valve to reduce the 
refrigerant flow, to reduce the torque being used to drive the A/C compressor. By reducing the maximum A/C compressor 
torque value, the ECM is able to reduce the load on the engine when it needs to maintain vehicle performance or cooling 
system integrity. 
 

Idle Speed Control 

In order to maintain A/C cooling performance, the ATCM requests an increase in engine idle speed if the evaporator 
temperature starts to rise while the refrigerant solenoid valve is already set to the maximum flow rate. The increase in 
engine idle speed is requested in three stages, using a medium speed CAN bus message to the Engine Control Module 
(ECM). For additional information, refer to   

Electronic Engine Controls

 (303-14A Electronic Engine Controls - 4.0L) 

For additional information, refer to   

Electronic Engine Controls

 (303-14B Electronic Engine Controls - 4.4L) 

For additional information, refer to   

Electronic Engine Controls

 (303-14C Electronic Engine Controls - 2.7L Diesel) 

 
 
The need for a change in idle speed is determined as follows: 
 

If the evaporator temperature increases by 3 °C (5.4 °F), or to 6 °C (10.8 °F) above the target operating 
temperature, over a 10 seconds period, the first stage of idle speed increase is requested.  
When the first stage of idle speed increase is set, if the evaporator temperature increases by 3 °C (5.4 °F), or 
increases to 12 °C (21.6 °F) above the target operating temperature, over a 9 seconds period, the second stage of 
idle speed increase is requested.  
When the second stage of idle speed increase is set, if the evaporator temperature increases by 3 °C (5.4 °F), or 
increases to 15 °C (27 °F) above the target operating temperature, over a 10 seconds period, the third stage of 
idle speed increase is requested.  
When an idle speed increase is set, if the evaporator temperature decreases by 3 °C (5.4 °F) over a 10 seconds 
period, the next stage down of idle speed increase is requested.  

Electrical Load Management 

The ATCM manages the vehicle electrical loads to: 
 

Maintain the vehicle battery in a healthy state of charge.  
Ensure adequate power is available for defrost demisting during engine warm-up.  
Ensure adequate power is available for A/C during extended periods with the engine at idle speed.  
To maintain system voltage within acceptable limits.  
To provide adequate power to meet customer expectations.  

Electrical load management is achieved by increasing the engine idle speed and controlling the electrical load of systems 
that do not affect the driveability or safety of the vehicle.