Blower Control

Blower control
The ATC ECU operates a blower relay, power transistor and power relay to run the blower at one of 31 stepped speeds. All speed steps are available in the automatic modes of blower control. In the manual mode, speed steps 3, 10, 16, 22 and 31 are used to provide slow, three intermediate and fast blower speeds. The ATC ECU energises the blower relay and modulates the power transistor to operate the blower for speed steps 1 to 30. For speed step 31, the ATC ECU energises the power relay, which switches the earth side of the blower motor direct to earth, bypassing the power transistor.

In the automatic, economy and defrost modes, blower speed is corrected for vehicle speed to compensate for the increase in ram effect on the inlet air as the vehicle speed increases. Correction begins at approximately 50 km/h, when blower speed is progressively decreased as vehicle speed increases, until a maximum decrease of 13 steps occurs at 123 km/h. Similarly, blower speed increases as vehicle speed decreases down to approximately 50 km/h.

In the automatic and economy modes, if the LH or RH temperature is set to LO or HI, the blower runs at maximum speed with correction only for vehicle speed. If both the LH and RH outlet air temperatures are set to a specific temperature, blower speed corrections are added to compensate for the heater coolant temperature, external air temperature, and the solar load acting on the vehicle:

^ During warm-up, the blower speed is set to 3 while the heater coolant temperature is below approximately 20 degrees C (68 degrees F). From approximately 20 degrees C (68 degrees F), the blower speed is progressively increased as the coolant temperature increases, until maximum speed is set at approximately 50 degrees C (122 degrees F).
^ During cool down, blower speed is set to 3, for 5 seconds after the system is switched on. Over the following 6 seconds, the blower speed is progressively increased up to maximum speed.
^ As the temperature in the cabin approaches the selected temperatures, blower speed is progressively reduced until, once the selected temperatures have been established, blower speed stabilises at approximately 6.
^ Solar heating correction is employed when air distribution is set to face level or to face and footwells. The correction progressively increases the blower speed, up to a maximum of 9 steps, with increasing values of solar heating.