Ch 6 page 23 ELECTRICAL SYSTEM The pressures, temperature, engine rpm, AC clutch signals and evaporator temperature, can be viewed on the VCU diagnostic page on the display in the cabin. There are a few items related to A/C on the diagnostic page, such as: A/C LP . Gives the pressure in the low pressure part of the A/C system. A/C HP . Gives the pressure in the high pressure part of the A/C system. A/C clutch input . Gives the status of the signal coming out of ATC to the VCU A/C clutch input (Y19,85). “On” means that the control panel asks for the activation of the compressor. Also shown is the voltage on the input pin of the VCU.
“On” corresponds to 0 - 3 Volt on the input pin, “Off” when voltage is higher than 3 Volt. A/C clutch output (Y10,6 & Y10,10). This indicates if the compressor should be On or Off. Both wires of the compressor clutch or connected to the VCU. Evaporator temperature . The VCU uses this to decide if the compressor can be on. If the temperature is below 2 to 3 °C then this is not allowed, to prevent freezing of the evaporator. Figure 30 Blower control The blower in the HVAC unit is connected to a speed controller unit mounted on top of the blower. The control panel gives a signal to the speed controller in the HVAC unit.
This signal is between 0 - 5 V that corresponds with the speed of the blower. Water valve This valve controls the amount of coolant water that goes through the heater core of the HVAC unit. It uses a motor to set the valve opening and a potentiometer to report back the position of the valve to the control panel. The motor is controlled by supplying power to pins 4 & 6 on the valve connector. Applying 24 V to pins 6-4 will open the valve. Applying 24 V to pins 4-6 will close it. Figure 31 SHOP MANUAL