Ch 6 page 22 ELECTRICAL SYSTEM HVAC Figure 29 There are several electrical parts involved in controlling the temperature and air stream in the cabin. There are: 1. HVAC unit . Inside are the heater core, evaporator, water valve, re-circulation flap motor, blower + control, pressure sensors, temperature sensors and the A/C thermostat. The electrically operated water valve is used to regulate the amount of engine coolant coming into the heater core. In this way the amount of heat is controlled. 2. Control panel (ATC) . This commands the water valve, the blower speed, the re-circulation flap and gives command to activate the compressor.
It reads the air temperature from sensors inside the HVAC unit. 3. VCU . This powers the control panel and the HVAC unit. It can also activate the A/C compressor clutch when the control panel signals this and both pressure and evaporator temperature allow it. A separate control is used to put the control panel in a preset mode. This is used in combination with an external engine heater (webasto). 4. Ambient temperature sensor . Used for the protection of the A/C compressor activation. 5. A/C compressor clutch . Electrical operated clutch that is part of the A/C compressor. When activated and the engine is running, the compressor will pump the coolant through the A/C system. 6.
Pressure sensors , both placed on low and high pressure tubes going into the HVAC unit. A/C compressor The control panel can decide that the A/C compressor should be activated. This can be either when the panel is operating in auto or manual mode. A signal called “Compressor Cmd” is pulled low (below 3V). This is passed to the VCU and will decide based on pressure, evaporator temperature if it can switch on the compressor. The VCU will do several checks to see if it can activate the compressor. It uses several sensor signals for this. These are low and high pressure sensors, an ambient temperature sensor outside the cabin and a temperature sensor inside the evaporator.
The compressor will be activated by the VCU if all of the following conditions are met: Low pressure is between 0.4 - 19 bar (5.8 - 276 psi). High pressure is between 2.5 - 19 bar (36 - 276 psi). Ambient temperature is higher than 3 °C (37 °F). The engine is running. A/C clutch has not been activated the last 15 seconds. This is to increase the lifetime of the compressor. SHOP MANUAL