Page created by T October 6th 2007:
HQ-WB Air Conditioning Temperature Control:
mb is correct, the GM A6 compressor as fitted to HQ-WB's runs whenever the air-con is switched on. They do not have a thermostat cut-out, or any other type of electrical temperature control system. The type of AC system fitted to these models uses a device called a Suction Throttling Valve (STV), other manufacturers refer to the same device as an Evaporator Pressure Regulator (EPR). This device is located between the output of the evaporator and the input of the compressor. The purpose of this device is to limit the minimum pressure in the evaporator (the part of the AC system that gets cold). If this device does not do its job then the pressure in the evaporator will get too low and the evaporator will turn into a big ice-block. Once this happens, air can no longer pass through the core and the system stops working. Since the temperature of the refrigerant is constantly proportional to the pressure, the STV is precisely set to control the pressure (and therefore the temperature) to just above freezing point. This pressure is 30 psi for R12, and 28 psi for R134a. The STV should be adjusted to match the type of refrigerant being used. This system works fine with the old GM A6 compressor, they are only a 3 cylinder compressor and basically just stop producing any substantial output pressure once the STV starts restricting (or throttling) the input. The compressor just chuggs away doing almost no work. Once the evaporator temperature starts to rise, the STV opens up and the compressor starts making some output pressure. If you remove the GM A6 compressor and replace it with a Sanden SD508 or SD708 etc, or a new type scroll compressor (eg TRF105), the low side (suction) capability of these compressors is actually great enough to "suck" the STV open, causing it to malfunction and therefore allowing the evaporator pressure to drop, uncontrolled, and then freezing up. There are two ways to overcome this. The most logical way is to use a generic automotive AC thermostat, wired in series with the compressor clutch. The thermostat probe must be in direct contact with the evaporator core to directly sense it's temperature. The thermostat should be set to a minimum of about 1 - 2 degrees C. The other way is to use an evaporator pressure switch. This can be fitted at the low side test port. The switch senses the evaporator pressure and cuts off the compressor clutch once the pressure gets down to the set point. These switches are designed with a damping feature, which prevents the compressor from cycling too quickly. The switches are pre-set, and must be chosen to suit the type of refrigerant being used. Brett.
It's a front air con compressor bracket from 253/308 HQ (and probably J/X/Z too). Jeff.