Relays and Switches

CCOT Refrigerant Lines And Switches:

High Pressure Relief Valve

The compressor is equipped with a pressure relief valve which is placed in the system as a mechanical safety device. Under certain conditions, refrigerant pressure on the compressor discharge side may exceed the designed operating level. To prevent system damage, the valve is designed to open automatically at approximately 3654 + 483 kPa (530 + 69 psi) and vent refrigerant to the atmosphere. Any condition that causes this valve to open should be corrected, and the refrigerant oil and refrigerant should he replaced as necessary.

Pressure Cycling Switch

Control of the refrigeration cycle (on and off operation of the compressor) is done with a pressure cycling switch which senses low-side (suction) pressure as an indicator of evaporator temperature. The switch is mounted on a fitting that is welded to a line. The switch fitting contains a standard Schrader-type valve so that the refrigerant charge does not have to be recovered from the system when replacing the switch.

With outside temperature at 16-26°C (60-80°F) and the system turned off, the equalized pressures within the charged A/C system are sufficient to close the contacts on the pressure cycling switch. When an air conditioning mode is selected, voltage is applied to the powertrain control module (PCM) through the switch. The PCM then energizes the A/C compressor control relay to energize the compressor clutch coil. As the compressor runs, low-side (suction) pressure decreases and high-side (discharge) pressure increases. When suction pressure drops to approximately 152 kPa (22 psi), the pressure cycling switch opens, de-energizing the compressor clutch. With the compressor stopped, system pressures again begin to equalize. When low-side pressure rises to approximately 296 kPa (43 psi) the switch contacts close, the clutch coil energizes and the compressor begins to run. This cycling process continues and maintains evaporator discharge air temperature at approximately 6°C (43°F).

Because the compressor is cycled on and off, some slight increases and decreases of engine speed/power may be noticed under certain conditions.

A/C Pressure Fan Switch

The A/C pressure fan switch is a normally closed switch that monitors high side pressure during A/C operation. When high side pressure reaches 1448 kPa (210 ± psi) the switch opens signaling the PCM to turn the primary cooling fan to high speed. The switch closes once high side pressure drops to approximately 1103 kPa (160 psi).

A/C High Side Temperature Sensor

The high side temperature sensor is located in the high pressure refrigerant liquid line between the condenser and orifice tube. The programmer monitors the refrigerant temperature and transfers that data to the PCM to use in conjunction with the coolant temperature to determine the need for cooling fans, and to prevent the A/C compressor from operating at high discharge pressures.

A/C Low Side Temperature Sensor

The low side temperature sensor is located in the low pressure refrigerant line between the orifice tube and evaporator. The programmer monitors this sensor to determine the low side pressure based on the pressure/temperature relationships of R-134a.

A/C Compressor Control Relay

The A/C compressor control relay cycles the compressor on and off by energizing/de-energizing the compressor clutch coil.

With the key on, 12 volts is applied to the relay common contact and one coil terminal. The second coil terminal is grounded by the PCM when the driver selects A/C or defrost modes, and the pressure cycling switch is closed.

When the relay is energized current passes through the relay contacts which energizes the compressor clutch coil.