SWITCH-PRESSURE 3142877 - Caterpillar

Water Temperature Sensor
Illustration 1 g00468069
The water temperature sensor detects the temperature of the engine coolant that flows into the heater core. The sensor then sends the signal back to the control panel. This sensor is located at the back of the heater core. The water temperature sensor can be removed by pulling out.
Table 1
Water Temperature    Resistance (Ohms)    
-30°C (-22°F)     91.4 kOhms    
25°C (77°F)     5 kOhms    
100°C (212°F)     0.321 kOhms    Air Temperature Sensor (Recirculated Air)
Illustration 2 g00468147
The sensor detects the temperature in the cab. The sensor then sends the signal back to the control panel. This sensor is located at the inlet for recirculated air. This sensor can be removed by removing one clamp.
Table 2
Air Temperature    Resistance (Ohms)    
0°C (32°F)     1.645 kOhms    
25°C (77°F)     5 kOhms    Evaporator (Frost) Sensor
Illustration 3 g00468174
(R) Resistance (Ohms) (T) Temperature (°C)The evaporator (frost) sensor is a semiconductor. When temperature increases, resistance decreases. On the other hand, when temperature decreases, resistance increases.The evaporator (frost) sensor is located at the discharge side of evaporator (13). The evaporator (frost) sensor detects the temperature of the cooled air as the air is discharged from evaporator (13). The sensor then sends the signal back to the amplifier in the control panel.The control panel switches the air conditioner clutch OFF when the temperature is under 3°C (37°F). When the air temperature becomes 4°C (39°F) or more, the air conditioner clutch is turned ON. This sensor also prevents the evaporator from freezing.Relay Switch
Illustration 4 g00468365
A relay switch is used to activate the air conditioner clutch. This relay is also used to deactivate the air conditioner clutch.
Table 3
Specifications for Relay    
Rated Voltage     24 VDC    
Rated Current     16 A    
Rated Current at Coil     0.1 A    The HI fan speed is also controlled by this type of relay.Reference: Refer to Systems Operation, "Air Conditioning and Heating System Schematic" for the location of these two relays.