307D, 311D, 312D, 313D, 315D, 318D, 319D, 320D, 323D, 324D, 325D, 326D, 328D, 329D, 330D, 336D, 340D, 345D, 349D, 374D, 390D and M325D Excavators Air Conditioning and Heating Air Conditioning and Heating System Sensor Operation Caterpillar


Air Conditioning and Heating System Sensor Operation
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1.1. Water Temperature Sensor
2.1. Air Temperature Sensor (Recirculated Air)
3.1. Evaporator (Frost) Sensor
4.1. Relay Switch

Water Temperature Sensor



Illustration 1g00468069

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 module. This sensor is located at the back of the heater core.

Air Temperature Sensor (Recirculated Air)



Illustration 2g00468147

The sensor detects the temperature in the cab. The sensor then sends the signal back to the control module. This sensor is located at the inlet for recirculated air.

Evaporator (Frost) Sensor



Illustration 3g01130938
(R) Resistance (Ω)
(T) Temperature °C (°F)

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 (8). 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 4g01130997

A relay switch is used to activate the air conditioner clutch. This relay is also used to deactivate the air conditioner clutch.

The HI fan speed is also controlled by this type of relay.

Reference: Refer to Schematic, "Air Conditioning Electrical System" for the location of these two relays.

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