Check Return Air Sensor (RAT)
electrical
Description
Return Air Temperature Sensor is out of -53 to +158°F (-47 to +70° C); the unit alarms and switches to supply air control, or enters Cargo Protect mode with 00123 active.
Activation Conditions
Return Air Temperature Sensor (RAT) is not within range of -53 to +158°F (-47 to +70° C).
Unit Behavior
If Alarm 00123 Check Air Supply Sensor is not active: Alarm and switch to supply air control. If Alarm 00123 Check Air Supply Sensor is active: Alarm and the System will enter Cargo Protect mode. Refer to Section 4.11.1.
Reset Conditions
Auto reset when sensor is in range or, alarm may be manually reset using the display mounted keys or by turning the unit off, then back on again.
Corrective Actions
1. Check Sensor a. Inspect sensor and connectors. Verify that there is no damage to sensor, and no damage, moisture or corrosion in connectors. b. Check sensor resistance. (Refer to Note 3 in Section 7.2) 10,000 Ohms @ 77°F (25°C). Refer to Section 8.9.16 for chart of resistances for different sensors. 2. Check Sensor Wiring a. Inspect connector pins and terminals at sensor connector 1-3MSM-1 and 27 (see wiring schematic). Verify that there is no physical damage to harness, and no damage, moisture or corrosion in connectors. b. Place system in PC mode. Refer to Note 2 in Section 7.2. Disconnect sensor from harness. Check for 3.0 ± 0.1 VDC at harness plug between pins. Voltage should be 3.0 ± 0.1 VDC volts at harness plug between pins. This verifies microprocessor output and wiring connections to the sensor.