Carrier 48/50PD05 manual Thermistor Troubleshooting, Air Temperatures, Refrigerant Temperatures

Thermistor Troubleshooting

The electronic control uses thermistors to sense temperatures used to control operation of the unit. Resistances at various temperatures are listed in Table 15-17. Thermistor pin connection points are shown in the Major System Components section. The general locations of the thermistors are shown the Major System Components section.

Air Temperatures

Air temperatures are measured with 10 kilo-ohm thermistors. This includes supply-air temperature (SAT), outdoor-air temperature (OAT), space temperature sensors (T55, T56, T58), and return air temperature (RAT).

The supply air temperature (SAT), return air temperature (RAT) and outdoor air temperature (OAT) thermistors use a snap-mount to attach through the unit sheet metal panels. The snap-mount tabs must be flattened on the tip end of the sensor to release for removal from the panel. (See Fig. 10.) To reinstall, make sure the snap-mount tabs extend out.

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Fig. 10 - SAT, RAT and OAT Thermistor Mounting

Refrigerant Temperatures

Condenser coil temperatures are measured with 5 kilo-ohm thermistors. These measurements provide an approximate saturated condensing temperature for each circuit (SCT.A). Fig. 11 shows the factory locations for the SCT thermistors on 48/50PD units. Ensure that thermistors are placed at the correct location and are snapped securely over the return bend so that contact is made between the thermistor and the tube.

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Fig. 11 - Saturated Condensing Temperature Thermistor

Location

Thermistor/Temperature Sensor Check

A high quality digital volt-ohmmeter is required to perform this check.

Connect the digital voltmeter across the appropriate thermistor terminals at the J8 terminal strip on the Main Base Board (see Major System Components section).

Using the voltage reading obtained, read the sensor temperature from Table 15-17.

To check thermistor accuracy, measure temperature at probe

location with an accurate thermocouple-type temperature-measuring instrument. Insulate thermocouple to avoid ambient temperatures from influencing reading. Temperature measured by thermocouple and temperature determined from thermistor voltage reading should be close, within 5°F, if care was taken in applying thermocouple and taking readings.

If a more accurate check is required, unit must be shut down and thermistor removed and checked at a known temperature (freezing point or boiling point of water) using either voltage drop measured across thermistor at the J8 terminal, or by determining the resistance with unit shut down and thermistor disconnected from J8. Compare the values determined with the value read by the control in the Temperatures mode using the Scrolling Marquee display.

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