Bryant Expert Guidelines on Unloader Test Procedures and Thermistor Functions for Models

Unloader Test Procedure

The unloader is the compressor internal mechanism, controlled by the DC solenoid, that modulates between high and low stage. If it is suspected that the unloader is not working, the following methods may be used to verify operation.

1.Operate the system and measure compressor amperage. Cycle the unloader on and off at 30 second plus intervals at the UI (from low to high stage and back to low stage). Wait 5 seconds after staging to high before taking a reading. The compressor amperage should go up or down at least 20 percent.

2.If the expected result is not achieved, remove the solenoid plug from the compressor and with the unit running and the UI calling for high stage, test the voltage output at the plug with a DC voltmeter. The reading should be 4 to 18 volts.

3.If the correct DC voltage is at the control circuit molded plug, measure the compressor unloader coil resistance. The resistance should be 32 to 60 ohms depending on compressor temperature. If the coil resistance is infinity, much lower than 32 ohms, or is grounded, the compressor must be replaced.

Temperature Thermistors

Thermistors are electronic devices which sense temperature. As the temperature increases, the resistance decreases. Thermistors are used to sense outdoor air (OAT) and coil temperature (OCT). Refer to Fig. 34 for resistance values versus temperature.

If the outdoor air or coil thermistor should fail, the control will flash the appropriate fault code. (See Table 17.)

IMPORTANT: The outdoor air thermistor and coil thermistor should be factory mounted in the final locations. Check to ensure thermistors are mounted properly per Fig. 38 and Fig. 39.

OAT Thermistor must be locked in place with spherical nib end facing to- wards the front of the control box

Fig. 38 – Outdoor Air Thermistor (OAT) Attachment

Thermistor Sensor Comparison

The control continuously monitors and compares the outdoor air temperature sensor and outdoor coil temperature sensor to ensure proper operating conditions. The comparison is:

SIn cooling if the outdoor air sensor indicates ≥ 10_F warmer than the coil sensor (or) the outdoor air sensor indicates ≥ 20_F cooler than the coil sensor, the sensors are out of range.

SIn heating if the outdoor air sensor indicates ≥ 35_F warmer than the coil sensor (or) the outdoor air sensor indicates ≥ 10_F cooler than the coil sensor, the sensors are out of range.

If the sensors are out of range, the control will flash the appropriate fault code as shown in Table 17.

The thermistor comparison is not performed during low ambient cooling or defrost operation.

Failed Thermistor Default Operation

Factory defaults have been provided in the event of failure of outdoor air thermistor (OAT) and/or outdoor coil thermistor (OCT).

If the OAT sensor should fail, low ambient cooling will not be allowed and the one-minute outdoor fan off delay will not occur. Defrost will be initiated based on coil temperature and time.

If the OCT sensor should fail, low ambient cooling will not be allowed. Defrost will occur at each time interval during heating operation, but will terminate after 5 minutes.

If there is a thermistor out of range error, defrost will occur at each time interval during heating operation, but will terminate after 5 minutes.

Count the number of short and long flashes to determine the appropriate flash code. Table 17 gives possible causes and actions related to each error.

OCT Thermistor must be secured tight on stub tube.

Fig. 39 – Outdoor Coil Thermistor (OCT) Attachment

Table 16—Two-Stage Compressor Resistances

(Winding Resistance at 70_F±20_)

Winding	286ANA024	286ANA036	286ANA048	286ANA060
Start (S-C)	2.74	1.98	1.55	0.74
Run (R-C)	0.80	0.75	0.48	0.36
Winding	226ANA024	226ANA036	226ANA048	226ANA060
	288ANA024	288ANA036	288ANA048	288ANA060
Start (S-C)	1.40	1.29	1.52	0.60
Run (R-C)	1.32	0.89	0.64	0.49

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