Trane WCY060G1, WCY042G1, WCY048G1 manual Demand Defrost System, Defrost Control, Fault Detection

DEMAND DEFROST SYSTEM

DEFROST CONTROL

The demand defrost control measures heat pump outdoor ambient temperature with a sensor located outside the outdoor coil. A second sensor located on the outdoor coil is used to measure the coil temperature. The difference between the ambient and the colder coil temperature is the difference or delta-T measurement. This delta-T measurement is representative of the operating state and relative capacity of the heat pump system. By measuring the change in delta-T, we can determine the need for defrost. The coil sensor also serves to sense outdoor coil temperature for termina- tion of the defrost cycle.

FAULT DETECTION

A fault condition is indicated by the flashing light on the defrost control inside the heat pump control box.

In normal operation, the defrost control light will flash once each second. If the light is flashing more than once per second or not-at- all, refer to the service manual for that unit.

PIN IDENTIFICATION (See Figure)

1.TEST_COMMON (Shorting any of the other pins to this pin causes the function of the other pin to be executed. Leaving this pin open results in the normal mode of operation.)

2.TST = Test (Shorting TEST_COMMON to this pin speeds up all defrost board timings.)

3.FRC_DFT = Forced Defrost (Short TEST_COMMON to this pin for two (2) seconds to initiate a forced defrost. Remove the short after defrost initiates.)

PIN IDENTIFICATION

FRC_DFT

TEST_COMMON

TST

DEFROST CONTROL CHECKOUT

Normal operation requires:

a.LED on board flashing 1 time/second.

b.24V AC between R & B

c.24V AC between Y & B with unit operating

d.Defrost initiation when FRC_DFT pin is shorted to TEST_COMMON pin.

If a defrost control problem is suspected, proceed to DEMAND DEFROST CONTROL CHECKOUT in this Service Facts.

▲WARNING: Do NOT connect 24 VAC to T1 (ODS-A) terminal. ODS-A thermistor WILL BE BLOWN.

PROCEDURE FOR TESTING SENSORS

Measure the temperature the subject sensor is exposed to. If the sensor is mounted on a tube, place the lead on an Annie A-8 (or equiv.) temperature tester on the same tube near the sensor and insulate the bulb.

Unplug the sensor and measure the resistance with a good quality ohmmeter (Simpson 260 or equiv.). Read the value as quickly as possible to prevent the meter current from changing the resistance reading.

Using the chart on the right, locate (as close as possible) the actual sensor temperature. The measured resistance should be relatively close to the resistance value shown in the chart.

Example:
Sensor temp.	= 19°F.
Measured Resistance	= 46K ohms

This sensor is good since the measured value is relatively close to the chart value.

FLASH RATE INDICATIONS:

TEMP °F	RESISTANCE
86	7.85K
81	8.85K
75	10.24K
70	11.59K
65	13.14K
59	15.32K
55	16.93K
50	19.41K
45	22.20K
41	24.76K
36	28.45K
32	31.84K
25	37.50K
19	46.44K
14	53.94K
10	57.64K
5	67.06K
0	78.05K

1	— Flash/Second	= Normal operation.
2	— Flashes/Second	= Fault A (Low ∆T) i.e.: Inoperative compressor, loss of charge, open ambient sensor, shorted coil sensor.
3	— Flashes/Second	= Fault B (20 defrosts terminated on time)
		Fault C (High ∆T) i.e.: SOV stuck in heating, shorted ambient sensor, open coil sensor, closed TXV,
		OD motor failure, OD fan on in defrost, undercharged unit.
4	— Flashes/Second	= Fault A & C or A & B

For additional information consult Pub. No. 34-1001 Heat Pump Defrost Controls.

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