CAUTION: Wear safety glasses and gloves when work- ing with refrigerants.
Apply heat with torch to solder joint and remove switch. Wear safety glasses when using torch. Have quenching cloth available. Oil vapor in line may ignite when switch is removed. Braze in 1/4-in. flare fitting and screw on replacement pressure switch.
HIGH-PRESSURE SWITCH
Located on discharge line, the high-pressure switch protects against high-discharge pressures caused by such events as over- charge, condenser-fan motor failure, system restriction, etc. It opens on pressure rise at about 435 psi. If system pressures go above this setting during abnormal conditions, the switch opens. Do not attempt to simulate these system abnormalities as high pressures pose a serious safety hazard. High-pressure switch is also checked with an ohmmeter similar to checking low-pressure switch. If system pressure is below 435 psi, the switch shows continuity. It is replaced in the same manner as low-pressure switch. Observe all safety precautions.
LIQUID-LINE PRESSURE SWITCH
Located on liquid line of heat pump only, the liquid-line pressure switch functions similar to conventional low-pressure switch. Because heat pumps experience very low suction pressures during normal system operation, a conventional low-pressure switch cannot be installed on suction line. This switch is installed in liquid line instead and acts as loss-of-charge protector. The liquid line is the low side of the system in heating mode. It operates identically to low-pressure switch except it opens at 7 psi when the heating piston is in the liquid valve or 27 psi when the heating piston is in the liquid line. Troubleshooting and removing this switch is identical to procedures used on other switches. Observe same safety precautions.
VIII. DEFROST THERMOSTATS
Defrost thermostat signals heat pump that conditions are right for defrost or that conditions have changed to terminate defrost. It is a thermally actuated switch clamped to outdoor coil to sense its temperature. Normal temperature range is closed at 30° ± 3 °F and open at 80° ± 5 °F.
NOTE: The defrost thermostat must be located on the liquid side of the outdoor coil on the bottom circuit and as close to the coil as possible.
IX. DEFROST-CONTROL BOARD
Solid-state defrost boards used on heat pumps replace electrome- chanical timer and defrost relay found on older defrost systems. The defrost-control board can be field-set to check need for defrost every 30, 50, or 90 minutes of operating time by connecting the jumper (labeled W1 on the circuit board) to the terminal for the defrost time desired. The board is set at factory for 90 minutes. The defrost period is field-selectable, depending upon geographic areas and defrost demands. Two types of defrost boards are used, and functions are described in the sections to follow.
Troubleshooting defrost control involves a series of simple steps that indicate whether or not board is defective.
NOTE: This procedure allows the service technician to check control board and defrost thermostat for defects. First troubleshoot to make sure unit operates properly in heating and cooling modes. This ensures operational problems are not attributed to the defrost- control board.
HK32FA003, 006 DEFROST CONTROL
This control board utilizes screw terminals for the low-voltage field wiring. The board has a feature that allows the heat pump to restart in defrost if room thermostat is satisfied during defrost. To troubleshoot the board, perform the following items.
1. Turn thermostat to OFF. Shut off all power to outdoor unit.
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2.Remove control-box cover for access to electrical compo- nents and defrost-control board.
3.Disconnect defrost-thermostat leads from control board and connect to ohmmeter. Thermostat leads are the black, insulated wires connected to DFT and R terminals on
control board. Resistance reading may be zero (indicating closed defrost thermostat) or infinity (∞ for open thermo- stat) depending on outdoor temperature.
4.Jumper between DFT and R terminals on control board as shown in Fig. 19.
5.Disconnect outdoor fan motor lead from OF2. Tape lead to prevent grounding.
6.Turn on power to outdoor unit.
7.Restart unit in heating, allowing frost to accumulate on outdoor coil.
8.After a few minutes in heating, liquid-line temperature at defrost thermostat should drop below closing set point of defrost thermostat of approximately 30°F. Using ohmmeter, check resistance across defrost-thermostat leads. Resistance of zero indicates defrost thermostat is closed and operating properly.
9.Remove protective cover from TP1 and TP2 speed-up terminals. Install jumper wire on speed-up terminals. This reduces the timing sequence to 1/60 of original time. (See Fig. 20.) Since Fig. 20 shows timing cycle set at 30 minutes, unit initiates defrost within approximately 30 sec; if setting is at 50 minutes, within 50 sec; 90 minutes, within 90 sec. When you hear the reversing valve change position, remove protective cover/jumper. Otherwise, control will terminate normal 10-minute defrost cycle in approximately 10 sec.
CAUTION: Exercise extreme caution when shorting speed-up pins. If pins are accidentally grounded, damage to the control board will occur.
10.Unit is now operating in defrost mode. Using voltmeter, check between C and W2 as shown in Fig. 21. Reading on voltmeter should indicate 24v. This step ensures defrost relay contacts have closed, energizing supplemental heat (W2) and reversing valve solenoid (O).
11.Unit should remain in defrost no longer than 10 minutes. Actual time in defrost depends on how quickly speed-up jumper is removed. If it takes 3 sec to remove speed-up jumper after unit has switched to defrost, only 7 minutes of defrost cycle remains.
12.After a few minutes in defrost (cooling) operation, liquid line should be warm enough to have caused defrost- thermostat contacts to open. Check resistance across defrost thermostat. Ohmmeter should read infinite resistance, indi- cating defrost thermostat has opened at approximately 80°F.
13.Shut off unit power and reconnect fan lead.
14.Remove jumper wire from speed-up terminal and reinsert cover on speed-up terminals. Failure to remove jumper causes unit to speed up operating cycles continuously.
15.Remove jumper between DFT and R terminals. Reconnect defrost thermostat leads.
16.Replace control-box cover. Restore power to unit.
If defrost thermostat does not check out following above items or incorrect calibration is suspected, check for a defective thermostat as follows.
1. Follow items 1-5 above.
