Service Manual

14
To fully troubleshoot defrost thermostat and control function
(FAST # 1173636 / 1177927 / 1190281):
1. Turn thermostat to OFF. Shut off all power to outdoor
unit.
2. Remove control box cover for access to electrical
components and defrost control board.
3. Disconnect defrost thermostat leads from control
board, and connect to ohmmeter. Thermostat leads
are black, insulated wires connected to DFT and R
terminals on control board. Resistance reading may
be zero (indicating closed defrost thermostat), or
Communicating ( for open thermostat) depending on
outdoor temperature.
4. Jumper between DFT and R terminals on control
board as shown in Fig. 8.
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 mode, allowing frost to
accumulate on outdoor coil.
8. After a few minutes in heating mode, liquid line
temperature at defrost thermostat should drop below
closing set point of defrost thermostat of
approximately 32_F. Check resistance across defrost
thermostat leads using ohmmeter. Resistance of zero
indicates defrost thermostat is closed and operating
properly.
9. Short between the speed- up terminals using a
thermostat screwdriver. This reduces the timing
sequence to 1/256 of original time. (See Table 2.)
Table 2—Defrost Control Speed- Up Timing Sequence
PARAMETER
MINIMUM
(MINUTES)
MAXIMUM
(MINUTES)
SPEED- U P
(NOMINAL)
30- minute cycle 27 33 7sec
60- minute cycle 56 66 14 sec
90- minute cycle 81 99 21 sec
10- minute cycle 9 11 2sec
5- minutes 4.5 5.5 1sec
UNIT DAMAGE HAZARD
Failure to follow this caution may result in equipment
damage or improper operation.
Exercise extreme caution when shorting speed- up
pins. If pins are accidentally shorted to other terminals,
damage to the control board will occur.
CAUTION
!
10. Unit is now operating in defrost mode. Check
between C and W2 using voltmeter. 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 2 sec
to remove speed- up jumper after unit has switched to
defrost, the unit will switch back to heat mode.
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, indicating defrost
thermostat has opened at approximately 65_F.
13. Shut off unit power and reconnect fan lead.
14. Remove jumper between DFT and R terminals.
Reconnect defrost thermostat leads. Failure to
remove jumper causes unit to switch to defrost every
30, 60, or 90 minutes and remain in defrost for full 10
minutes.
15. 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 defective thermostat as follows:
1. Follow items 1- 5 above.
2. Route sensor or probe underneath coil (or other
convenient location) using thermocouple temperature
measuring device. Attach to liquid line near defrost
thermostat. Insulate for more accurate reading.
3. Turn on power to outdoor unit.
4. Restart unit in heating.
5. Within a few minutes, liquid line temperature drops
within a range causing defrost thermostat contacts to
close. Temperature range is from 33_Fto27_F.
Notice temperature at which ohmmeter reading goes
from  to zero ohms. Thermostat contacts close at
this point.
6. Short between the speed- up terminals using a small
slotted screwdriver.
7. Unit changes over to defrost within 21 sec (depending
on timing cycle setting). Liquid line temperature rises
to range where defrost thermostat contacts open.
Temperature range is from 60_Fto70_F. Resistance
goes from zero to  when contacts are open.
8. If either opening or closing temperature does not fall
within above ranges or thermostat sticks in 1 position,
replace thermostat to ensure proper defrost
operation.
NOTE: With timing cycle set at 90 minutes, unit initiates
defrost within approximately 21 sec. When you hear the
reversing valve changing position, remove screwdriver
immediately. Otherwise, control will terminate normal
10- minute defrost cycle in approximately 2 sec.
DEFROST CONTROL (Fast # 1174185)
This defrost control is used in most Performance Series
heat pumps with R- 410A refrigerant. Its features include
selectable defrost intervals of 30, 60, 90, & 120 minutes,
Quiet Shift, compressor time delay, deluxe defrost speed up
capability. This section describes the sequence of operation
and trouble shooting methods for this control.