22
18-BB35D1-16B-EN
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.)
DDeeffrroosstt CCoonnttrrooll CChheecckkoouutt
Normal operation requires:
• LED on board flashing 1 time/second.
• 24V AC between R & B
• 24V AC between Y & B with unit operating
• Defrost initiation when FRC_DFT pin is shorted to
TEST_COMMON pin.
If a defrost control problem is suspected, proceed to
the Demand Defrost Control Checkout table.
TTeesstt SSeennssoorrss
Using the chart below, 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.
1 — Flash/
Second
= Normal operation.
2 — Flashes/
Second
= Fault A (Low ΔT): Inoperative compressor,
loss of charge, open ambient sensor, shorted
coil sensor.
3 — Flashes/
Second
= Fault B (20 defrosts terminated on time)
Fault C (High ΔT): 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.
TEMP (ºF)
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
Table 12. Demand Defrost Control Checkout Table
SYMPTOMS CHECKS
YES/NO
ACTIONS
1. LED off.
1.
24V R-B & Y-B at board, with unit
running?
N
Repair low voltage wiring.
Y
Complete Check #2.
2.
Short FRC_DFT pin to TEST_COMMON
pin. Is defrost cycle initiated?
N
Replace defrost control.
Y LED is bad but control will still function.
2.
LED flashing very rapidly
(greater than 4 times/ sec) or
appears to be on continuously.
TEST_COMMON pin shorted to TST pin?
Y Remove short.
N
Replace defrost control.
UUnniitt SSttaarrttuupp
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