Page 15
Installer’s Guide
FLASH RATE INDICATIONS:
1 — Flash/Second
= Normal operation.
2 — Flashes/Second
= Fault A (Low DT) i.e.: Inoperative compres-
sor, loss of charge, open ambient sensor,
shorted coil sensor.
3 — Flashes/Second
= Fault B (20 defrosts terminated on time)
Fault C (High DT) 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. 34-1005-03 Heat Pump
Defrost Controls.
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.
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 termination 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 func-
tion 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.)
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 CON-
TROL CHECKOUT in this Service Facts.
Figure 8. Pin Identification
FRC_DFT
TEST_COMMON
TST
Do NOT connect 24 VAC to T1 (ODS-A) terminal. ODS-A thermistor WILL
BE BLOWN.
▲
WARNING
!
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
