Service Manual
59
Troubleshooting units for proper switching
between low & high stages - *CA7, *CA9,
*CH6, *CH9
Check the suction pressures at the service valves. Suction
pressure should be reduced by 3- 10% when switching from
low to high capacity.
NOTE: The liquid pressures are very similar between low
and high stage operation, so liquid pressure should not be
used for troubleshooting.
Compressor current should increase 20- 45% when
switching from low to high stage. The compressor solenoid
when energized in high stage, should measure 24vac.
When the compressor is operating in low stage the 24v DC
compressor solenoid coil is de- energized. When the
compressor is operating in high stage, the 24v DC solenoid
coil is energized. The solenoid plug harness that is
connected to the compressor HAS an internal rectifier that
converts the 24v AC signal to 24v DC. DO NOT INSTALL A
PLUG WITHOUT AN INTERNAL RECTIFIER.
Unloader Test Procedure - *CA7, *CA9, *CH6,
*CH9
The unloader is the compressor internal mechanism,
controlled by the DC solenoid, that modulates between high
and low stage. If it is suspected that the unloader is not
working, the following methods may be used to verify
operation.
1. Operate the system and measure compressor
amperage. Cycle the unloader on and off at 30
second plus intervals at the UI (from low to high stage
and back to low stage). Wait 5 seconds after staging
to high before taking a reading. The compressor
amperage should go up or down at least 20 percent.
2. If the expected result is not achieved, remove the
solenoid plug from the compressor and with the unit
running and the UI calling for high stage, test the
voltage output at the plug with a DC voltmeter. The
reading should be 24 volts DC.
3. If the correct DC voltage is at the control circuit mold-
ed plug, measure the compressor unloader coil resis-
tance. The resistance should be approximately 330 or
1640 ohms depending on unloader coil supplier. If the
coil resistance is infinite or is grounded, the compres-
sor must be replaced.
Temperature Thermistors
Thermistors are electronic devices which sense
temperature. As the temperature increases, the resistance
decreases. Thermistors are used to sense outdoor air (OAT)
and coil temperature (OCT). Refer to Fig. 40 for resistance
values versus temperature.
If the outdoor air or coil thermistor should fail, the control will
flash the appropriate fault code. (See Table 17)
IMPORTANT: The outdoor air thermistor and coil
thermistor should be factory mounted in the final
locations. Check to ensure thermistors are mounted
properly per Fig. 43 and Fig. 44.
Thermistor Sensor Comparison
The control continuously monitors and compares the
outdoor air temperature sensor and outdoor coil
temperature sensor to ensure proper operating conditions.
The comparison is:
S In cooling if the outdoor air sensor indicates 10_F
warmer than the coil sensor (or) the outdoor air
sensor indicates 20_F cooler than the coil
sensor, the sensors are out of range.
S In heating if the outdoor air sensor indicates 35_F
warmer than the coil sensor (or) the outdoor air
sensor indicates 10_F cooler than the coil
sensor, the sensors are out of range.
If the sensors are out of range, the control will flash the
appropriate fault code as shown in Table 17.
The thermistor comparison is not performed during low
ambient cooling or defrost operation.
Failed Thermistor Default Operation
Factory defaults have been provided in the event of failure
of outdoor air thermistor (OAT) and/or outdoor coil thermistor
(OCT).
If the OAT sensor should fail, low ambient cooling will not be
allowed and the one- minute outdoor fan off delay will not
occur. Defrost will be initiated based on coil temperature and
time.
If the OCT sensor should fail, low ambient cooling will not be
allowed. Defrost will occur at each time interval during
heating operation, but will terminate after 5 minutes.
If there is a thermistor out of range error, defrost will occur at
each time interval during heating operation, but will
terminate after 5 minutes.
Count the number of short and long flashes to determine the
appropriate flash code. Table 17 gives possible causes and
actions related to each error.
A150065
Fig. 43 – Outdoor Air Thermistor (OAT) Attachment
OCT Thermistor
must be secured
tight on stub tube.
A05408
Fig. 44 – Outdoor Coil Thermistor (OCT) Attachment