Installation guide

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 across pin
numbers PL5-2 HI and PL5-5 C. 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.
NOTE: DO NOT INSTALL A PLUG WITHOUT AN
INTERNAL RECTIFIER,
Unloader Test Procedure
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 Wall Control (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 step one does not give the expected results, remove the
solenoid plug from the compressor and, with the unit
running and the Wall Control (or Thermostat) 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 molded
plug, measure the compressor unloader coil resistance.
The resistance should be approximately 330 or 1640 ohms
depending on unloader coil supplier. If the coil resistance is
infinite or is grounded, the compressor must be replaced.
MAJOR COMPONENTS
2-Stage Control
The 2-stage control board controls the following functions:
-- Compressor high and low stage operation
-- Outdoor fan motor operation
-- Compressor external protection
-- Pressure switch monitoring
-- Time delays
Field Connections
On models with non-communicating (Thermostat other than
Observer Wall Control) system, the 2-stage control receives
24vac low-voltage control system inputs through the Y1, and Y2
connections located at the bottom of the control board (see Fig.
10). The OD units can be controlled using a standard 2-stage
thermostat or Observer Wall Control.
2-Stage Compressor
The 2-stage compressor contains motor windings that provide
2-pole (3500 RPM) operation. Refer to Table 4 for correct winding
resistance.
Compressor Internal Relief
The compressor is protected by an internal pressure relief (IPR)
which relieves discharge gas into compressor shell when
differential between suction and discharge pressures exceeds
550 - 625 psig The compressor is also protected by an internal
overload attached to motor windings.
Compressor Control Contactor
The contactor has a 24 volt coil. The electronic control board
controls the operation of the appropriate contactor.
TEMPERATURE THERMISTORS
Thermistors are electronic devices which sense temperature. As
the temperature increases, the resistance decreases. Thermistors
are used to sense outdoor ambient (OAT) and coil temperature
(OCT). Refer to Fig. 9 for resistance values versus temperature. If
the outdoor ambient or coil thermistor should fail, the control will
flash the appropriate fault code (see Table 6.)
IMPORTANT: Outdoor air thermistor and coil thermistor are
factory mounted in the final locations. Check to insure thermistors
are mounted properly per Fig.7 and Fig.8.
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:
-- If the outdoor air sensor indicates 10°F (5.56°C) warmer than
the coil sensor (or) the outdoor air sensor indicates 20°F (11.1 °C)
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 6.
-- 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 and/or coil thermistor. If the OAT sensor
should fail, low ambient cooling will not be allowed and the one
minute outdoor fan-off delay will not occur. If the OCT sensor
should fail, low ambient cooling will not be allowed.
OAT Thermistor must be locked in place with
spherical nib end facing towards the front of the
control box.
10 Specificationssubjectto change without notice. 421 01 1702 04