Thermo King SB-210, Manual

Page: 106 / 159

Refrigeration Maintenance
106
2. Use the Service Test Mode to run the unit in
high speed cool.
3. Raise the discharge pressure of the
compressor first by blocking the condenser
coil air flow by covering the condenser grille
with a piece of cardboard. If this does not raise
the discharge pressure to the cutout level of
the HPCO, increase the engine speed by
overriding the throttle solenoid. This should
increase the discharge pressure enough to
cause the HPCO to cut out.
4. If the HPCO does not open to de-energize the
fuel solenoid and stop the engine, it must be
replaced.
Three-Way Valve Condenser
Pressure Bypass Check Valve
A three-way valve condenser pressure bypass
check valve is used in this unit. This check valve
controls the bypass flow of refrigerant gas
between the condenser inlet line and the
compressor discharge line.
The check valve is closed when the unit is running
on cool, or whenever the discharge pressure is
higher than the condenser pressure.
When the unit is running on defrost or heat, if the
condenser pressure is higher than the discharge
pressure, the check valve opens and the condenser
pressure is bled off until it drops to the level of the
discharge pressure. The purpose of the valve is to
improve the three-way valve response time when
shifting from heat to cool.
If a three-way valve does not shift back to cool
immediately after the pilot solenoid closes, and
finally shifts to cool when the temperature rise
puts the unit into high speed, the three-way valve
end cap should be checked. See “End Cap
Checks” in the Refrigeration Service Operations
Chapter.
To check the operation of the condenser pressure
bypass check valve:
1. Remove the condenser pressure bypass check
valve cap from the three-way valve.
2. Using a screwdriver, gently turn the check
valve stem in until the valve is front seated.
3. Install a gauge manifold set on the
compressor.
4. Close (front seat) the receiver tank outlet
valve.
5. Operate the unit on cool and pump down the
low side to 20 in. Hg (-68 kPa) of vacuum.
6. Stop the unit. The condenser and suction
pressures should remain stable, indicating no
leaks.
7. Shift the three-way valve to the heat position.
The low side gauge will raise slightly. The
high side gauge will drop to approximately
zero. The gauges will equalize.
CAUTION: If the discharge pressure
reaches 477 psig (3289 kPa), shut the unit
off immediately. Do not allow the
discharge pressure to exceed 477 psig
(3289 kPa).
1. Check Valve
2. Heating/Defrost Position
3. Cooling Position
Figure 93: Three-way Valve Condenser
Pressure Bypass Check Valve