Thermo King SB-III 30 SR+, Manual

Page: 72 / 120

Refrigeration Maintenance (Rev. 04/02)
62
MODULATION VALVE (Optional)
The modulation valve is normally open. As a controlled
electrical signal is applied to the coil the armature over-
comes spring pressure and the valve moves a precise
amount in the closed direction. As the signal strength is
increased, the valve closes more. This controls the flow of
refrigerant to the compressor. The µPV Microprocessor
controller monitors the return and discharge air temperature
to control the signal and the position of the modulation
valve.
Unlike other valves, the pressure from the flow of fluid
going past the valve has no opening or closing force
because of the valve and seat design. The forces cancel each
other. The operation of this valve allows a very exact tem-
perature to be maintained in the box by controlling the
pumping ability of the compressor.
The modulation valve is not repairable but the electric coil
can be replaced.
1. Outlet
2. Electric Coil
3. Armature
4. Inlet
5. Opening Spring
6. Seats
Modulation Valve
AEA712
1
2
3
4
5 6
Refer to the appropriate Microprocessor Operation and
Diagnosis Manual for information about testing the modula-
tion valve. See the Refrigeration Service Operations section
of this manual for removal and installation procedures.
HOT GAS SOLENOID (Optional)
The hot gas solenoid is used in conjunction with the modu-
lation valve to reduce the capacity of the unit during modu-
lation. This normally closed solenoid valve is located in the
refrigeration line that connects the discharge line to the hot
gas line. The hot gas solenoid is energized (opened) at full
modulation. The hot gas solenoid is de-energized (closed)
when modulation is discontinued.
Refer to the appropriate Microprocessor Operation and
Diagnosis Manual for information about testing the hot gas
solenoid. See the Refrigeration Service Operation chapter of
this manual for removal and installation procedures.