Thermo King Super II SR+, Manual

Page: 87 / 145

Refrigeration Maintenance (Rev 6/98)
77
the valve is to improve heating/defrosting ability and three-
way valve response.
To check the operation of the 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
-68 kPa (20 in. Hg) 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. Low side
gauge will raise slightly. High side gauge will drop to
approximately zero. Gauges will equalize.
8. Gauges will remain in this position approximately zero
if the three-way valve seals properly toward the con-
denser and the condenser pressure bypass check valve
seals properly.
9. Back seat condenser pressure bypass check valve stem
against snap ring. Both gauges should rise indicating
the condenser pressure bypass check valve is properly
releasing condenser pressure into the discharge tube
and evaporator.
10. Replace the cap on the condenser pressure bypass
check valve.
NOTE: Valve stem MUST be back seated during nor-
mal unit operation.
11. Open the receiver tank return outlet valve, remove the
gauges and return the unit to normal operation.
MODULATION VALVE
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 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.