CSR-20 & CSR-40, September 1999
Refrigeration System Diagnosis and Service
Modulation Valve Repair or Replacement
6-17
Modulation Valve Repair or Replacement
The modulation valve is used to control the flow of refrigerant
to the compressor when the unit is operating in the Modulation
mode. As the supply air temperature approaches setpoint, the
controller sends an electrical signal to the coil of the valve.
The armature overcomes the spring tension and the valve clos-
es a precise amount. This throttles the suction gas returning to
the compressor and reduces cooling capacity. As the signal is
increased, the valve closes an additional amount. Due to valve
design, the flow of refrigerant gas exerts no opening or closing
forces on the valve spool allowing very precise operation.
Service of the modulation valve includes replacement of
the coil, replacement of the enclosing tube assembly or
replacement of the complete valve.
Tools Required:
• Digital Multimeter (P/N 204-615)
• Modulation Valve Coil (P/N 44-5175) or Modulation Valve
Repair Kit (P/N 60-203) (kit includes coil)
• Scissors (with duct tape), pocket knife or other thin-pointed
instrument
• Adjustable Wrench
• 1.5 inch Wrench
• Torque Wrench
Coil Checkout Procedure
NOTE: In most cases, only the coil requires replace-
ment.
1.
Unplug the modulation valve coil lead wire harness.
2.
Using a FLUKE multimeter, test each lead resistance to
ground. Low resistance indicates a short is present.
Repair or replace any damaged or exposed wires.
3.
Check the coil resistance. If the coil resistance is below 5
ohms, replace the coil (good coils have a resistance of 7.6
ohms at 25 C (77 F) or 6.9 ohms at 4.4 C (40 F).
NOTE: The ohmmeter will display a higher coil
resistance if the modulation valve was energized
for a long period of time just prior to testing the
coil resistance.
4.
To return the unit to service, plug the modulation valve
lead connector into the unit wire harness.
Enclosure Tube Replacement
If the modulation fails to operate properly, remove the coil
housing and inspect the solenoid coil sleeve and enclosure tube
assembly for rust or corrosion. Rust or corrosion can damage
the enclosure tube, preventing the piston inside the tube from
opening and closing the valve properly. If the solenoid coil
sleeve is badly corroded, replace the entire enclosure tube and
coil assembly.
Replacing the Enclosure Tube Assembly
CAUTION: When replacing the enclosing tube
assembly, DO NOT remove the valve piston, top
return spring or bottom return spring from the valve
body. These components must be factory installed
and adjusted to ensure proper valve operation.
1.
Remove the compressor compartment bracket.
2.
Recover the refrigerant charge from the unit (see
“Refrigerant Recovery” in this chapter).
3.
Disconnect the unit from the three-phase power supply.
4.
Unplug the coil lead wire harness.
5.
Remove the coil, coil sleeve and bottom housing plate
from the enclosure tube.
Modulation Valve
1. Outlet
2. Inlet
3. Enclosure Tube
4. Armature
5. Electric Coil
6. Piston
7. Closing Spring
8. Valve Seats
9. Opening Spring
