Thermo King 914 724, Руководство пользователя

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Electrical Maintenance (Rev. 1/99)
38
Defrost Termination Switch Bench Test
1. Connect a test light between the screw terminal on the
switch and the positive battery terminal.
NOTE: Attempting to test the electronic defrost termi-
nation switch with an ohmmeter is generally not satis-
factory because of the low voltage available at the
meter leads.
2. Connect the ground wire of the switch to the negative
battery terminal.
3. Raise the temperature of the defrost termination switch
above 52 F (11.1 C). The light should be off, indicating
an open switch.
4. Cool the defrost termination switch below 42 F (5.6 C).
The light should come on, indicating the switch has
closed.
NOTE: Allow adequate time for the temperature
change to saturate the defrost termination switch
before performing the test.
Defrost Relay
The defrost relay controls the operation of the defrost cycle.
When the defrost timer or the Manual Defrost switch com-
pletes the circuit through the defrost termination switch to
ground, the defrost relay is energized. This energizes the 26
circuit and de-energizes the fan relay. The defrost relay
stays energized until the defrost cycle is terminated by the
defrost termination switch or the automatic defrost timer.
Pilot Solenoid
The pilot solenoid is an electrical valve that controls the
operation of the three-way valve in the refrigeration system.
The pilot solenoid is energized by the 26 circuit. When the
pilot solenoid coil is energized, it lifts the pilot solenoid
valve piston and allows refrigerant pressure to shift the
three-way valve. This routes hot refrigerant gas to the evap-
orator.
Testing the Defrost System
To test the defrost system, run the unit on Cool until the
evaporator coil temperature is below 42 F (5.6 C) and press
the Manual Defrost switch.
If the unit shifts to defrost momentarily but shifts out of
defrost when the switch is released, check the defrost timer.
If the unit will not shift to defrost, or if the defrost cycle will
not terminate, see the following defrost checkout procedure.
CAUTION: Do not forget to remove the jumper
wires from the unit after checking or testing unit
components.
Unit Does Not Defrost
1. Check the evaporator temperature:
Make sure that the evaporator temperature is actually
below 42 F (5.6 C) if the unit will not defrost. Use a test
thermometer to check the evaporator temperature.
2. Check the defrost termination switch:
If the unit fails to defrost, place a jumper wire between
the 12 wire at the screw terminal on the defrost termi-
nation switch and a good ground. Press the Manual
Defrost switch.
If the unit shifts to defrost, the defrost termination
switch is defective.
If the unit does not shift to defrost, check for an open
circuit in the 12 circuit that goes back to the Manual
Defrost switch. If the 12 circuit is not open, go to
step 3.
3. Check the Manual Defrost switch:
Place a jumper wire between a good ground (CH) and
the 11 wire in the four pin connector that connects the
control relay harness to the main wire harness. This
connector is located inside the unit control box.