Thermo King 914 724, Manual

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Electrical Maintenance (Rev. 1/99)
39
If the unit shifts to defrost immediately, check for an
open circuit in the 11 circuit that goes back to the
Manual Defrost switch. If the 11 circuit is not open, the
Manual Defrost switch is defective.
If the unit does not shift to defrost, go to step 4.
4. Check the defrost timer:
Remove the defrost timer from its socket and insert the
defrost jumper into socket slots 1, 2, and 3. Press the
Manual Defrost switch.
If the unit shifts to defrost, the defrost timer is
defective.
If the unit does not shift to defrost, go to step 5.
5. Place a jumper wire between a good ground and the
11A wire at the defrost relay socket (B terminal).
If the unit shifts to defrost, check for an open circuit in
the 11A circuit between the defrost timer socket and the
four pin connector that connects the control relay har-
ness to the main wire harness.
If the unit does not shift to defrost, check for battery
voltage on the 8 wire at the defrost relay socket (A ter-
minal).
If battery voltage is present on the 8 wire, the defrost
relay is defective.
If battery voltage is not present, check for an open cir-
cuit in the 8 circuit between the defrost relay socket and
the terminal board.
Unit Will Not Terminate Defrost
NOTE: The defrost timer should automatically terminate
the defrost cycle 45 minutes after it is initiated.
Unit Sticks in Defrost Indefinitely
1. Check the defrost timer:
If the unit has been stuck in defrost for more than 45
minutes, remove the defrost timer from its socket.
If the unit shifts out of defrost, the defrost timer is
defective.
If the unit remains in defrost, go to step 2.
2. Remove the defrost relay from its socket and check the
11A wire (slot B in the defrost relay socket) for conti-
nuity to ground.
If the 11A wire is not grounded, the defrost relay is
defective.
If the 11A wire is grounded, find and repair the short to
ground.
Unit Continually Fails to Terminate Defrost (In
Less than 45 Minutes)
If the unit continually fails to complete the defrost cycle in
less than 45 minutes and cycles between cool and defrost,
check the evaporator coil temperature, three-way valve seal,
refrigerant charge, defrost termination switch, and the 11
and 12 wires.
1. Check the Evaporator Temperature:
Be sure the evaporator temperature is actually above 52
F (11.1 C) if the unit will not terminate defrost. Use a
test thermometer to check the temperature. If the evap-
orator temperature does not rise enough to bring the
unit out of defrost, the unit may be low on refrigerant,
or the condenser seal in the three-way valve may be
leaking.
To check the three-way valve or the condenser check
valve, refer to the procedures described in TK 5984,
Diagnosing the Thermo King Refrigeration System.
To check the refrigerant charge, refer to Refrigerant
Charge in the Refrigeration Maintenance chapter.