Thermo King 920000, Manual

Page: 63 / 190

Electrical Maintenance
63
1. Check the evaporator temperature:
Be sure the evaporator temperature is actually
above 52 F (11 C) if the unit will not terminate
Defrost. Use a test thermometer to check the
temperature. If the evaporator temperature
does not rise enough to bring the unit out of
Defrost, the CIS, LLS, HGS, PV, RTPS, or the
RTPC may be faulty, or the unit may be low
on refrigerant.
To check the refrigerant charge, refer to
Refrigerant Charge in the Refrigeration
Maintenance chapter.
If the fan relay fails to open and/or the
evaporator fan continues to run, the
evaporator temperature may not rise enough to
terminate Defrost. To test the evaporator fans,
start the condensing unit and run the Zone 1
evaporator in Defrost. The evaporator fans
should not be running. If the fans are running,
remove the fan relay, the fans should stop
running. If the fans stop running, the relay is
defective. If the fans do not stop, there is a
short in the FM circuit.
2. Check the operation of the defrost termination
switch.
If the unit will not come out of Defrost,
disconnect the 12 wire from the defrost
termination switch. If the unit shifts back to
Cool, the defrost termination switch is not
opening and should be replaced. If the unit
remains in Defrost, move to step 3.
3. Check the 12 circuit for a ground:
If the unit remains in Defrost, use an
ohmmeter to check the 12 wire for a ground. If
a 12 wire is grounded, find the grounded
portion and repair it.
Zone 2 Evaporator Defrost Cycle
Diesel Operation
The Defrost cycle can be initiated with the
M
ANUAL
D
EFROST
key, by the air switch, or by the defrost
timer in the TG-V controller when the defrost
termination switch is closed.
Initiating the Defrost cycle energizes the Zone 2
defrost relays, RD and RDR4. When RD is
energized, it energizes the RHGS circuit, which
energizes the RHGS and the 26 circuit. The 26
circuit energizes the CIS (and the RTPS and PV
when the RTPC is closed). When RDR4 is
energized, it opens the RLLS-REFR circuit,
which de-energizes the Zone 2 evaporator fans.
During Defrost the Zone 2 TG-V Controller
energizes the R10 circuit, which forces the
condensing unit to run in high speed.
The CIS and RHGS divert heated refrigerant
vapor to the evaporator coil, melting frost.
De-energizing the evaporator fans keeps the heat
in the evaporator and prevents warm air from
passing over the load.
The defrost termination switch will open and
de-energize the defrost relays when the frost has
melted and the evaporator temperature rises above
52 F (11 C). The TG-V Controller then determines
the mode of operation.
Zone 2 Evaporator Defrost Cycle
Electric Operation
The Defrost cycle on electric standby operation is
similar to the Defrost cycle on diesel operation.
Zone 2 Evaporator Defrost Cycle
Checkout Procedure
To check the Defrost cycle, run the Zone 2
evaporator on Cool until the evaporator coil
temperature is below 42 F (6 C). Press the
M
ANUAL
D
EFROST
key.
If the Zone 2 evaporator shifts to Defrost
momentarily but shifts out of Defrost when the
key is released, check the TG-V controller. See
“Field Test Procedure for Multi-Temperature
In-Cab TG-V Controllers” on page 72.
If the Zone 2 evaporator will not shift to Defrost,
or if the Defrost cycle will not terminate, see the
following Defrost checkout procedures.