Thermo King MD-MT, Operator'S Manual

Page: 66 / 192

Electrical Maintenance
66
Zone 1 Evaporator Sticks In Defrost
Indefinitely
If the unit remains stuck in Defrost indefinitely,
check the defrost relay, the 11 circuit and the
TG-V Controller.
NOTE: If the Multi-Temp TG-V Controller has
recorded Alarm Code 14, check the 11 circuit for
a ground first.
1. Unplug the defrost relay and check the 11
circuit:
Unplug the defrost relay and check the B slot
in the defrost relay socket for continuity to CH
(ground). If there is no continuity from the B
slot to CH (ground), replace the defrost relay.
If there is continuity from the B slot to CH
(ground), move to step 2.
2. Disconnect the cab box harness and check the
11 circuit:
Disconnect the cab box harness from the back
of the TG-V Controller and check the 11
circuit for continuity to CH (ground). If the 11
circuit has continuity to CH (ground), check
the 11 circuit for a short to CH (ground). If the
11 does not have continuity to CH (ground),
move to step 3.
3. Check the TG-V Controller:
The integral defrost timer in the TG-V
Controller should terminate the Defrost cycle
after 45 minutes regardless of the evaporator
temperature or amount of frost or ice on the
evaporator coil. If not, test the TG-V
Controller and replace it if necessary. See
“Field Test Procedure for Multi-Temperature
In-Cab TG-V Controllers” on page 75.
Zone 1 Evaporator 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, CIS, HGS, PV, RTPS, RTPC,
refrigerant charge, fan relay, defrost termination
switch, and the 12 circuit.
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.