Thermo King MD-MT, Operator'S Manual

Page: 69 / 192

Electrical Maintenance
69
Zone 2 Evaporator Continuously 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, RHGS, CIS, PV, RTPS, RTPC,
refrigerant charge, Zone 2 fan relay, defrost
termination switch and the R12 wire.
1. Check the evaporator temperature:
Make 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 out of Defrost, the CIS,
RHGS, PV, RTPS, or the RTPC may be faulty,
or the unit may be low on refrigerant.
If the Zone 2 fan relay fails to open and/or the
Zone 2 evaporator fans continue to run, the
evaporator temperature may not rise enough to
terminate Defrost. To test the Zone 2
evaporator fans, start the condensing unit and
run the Zone 2 evaporator in Defrost. The
Zone 2 evaporator fans should not be running.
If the fans are running, remove the Zone 2 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 RFM circuit.
2. Check the operation of the defrost termination
switch.
If the unit will not come out of Defrost,
disconnect the R12 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 R12 circuit for a ground:
If the unit remains in Defrost, use an
ohmmeter to check the R12 circuit for a
ground. If the R12 circuit is grounded, find the
grounded portion and repair it.
Defrost System Components
Defrost Termination Switches
Each electronic defrost termination switch uses
solid state components to control the Defrost
cycle. A switch is mounted in each evaporator and
controls the Defrost cycle in response to the
evaporator coil temperature. The switch is closed
when the evaporator coil temperature is below
42 F (6 C), completing the Defrost circuit to
ground and preparing the electrical system for the
Defrost cycle.
When the unit does shift into a Defrost cycle, the
fans stop, and heat from the hot refrigerant gas
melts the frost from the evaporator coil. The
switch opens and terminates the Defrost cycle
when the evaporator coil temperature rises to 52 F
(11 C).
Installation:
The proper polarity must be observed
when installing the defrost termination switch.
The wire from the switch is negative and must be
attached to the chassis ground of the unit. This
chassis ground wire must be grounded on a screw
separate from the switch mounting screws or an
improper ground may result. The 12 wire from the
unit attaches to the terminal mounted solid on the
switch. If the polarity is reversed on the device, it
will conduct continuously and act like a switch
that is stuck closed.
Figure 24: Defrost Termination Switch
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 termination switch with an ohmmeter
is generally not satisfactory because of the
low voltage available at the meter leads.