Thermo King 914 724, Manual

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Electrical Maintenance (Rev. 1/99)
37
To check the defrost cycle, run the unit in cool to drop the
evaporator coil to a temperature below 42 F (5.6 C). Press
the manual defrost switch. The unit should shift from the
cool to defrost. If the unit continues to cool, double check
the evaporator coil temperature, and refer to Testing the
Defrost System.
Electric Standby Operation (Model 20
Only)
Defrost operates essentially the same on electric standby as
it does on engine operation.
NOTE: It takes more time to complete a defrost cycle in
low ambient temperatures (below 35 F [2 C]) than it does
in high ambient temperatures (above 70 F [21 C]). There-
fore, consider the ambient temperature before deciding
that a unit is not defrosting properly.
Defrost Components
Defrost Timer
The defrost timer initiates and, if necessary, terminates the
defrost cycle.
The timer is wired to initiate a defrost cycle every 4 hours.
The timing cycle begins when the defrost termination
switch closes. Every time the defrost termination switch
opens, the timer resets to zero.
At the initiation of the defrost cycle, a 45 minute timer is
activated. If the defrost termination switch does not open
and terminate the defrost cycle, the timer will terminate the
defrost cycle 45 minutes after it started.
It is possible that the defrost timer could fail and not allow
the unit to defrost. In this case, bypass the defrost timer by
removing it from its socket and placing the defrost jumper
in the defrost timer socket slots 1, 2, and 3. Pressing the
Manual Defrost switch should then cause the unit to shift to
defrost until the defrost termination switch opens.
Manual Defrost Switch
A Manual Defrost switch is located in the remote or cab
control box. Pressing the Manual Defrost switch initiates
the defrost cycle if the defrost termination switch is closed.
Defrost Termination Switch
The electronic defrost termination switch uses solid state
components to control the defrost circuit. The switch is
mounted in the 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
(5.6 C), completing the defrost circuit to ground (CH) and
preparing the electrical system for the defrost cycle.
When the unit does shift into a defrost cycle, the evaporator
fan stops, and heat from the hot refrigerant gas melts the
frost from the evaporator coil. The switch opens and termi-
nates the defrost cycle when the evaporator coil temperature
rises to 52 F (11.1 C).
Installation
The proper polarity must be observed when installing the
defrost termination switch. The wire from the switch is neg-
ative and must be attached to the chassis ground of the unit.
This chassis ground wire cannot be attached to either of the
switch mounting screws or an improper ground may result.
The 12 wire from the unit attaches to the screw terminal that
is mounted solidly on the switch. If the polarity is reversed
on the device, it will conduct continuously and act like a
switch that is stuck closed.
1. Screw Terminal 2. Ground Wire
Defrost Termination Switch
2
1