Thermo King MD-MT, Operator'S Manual

Page: 70 / 192

Electrical Maintenance
70
2. Connect the negative lead of the switch to the
negative battery terminal.
3. Raise the temperature of the defrost
termination switch above 52 F (11 C). The
light should be off indicating an open switch.
4. Cool the defrost termination switch below
42 F (6 C). The light should come on
indicating the switch has closed.
NOTE: Allow adequate time for the
temperature change to saturate the switch
before performing the test.
Defrost Relays
The defrost relays control the operation of each
evaporator on the Defrost cycle. When the air
switch, the TG-V defrost timer, or the Manual
Defrost switch complete the circuit through the
defrost termination switch to ground, the defrost
relays energize to initiate Defrost. A holding
circuit in the TG-V keeps the unit on Defrost until
the defrost termination switch opens.
Air Switches
An air switch automatically places each
evaporator on Defrost when ice accumulation on
the evaporator coil builds up to a point where the
air flow across the coil is restricted.
Restricted air flow results in a pressure difference
between the evaporator coil inlet and outlet. The
air switch senses the pressure differential across
the coil and initiates the Defrost cycle.
The air switch is preset at the factory. Normally
readjustment is not necessary unless the switch
has been tampered with or does not function
properly due to factors affecting air circulation,
such as bulkhead construction and duct work.
Air Switch Testing and Adjustment:
Before testing
or adjusting the air switch, check the clear plastic
tubing and black plastic tubing to the evaporator
coil.
Make sure they are not obstructed or crushed.
Check the probes in the evaporator housing to be
sure they are not obstructed.
1. Remove plastic sensing tubing from both sides
of the air switch.
2. Disconnect one wire at the switch terminal.
Connect a test light or continuity tester to the
two terminals used on the switch.
3. Connect the test equipment (P/N 204-442 and
P/N 204-494) to the hose fitting on the side of
the air switch stamped BLACK.
4. Pressurize the hose until the continuity tester
indicates a completed circuit. Now read the
dial of the test gauge. This is the setpoint of
the air switch. The correct settings is
1.00 ± 0.05 in. (25.4 ± 1.3 mm) H
2
O. Release
the pressure.
5. If the switch is out of calibration, pressurize
the hose again until the tester indicates the
correct reading. Turn the adjustment screw
until the switch closes and the continuity
tester indicates a completed circuit with the
correct reading. Release the pressure.
6. Repeat the test procedure several times to be
sure the setting is correct.
7. Remove the test equipment. Connect the wire
and air sensing tubes to the switch. The
BLACK hose from the high pressure or air
inlet side of the evaporator coil goes on the
hose fitting on the side of the air switch
stamped BLACK. The CLEAR hose from the
low pressure or air outlet side of the
evaporator coil goes on the hose fitting on the
side of the air switch stamped CLEAR.
NOTE: Route hoses for continuous slope to
avoid condensate traps.
If too much frost continues to accumulate before
Defrost is initiated, decrease the pressure setting.
Turn the adjustment screw counterclockwise.
If Defrost occurs with too little frost
accumulation, increase the pressure setting. Turn
the adjustment screw clockwise.