Thermo King 920000, Manual

Page: 68 / 190

Electrical Maintenance
68
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.
Measuring Evaporator Coil Pressure
Differential
If the air switch is initiating Defrost too often
even though the air switch is adjusted properly,
the tubing is routed correctly, and the probes are
positioned correctly, the air flow through the
evaporator may be restricted.
1. Run the unit through a Defrost cycle to clear
the evaporator coil of frost.
2. Remove the plastic sensing tubes from the air
switch.
3. Connect a Magnehelic pressure gauge (P/N
204-442) to the plastic sensing tubes.
4. Run the unit in High Speed Cool and measure
the evaporator coil pressure differential. If the
pressure differential of the clear coil is more
than one half of the air switch setpoint, the
evaporator coil is restricted and must be
cleaned.
5. Remove the test equipment. Connect the 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.
Fan Speed Control
The S-2 and S-3 evaporators are equipped with a
fan speed control circuit. The speed control circuit
is connected to the +12 Fan circuit, the Fan SC
circuit, and ground. The voltage on the Fan SC
circuit controls the evaporator fan speed. The
speed control circuit limits the voltage on the Fan
SC circuit to 7.5 volts, and the fans run at
approximately 1900 RPM.
Circuit Boards
The printed circuit boards are generally fail safe
and should require no repair. Some things to note
are:
1. The current paths on the circuit boards can be
damaged (scratched, etc.) and cause an open
circuit.
2. Do not use flat washers between the circuit
breakers and the circuit board. Flat washers at
this location can contact a current path and
cause a malfunction.
3. A feed thru hole may burn out between the top
and bottom surfaces.
A circuit board cannot be repaired. It must be
replaced if it is damaged.
CYCLE-SENTRY Option
Refer to the CYCLE-SENTRY IV Operation and
Diagnosis Manual for all operation and diagnosis
procedures. The RD-MT is not presently included
in that manual but the systems are similar.