Thermo King MD-MT, Operator'S Manual

Page: 71 / 192

Electrical Maintenance
71
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.
AGA271
1
2
3
4
1. Adjustment Screw 3. Squeeze Bulb (P/N 204-494)
2. Continuity Tester 4. Pressure in Inches of Water (P/N 204-442)
Figure 25: Air Switch Testing and Adjustment