Thermo King 920000, Manual

Page: 64 / 190

Electrical Maintenance
64
Zone 2 Evaporator Does Not Defrost
1. Check the evaporator temperature:
Make sure the evaporator temperature is
actually below 42 F (6 C) if the unit will not
Defrost. Use a test thermometer to check the
evaporator temperature.
2. Check the operation of the defrost termination
switch:
If the unit fails to Defrost, place a jumper wire
between the R12 circuit and the CH circuit at
the defrost termination switch. Press the
M
ANUAL
D
EFROST
key. If the unit shifts to
Defrost, check for a defective defrost
termination switch. If the unit still fails to
Defrost, move to step 3.
3. Check for an open in the R12 circuit between
the defrost termination switch and the TG-V
controller. If the 12 circuit is not open, move
to step 4.
4. Check for an open in R11 circuit between the
TG-V Controller and the defrost relay. If the
R11 circuit is not open, move to step 5.
5. Check the defrost relays:
If the unit fails to Defrost, check the R8NN
circuit at RD and the 88 circuit at RDR4 for
12 volts dc. If the circuit has 12 volts, the
defrost relay is defective and should be
replaced. A lack of voltage indicates an open
in the R8NN or 88 circuit.
6. If the defrost relays energize, the defrost
indicator light comes on and the fans stop, but
the unit continues to Cool; check the CIS, the
26 circuit, the RHGS, and the RHGS circuit.
1 2 3 4
1. Zone 2 Defrost Relay (RD) 3. Zone 1 Fan Relay (FR)
2. Zone 2 Fan Relay (RFR) 4. Zone 1 Defrost Relay (DR)
Figure 20: Defrost Relay Locations