Thermo King 914 724, Manual

Page: 52 / 111

Electrical Maintenance (Rev. 1/99)
42
6. Attach a test lead to the A terminal and to the positive
terminal of the power source.
7. Attach a test lead to the 4 terminal and to the positive
terminal of the power source.
8. The test light should come on for approximately 10 sec-
onds. This is the speed-up mode of the 45 minute maxi-
mum defrost interval.
9. The test light should then turn off for approximately 1
to 3 minutes depending on which terminal 7 is con-
nected to. This is the speed-up mode of the interval
between defrost cycles.
10. A good timer will alternate between 10 second “on”
time and the 1 to 3 minute “off” time until the 6 termi-
nal is disconnected from the negative terminal of the
power source. Replace the timer if it fails to function
properly.
CIRCUIT BREAKERS
Units with 12 volt systems use 40 amp circuit breakers.
Units with 24 volt systems use 25 amp circuit breakers.
CB2
This auto reset circuit breaker is located in the unit control
box. Electrically it is located between the 2 circuit and the
2A circuit. It protects the unit control circuits during truck
engine operation.
If CB2 opens, check the unit for a grounded wire, a defec-
tive solenoid, a defective relay, or a defective fan motor.
CB1 (Model 20 Only)
This auto reset circuit breaker is located in the unit control
box. Electrically it is located between the 2R circuit and the
2B circuit. It protects the unit control circuits during electric
standby operation.
If CBI opens, check the unit for a grounded wire, a defec-
tive solenoid, a defective relay, or a defective fan motor.
CONDENSER FAN PRESSURE SWITCH
(CFPS)
The CFPS is located in the discharge line. Electrically it is
located between the J circuit and the CH circuit. It closes to
energize the condenser fan when the head pressure rises to
180 ± 10 psi (1241 ± 69 kPa). It opens to de-energize the
condenser fan when the head pressure drops to 130 ± 10 psi
(896 ± 69 kPa). To check the CFPS:
1. Install a manifold gauge set.
2. Connect a voltmeter between the J circuit and the CH
circuit where the CFPS wires are connected to the main
wire harness.
3. Run the unit on cool, check the voltmeter reading, and
watch the condenser fan.
a. When the head pressure is below 180 ± 10 psi
(1241 ± 69 kPa), the CFPS should be open, the
voltmeter should indicate battery voltage, and the
condenser fan should not be running.
If the voltage is low and the condenser fan is run-
ning, the CFPS is defective.
If the voltage is low and the condenser fan is not
running, check the power circuit (J, CF, and 8)
back through the condenser fan motor and the fan
relay to the battery. Also check the ground circuit
(CH) back to the battery.
b. When the head pressure rises above 180 ± 10 psi
(1241 ± 69 kPa), the CFPS should close, the volt-
meter should indicate a very low voltage, and the
condenser fan should be running.
c. When the head pressure drops below 130 ± 10 psi
(896 ± 69 kPa), the CFPS should open, the voltme-
ter should indicate battery voltage, and the con-
denser fan should not be running.
If the CFPS does not open or close at the proper pres-
sures, it is defective and must be replaced.