Thermo King 914970, Manual

Page: 87 / 186

Electrical Maintenance
87
Engine Reset Switch
The engine is protected by a manual reset switch.
The reset switch is attached to a two sensors. One
sensor switch is in the engine oil system, the other
is in the engine cooling system.
If either sensor switch is grounded due to an
abnormal condition (low oil pressure, or high
water temperature), the reset switch will trip and
stop the engine in about 40 seconds.
The reset switch must be replaced if it is
defective.
Conditions that cause the reset switch to trip:
• Engine coolant (water) temperature over
220 F (104 C).
• Engine oil pressure below 10 psig
(69 kPa).
• Lack of fuel to the engine. The low oil
pressure switch will cause the reset switch
to trip after the engine stops.
NOTE: If the On/Off switch(es) are in the
On position, if the CYCLE-SENTRY switch
is in the Cont Run position, and if the engine
is not running; the low oil pressure switch
will cause the reset switch to trip.
• High pressure in the refrigeration system.
The low oil pressure switch will cause the
reset switch to trip after the high pressure
cutout stops the engine.
• Reset switch becomes defective. The
switch may get to a point where it will
open due to vibration.
• A ground fault in the 20 or 20A wires to
the sensor switches is also a possible
cause.
NOTE: A ground or shorter circuit in the
electrical system does not cause the reset
switch to pop out.
Low Oil Pressure Switch (LOP)
The engine oil pressure should rise immediately
after the engine is started. The LOP will trip the
reset switch and stop the engine if the oil pressure
drops below 10 ± 2 psig (69 ± 14 kPa). A
continuity tester is needed to check the LOP.
1. Remove the 20A wire from the LOP.
2. The continuity tester should indicate a
complete circuit between the terminal and
ground.
3. Start the engine. The tester should show an
open circuit between the terminal and ground.
Repair consists of replacing the LOP.
Defrost Air Switch Checkout
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 in proper
position, and make sure they are not obstructed.
1. Remove plastic sensing tubing from both sides
of the defrost air switch.
2. Disconnect one wire at switch terminal.
Connect test light or continuity tester to the
two terminals used on the switch.
3. Install test equipment (see Tool Catalog) onto
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 (correct reading is
0.70 ± 0.05 in. [17.8 ± 1.3 mm] H
2
O).
Release the pressure.