Thermo King MD-MT, Operator'S Manual

Page: 61 / 192

Electrical Maintenance
61
Glow Plugs
Glow plugs heat the combustion chamber to aid in
quick starting. The glow plugs are energized when
the Preheat/Start switch is held on the Preheat or
the Start position or when the CYCLE-SENTRY
module initiates a unit start-up
(CYCLE-SENTRY switch in the Auto Start-Stop
position).
An open glow plug (burned out) can be detected
with an ammeter in the 2A circuit. The ammeter
should show 20 to 28 amps while the
Preheat/Start switch is held in the Preheat position
and the On/Off switch is in the Off position. A
current draw of 20 to 28 amps means the glow
plugs are working. If the current draw drops
below 20 amps on Preheat, at least one glow plug
is bad.
To isolate an open circuit glow plug, remove the
H wires and test each glow plug individually with
an ohmmeter or a jumper wire and ammeter. Each
glow plug should have a resistance of 1.5 ohms or
a current draw of about 8.3 amps.
A shorted glow plug will be indicated by the
ammeter showing a very high current draw when
the Preheat switch is pressed, the 50 amp circuit
breaker tripping, or the control circuit burning out.
Check each glow plug, a shorted glow plug will
have very low resistance.
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.
High Water Temperature Switch
(HWT)
The HWT will close and trip the reset switch if
the coolant temperature is greater than 220 F
(104 C). Use a continuity tester to check the
switch, and use a coolant temperature gauge to
check the temperature.
1. Remove the 20A wire from the HWT.
2. Run the unit until it reaches normal operating
temperature, approximately 180 F (82 C).
There should be no continuity from the HWT
to ground.
3. If the engine runs hot and the reset switch
does not trip, run the unit until it reaches 220 F
(104 C). The HWT should have continuity to
ground.
Replace the switch as necessary.