Guardian 4721, Repair Manual

Page: 18 / 115

PART 1
Page 14
GENERAL INFORMATION
SECTION 1.5
ENGINE-GENERATOR PROTECTIVE DEVICES
HIGH COOLANT TEMPERATURE SHUTDOWN:
The engine is equipped with a coolant temperature
switch. Should engine coolant temperature exceed
approximately 284° F. (140° C.), the engine will be
shut down automatically by control board action.
LOW COOLANT LEVEL SENSOR:
It is possible that engine coolant level might drop low
enough so that the high temperature switch is no
longer immersed in the liquid coolant. If this happens
engine temperatures could increase rapidly but the
temperature switch would not sense the high temper-
ature condition and the engine would continue to run.
To prevent this occurrence, a low coolant level sensor
is provided. The sensor is immersed in cooling sys-
tem liquid. If coolant level drops below the level of the
low coolant level sensor, the device will complete a
Wire 85 circuit to ground. Engine shutdown will occur.
OVERSPEED SHUTDOWN:
The control board on liquid cooled units receives AC
frequency (rpm) signals directly from the stator AC
power windings, via sensing leads S15 and S16.
Should AC frequency exceed approximately 72 Hz,
circuit board action will initiate an automatic engine
shutdown.
NOTE: For units rated 1800 rpm, 72 Hz is equal to
2160 rpm.
NOTE: The control board also uses the sensing
lead signals (S15, S16) (a) to terminate cranking
at about 50% of rated frequency, and (b) as an
“engine running” signal. The circuit board will not
initiate transfer of electrical loads to the
"Standby" source until sensing voltage and fre-
quency is greater than 50 % of the unit’s rated
values.
OVERCRANK SHUTDOWN:
Automatic engine cranking and startup normally
occurs when the control board senses that utility
source voltage has dropped below approximately 60
percent of its nominal rated voltage and remains at
that low level longer than six (6) seconds. At the end
of six (6) seconds, control board action will energize a
crank relay and a run relay (both relays are on the
control board). On closure of the crank relay contacts,
control board action will deliver 12 volts DC to a con-
trol contactor (CC). The control contactor will ener-
gize and battery power will be delivered across its
closed contacts to the starter motor (SM). The engine
will then crank.
During a manual startup (AUTO-OFF-MANUAL
switch at "Manual"), action is the same as during an
automatic start, except that cranking will begin imme-
diately when the switch is set to "Manual".
Control board action (during both a manual and an
automatic start) will hold the crank relay energized for
about 7-9 seconds. The relay will then de-energize for
about 7-9 seconds, and then energize again. In this
manner, the engine will be cranked for 7-9 seconds,
will rest for 7-9 seconds, and will crank again, and so
on until the engine starts.
If the engine has not started after approximately 90 sec-
onds of these crank-rest cycles, cranking will automati-
cally terminate and shutdown will occur. The control
board uses AC signals from the stator battery charge
windings as an indication that the engine has started.