Guardian 4721, Repair Manual

Page: 70 / 115

PART 4
CIRCUIT CONDITION- CRANKING AND INITIAL STARTUP
• The control board’s 15-second timer will time for 15 seconds. If “Utility” source voltage is still low after 15 sec-
onds, a crank relay (K1) and a run relay (K2) on the circuit board will energize simultaneously.
• When the crank relay (K1) energizes, its contacts will close to deliver 12 volts DC to a control contactor (CC)
coil via Wire 56. The contactor energizes and its contacts close to deliver battery power to the engine starter
motor (SM). Starter motor (SM) energizes and the engine is cranked.
• On the first crank cycle circuit board action will hold crank relay (K1) energized for about 7-9 seconds, will de-
energize the relay for 7-9 seconds, energize it again for 7-9 seconds, and so on. Thus, the engine will crank for
7-9 seconds, rest for 7-9 seconds, and so on. This cyclic crank/rest action will continue until either the engine
starts, or has cycled through six (6) crank cycles. If the engine has not started in approximately 90 seconds,
cranking will end and the fault indicator lamp will light (“overcrank” condition).
• When the engine is cranking, a DC output is delivered to the generator rotor. This is “field boost” current, which
is delivered cyclically (during cranking only). See “Field Boost Circuit’ on Page 2.1-4.
• When the engine run relay (K2) is energized by circuit board action, 12 volts DC is delivered to a fuel solenoid,
and to the engine ignition system. The fuel solenoid (FS) is energized open, to allow fuel flow to the fuel sys-
tem. Ignition occurs.
• With fuel available to the carburetor and with engine ignition occurring, the engine will start.
• Generator AC output voltage and frequency signals are available to the control board, via Wires S15 and S16.
• When generator AC output frequency reaches approximately 50% of the rated AC frequency, an “engine warm-
up timer” and an “engine minimum run timer” on the control board are both turned on.
Page 66
DC CONTROL
LIQUID COOLED
ENGINE UNITS
SECTION 4.2
OPERATIONAL ANALYSIS