Kohler command pro ECH749 Service Manual page 61

ECU requires a minimum of 7.0 volts to operate.
Adaptive memory in ECU is operational whenever
required voltage is present, however adapted values
are lost if power supply is disrupted for any reason.
ECU will "relearn" adapted values if engine is operated
for 10-15 minutes at varying speeds and loads after oil
temperature exceeds 55°C (130°F).
To prevent engine over-speed and possible failure,
a "rev-limiting" feature is programmed into ECU.
If maximum RPM limit (4500) is exceeded, ECU
suppresses injection signals, cutting off fuel fl ow. This
process repeats itself in rapid succession, limiting
operation to preset maximum.
Engine speed sensor is essential to engine operation;
constantly monitoring rotational speed (RPM) of
crankshaft. A ferromagnetic 60-tooth ring gear with
two consecutive teeth missing is mounted on fl ywheel.
Inductive speed sensor is mounted 1.5 ± 0.25 mm
(0.059 ± 0.010 in.) away from ring gear. During rotation,
an AC voltage pulse is created within sensor for each
passing tooth. ECU calculates engine speed from time
interval between consecutive pulses. Two-tooth gap
creates an interrupted input signal, corresponding to
specifi c crankshaft position (84° BTDC) for cylinder #1.
This signal serves as a reference for control of ignition
timing by ECU. Synchronization of inductive speed
pickup and crankshaft position takes place during fi rst
two revolutions each time engine is started. Sensor must
be properly connected at all times. If sensor becomes
disconnected for any reason, engine will quit running.
Throttle position sensor (TPS) is used to indicate throttle
plate angle to ECU. Since throttle (by way of governor)
reacts to engine load, angle of throttle plate is directly
related to load on engine.
Mounted on throttle body/intake manifold and
operated directly off end of throttle shaft, TPS works
as a potentiometer, varying voltage signal to ECU in
direct correlation to angle of throttle plate. This signal,
along with other sensor signals, is processed by ECU
and compared to internal pre-programmed maps to
determine required fuel and ignition settings for amount
of load.
Correct position of TPS is established and set at factory.
Do not loosen TPS or alter mounting position unless
absolutely required by fault code diagnosis or throttle
shaft service. If TPS is loosened or repositioned,
appropriate "TPS Initialization Procedure must be
performed to reestablish baseline relationship between
ECU and TPS.
Engine (oil) temperature sensor is used by system to
help determine fuel requirements for starting (a cold
engine needs more fuel than one at or near operating
temperature).
Mounted in oil fi lter adapter housing, it has a
temperature-sensitive resistor that extends into oil
fl ow. Resistance changes with oil temperature, altering
voltage sent to ECU. Using a table stored in its memory,
ECU correlates voltage drop to a specifi c temperature.
Using fuel delivery "maps", ECU then knows how much
fuel is required for starting at that temperature.
Oxygen sensor functions like a small battery, generating
a voltage signal to ECU based upon difference in oxygen
content between exhaust gas and ambient air.
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Tip of sensor, protruding into exhaust gas, is hollow.
Outer portion of tip is surrounded by exhaust gas, with
inner portion exposed to ambient air. When oxygen
concentration on one side of tip is different than that of
other side, a voltage signal between 0.2 and 1.0 volts
is generated between electrodes and sent to ECU.
Voltage signal tells ECU if engine is straying from ideal
14.7:1 fuel mixture, and ECU then adjusts injector pulse
accordingly.
Oxygen sensor can function only after being heated by
exhaust temperatures to a minimum of 375°C (709°F).
A cold oxygen sensor will require approximately 1-2
minutes at moderate engine load to warm suffi ciently
to generate a voltage signal. Proper grounding is also
critical. Oxygen sensor grounds through metal shell,
so a good, solid, unbroken ground path back through
exhaust system components, engine, and wiring harness
is required. Any disruption or break in ground circuit can
affect output signal and trigger misleading fault codes.
Keep that in mind when doing any troubleshooting
associated with oxygen sensor. Oxygen sensor can also
be contaminated by leaded fuel, certain RTV and/or
other silicone compounds, carburetor cleaners, etc. Use
only those products indicated as "O
Electrical relay is used to supply power to injectors,
coil, and fuel pump. When key switch is turned "ON"
and all safety switch requirements met, relay provides
12 volts to fuel pump circuit, injectors, and ignition
coils. Fuel pump circuit is continuously grounded, so
pump is immediately activated and pressurizes system.
Activation of ignition coils and fuel injectors is controlled
by ECU, which grounds their respective ground circuits
at proper times.
Fuel injectors mount into throttle body/intake manifold,
and fuel rail attaches to them at top end. Replaceable
O-Rings on both ends of injector prevent external fuel
leakage and also insulate it from heat and vibration. A
special clip connects each injector to fuel rail and holds
it in place. O-Rings must be replaced anytime injector is
removed.
When key switch is on and relay is closed, fuel rail
is pressurized, and voltage is present at injector.
At proper instant, ECU completes ground circuit,
energizing injector. Valve needle in injector is opened
electromagnetically, and pressure in fuel rail forces fuel
down through inside. "Director plate" at tip of injector
contains a series of calibrated openings which directs
fuel into manifold in a cone-shaped spray pattern.
Injector is opened and closed once for each crankshaft
revolution, however only one-half total amount of fuel
needed for one fi ring is injected during each opening.
Amount of fuel injected is controlled by ECU and
determined by length of time valve needle is held open,
also referred to as "injection duration" or "pulse width". It
may vary in length from 1.5-8 milliseconds depending on
speed and load requirements of engine.
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