Kohler command pro ECH749 Service Manual page 32

EFI SYSTEM-ECH
Earlier engines have a separate intake air temperature
(IAT) sensor (located in throttle body) and a manifold
absolute pressure (MAP) sensor. Later engines have
a combined temperature/manifold absolute pressure
(TMAP) sensor.
Intake Air Temperature (IAT) sensor is a thermally
sensitive resistor that exhibits a change in electrical
resistance with a change in its temperature. When
sensor is cold, resistance of sensor is high. As sensor
warms up, resistance drops and voltage signal
increases. From voltage signal, ECU can determine
temperature of intake air.
Purpose of an air temperature sensor is to help ECU
calculate air density. Higher air temperature less dense
air becomes. As air becomes less dense ECU knows
that it needs to lessen fuel fl ow to achieve correct air/fuel
ratio. If fuel ratio was not changed engine would become
rich, possibly losing power and consuming more fuel.
Manifold absolute pressure (MAP) sensor provides
immediate manifold pressure information to ECU.
MAP measures difference in pressure between outside
atmosphere and vacuum level inside intake manifold
and monitors pressure in manifold as primary means
of detecting load. Data is used to calculate air density
and determine engine's mass air fl ow rate, which in
turn determines required ideal fueling. MAP also stores
instant barometric pressure reading when key is turned
ON.
Later engines have a Temperature/Manifold Absolute
Pressure (TMAP) sensor. This is an integrated sensor
that checks both intake air temperature and manifold
absolute pressure. This combined sensor is located in
intake manifold.
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.
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 up to 1.0 volt is generated
and sent to ECU. Voltage signal tells ECU if engine is
straying from ideal fuel mixture, and ECU then adjusts
injector pulse accordingly.
Oxygen sensor functions after being heated to a
minimum of 400°C (752°F). A heater inside sensor heats
electrode to optimum temperature in about 10 seconds.
Oxygen sensor receives ground through wire, eliminating
need for proper grounding through muffl er. If problems
indicate a bad oxygen sensor, check all connections and
wire harness. Oxygen sensor can also be contaminated
by leaded fuel, certain RTV and/or other silicone
compounds, fuel injector cleaners, etc. Use only those
products indicated as O2 Sensor Safe.
32
Fuel injectors mount into intake manifold, and high
pressure fuel line 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 high
pressure fuel line and holds it in place. O-rings and
retaining clip must be replaced any time fuel injector is
separated from its normal mounting position.
When key switch is on, fuel pump module will pressurize
high pressure fuel line to 39 psi, 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 high
pressure fuel line 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.
Injectors have sequential fueling that open and close
once every other crankshaft revolution. 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. Time injector is open
(milliseconds) may vary in duration depending on speed
and load requirements of engine.
A high-voltage, solid-state, battery ignition system is
used with EFI system. ECU controls ignition output and
timing through transistorized control of primary current
delivered to coils. Based on input from crankshaft
position sensor, ECU determines correct fi ring point
for speed at which engine is running. At proper instant,
it interrupts fl ow of primary current in coil, causing
electromagnetic fl ux fi eld to collapse. Flux collapse
induces an instantaneous high voltage in coil secondary
which is strong enough to bridge gap on spark plug.
Each coil fi res every other revolution.
EFI engines are equipped with either a 20 or 25 amp
charging system to accommodate combined electrical
demands of ignition system and specifi c application.
Charging system troubleshooting information is provided
in Electrical System.
An electric fuel pump module and a lift pump (two types)
are used to transfer fuel in EFI system. Types of lift
pumps are: a pulse fuel pump, a mechanical fuel pump,
or a low pressure electric fuel pump. Pumping action
is created by either oscillation of positive and negative
pressures within crankcase through a hose, or by direct
lever/pump actuation off rocker arm movement. Pumping
action causes diaphragm on inside of pump to pull fuel
in on its downward stroke and to push it into fuel pump
module on its upward stroke. Internal check valves
prevent fuel from going backward through pump. Fuel
pump module receives fuel from lift pump, increases and
regulates pressure for fuel injectors.
Fuel pump module is rated for a minimum output of 13.5
liters per hour and regulated at 270 kilopascals (39 psi).
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