Doosan G20P-5, Руководство по обслуживанию

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G424F(FE) Service Manual Chapter 5. Engine Management System (EMS) 144
and length. There are fewer parts to malfunction on
the DIS system when compared to a distributor type
system.
DIS COMPONENTS
In a typical DIS ignition system, a crankshaft
position sensor generates a basic timing signal by
reading notches on the crankshaft, flywheel, or
harmonic balancer. The crank sensor signal goes to
the small engine control module (SECM), where it is
used to determine firing order and turn the individual
ignition coils on and off.
The operation of the ignition system is essentially
the same as any other ignition system. Each coil has
a low primary resistance (0.4 to 0.6 ohms) and steps
up the primary system voltage from 12 volts to as
much as 40,000 volts to produce a spark for the
spark plug.
It is a direct connection that delivers the hottest
spark possible. Resistor plugs are generally used to
suppress electromagnetic interference (EMI).
MISFIRES
DIS problems can include many of the same
ailments as other ignition systems such as misfiring,
hard starting, or a no start. Spark plugs can still be
fouled by oil or fuel deposits, as well as pre-ignition
and detonation.
If the crankshaft position sensor fails, the loss of the
basic timing signal will prevent the system from
generating a spark and the engine will not start or
run. A failed driver circuit within the SECM can kill
individual coils and prevent two cylinders from firing.
It is important to remember that ignition misfire can
also be caused by other factors such as worn or
fouled spark plugs, loose or damaged coil connector
or terminals, dirty fuel injectors, low fuel pressure,
intake vacuum leaks, loss of compression in a
cylinder, even contaminated fuel. These other
possibilities should all be ruled out before a DIS unit
is replaced.
A DIS engine that cranks but fails to start, in many
cases, will often have a problem in the crankshaft or
camshaft position sensor circuits. Loss of sensor
signals may prevent the SECM from properly
synchronizing, thereby preventing the engine from
starting and running.
Exhaust System
Heated Exhaust Gas Oxygen Sensors (HEGO)
The MI-07 system utilizes two HEGO (O2) sensors.
One sensor is a pre-catalyst sensor that detects the
amount of oxygen in the exhaust stream and is
considered the primary control point. Based upon
the O2 sensor feedback, the MI-07 system supplies
a stoichiometric air-fuel ratio to the catalytic
converter. The catalytic converter then reduces
emissions to the required levels. The second sensor
is a post-catalyst sensor that detects the amount of
oxygen after the catalyst. This sensor is used as a
secondary control point to adjust the pre-catalyst
setpoint to ensure proper catalyst conversion
efficiency.
Figure 18. HEGO (O2) Sensor
Once a HEGO sensor reaches approximately 600°F
(316°C), it becomes electrically active. The
concentration of oxygen in the exhaust stream
determines the voltage produced. If the engine is
running rich, little oxygen will be present in the
exhaust and voltage output will be relatively high.
Conversely, in a lean situation, more oxygen will be
present and a smaller electrical potential will be
noticed.
In order for the sensor to become active and create
an electrical signal below 600°F (316°C) a heated
element is added to the sensor housing. Two wires
provide the necessary 12 Vdc and ground signal for
the heater element. A fourth wire provides an
independent ground for the sensor. The pre-catalyst
sensor heater is powered by the main power relay
and is always powered. The post-catalyst sensor
heater is powered from an additional relay that is
controlled by the SECM. This relay is only energized
when the SECM calculates that water condensation
in the exhaust system and catalytic muffler prior to
the sensor should be evaporated. This is to avoid
thermal shock of the sensor that could prematurely
fail the sensor.