Isuzu engine C22NE / 22LE / 20LE. Manual - part 114

DRIVEABILITY AND EMISSIONS  6E1-285

GENERAL DESCRIPTION

ECM AND SENSORS

58X REFERENCE ECM INPUT

The engine control module (ECM) uses this signal from the
crankshaft position (CKP) sensor to calculate engine RPM and
crankshaft position at all engine speeds. The ECM also uses
the pulses on this circuit to initiate injector pulses. If the ECM
receives a number of pulses other than the expected amount,
DTC 19, will set. The engine will not start and run without using
the 58X reference signal.

A/C REQUEST SIGNAL

This signal tells the ECM when the A/C mode is selected at the
A/C control switch. The ECM uses this signal to adjust the idle
speed before turning ON the A/C clutch. The A/C compressor
will be inoperative if this signal is not available to the ECM.
Refer to A/C Clutch Circuit Diagnosis for A/C wiring diagrams
and diagnosis for the A/C electrical system.

CRANKSHAFT POSITION (CKP) SENSOR

The crankshaft position (CKP) sensor provides a signal used
by the engine control module (ECM) to calculate the ignition
sequence and ECM uses this signal as a trigger for fuel
injection timing and spark timing. The CKP sensor initiates the
58X reference pulses which the ECM uses to calculate RPM
and crankshaft position.
Refer to Electronic Ignition System for additional information.

6E1-286  DRIVEABILITY AND EMISSIONS

ENGINE COOLANT TEMPERATURE

(ECT) SENSOR

The engine coolant temperature (ECT) sensor is a thermistor
(a resistor which changes value based on temperature)
mounted in the engine coolant stream. Low coolant
temperature produces a high resistance of about 100,000
ohms at 

−40°C (−40°F). High temperature causes a low

resistance of about 70 ohms at 130°C (266°F).
The ECM supplies a 5-volt signal to the ECT sensor through
resistors internal to the ECM and then measures the voltage
after the internal resistor. This signal voltage will be high when
the engine is cold and low when the engine is hot. By
measuring the voltage, the ECM calculates the engine coolant
temperature. Engine coolant temperature affects most of the
systems that the ECM controls.
The Scan Tool displays engine coolant temperature in degrees.
After engine start-up, the temperature should rise steadily to
about 92°C (197°F). It then stabilizes when the thermostat
opens. If the engine has not been run for several hours
(overnight), the engine coolant temperature and intake air
temperature displays should be close to each other. A hard
fault in the engine coolant sensor circuit will set DTC 14 or 15.

FUEL CONTROL HEATED OXYGEN SENSOR

(IF APPLICABLE)

The fuel control heated oxygen sensor is mounted in the
exhaust stream where it can monitor the oxygen content of the
exhaust gas. The oxygen present in the exhaust gas reacts
with the sensor to produce a voltage output. This voltage
should constantly fluctuate from approximately 60mV to
900mV. The heated oxygen sensor voltage can be monitored
with a Scan Tool. By monitoring the voltage output of the
oxygen sensor, the ECM calculates the pulse width command
for the injectors to produce the proper combustion chamber
mixture.
• Low HO2S voltage is a lean mixture which will result in a rich

command to compensate.

• High HO2S voltage is a rich mixture which will result in a

lean command to compensate.

When HO2S is not ready for use as a feedback for fuel control
system, Diagnostic Trouble Code 13 will be set and the scan
tool will display a voltage between 340mv and 540mv.

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INTAKE AIR TEMPERATURE (IAT) SENSOR

The intake air temperature (IAT) sensor is a thermistor which
changes its resistance based on the temperature of air entering
the engine. Low temperature produces a high resistance of
about 100,000 ohms at 

−40°C (−40°F). High temperature

causes low resistance of about 70 ohms at 130°C (266°F). The
ECM supplies a 5-volt signal to the sensor through a resistor
internal to the ECM, and then monitors the signal voltage. The
voltage will be high when the incoming air is cold. The voltage
will be low when the incoming air is hot. By measuring the
voltage, the ECM calculates the incoming air temperature. the
IAT sensor signal is used to adjust spark timing according to
the incoming air density.
The Scan Tool displays the temperature of the air entering the
engine. The temperature should read close to the ambient air
temperature when the engine is cold and rise as underhood
temperature increases. If the engine has not been run for
several hours (overnight), the IAT sensor temperature and
engine coolant temperature should read close to each other. A
failure in the IAT sensor circuit will set DTC 69 or DTC 71.

MANIFOLD ABSOLUTE PRESSURE

(MAP) SENSOR

The MAP sensor responds to changes in intake manifold
pressure (vacuum). the MAP sensor signal voltage to the ECM
varies from below 2 volts at idle (high vacuum) to above 4 volts
with the ignition ON, engine not running or at wide-open throttle
(low vacuum).
The MAP sensor is used to determine the following.
• Engine vacuum level for other diagnostics.
• Barometric pressure (BARO).
If the ECM detects a voltage that is lower than the possible
range of the MAP sensor, DTC 34 will be set. A signal voltage
higher than the possible range of the sensor will DTC 33.

ENGINE CONTROL MODULE (ECM)

The engine control module (ECM) is located in the passenger
compartment at the Instrument panel lower center cover
assembly. The ECM controls the following:
• Fuel metering system.
• Ignition timing.
• On-board diagnostics for electrical functions.
The ECM constantly observes the information from various
sensors. The ECM controls the systems that affect vehicle
performance. The ECM performs the diagnostic function of the
system. It can recognize operational problems, alert the driver
through the Check Engine lamp, and store diagnostic trouble
codes (DTCs). DTCs identify the problem areas to aid the
technician in making repairs.

6E1-288  DRIVEABILITY AND EMISSIONS

ECM FUNCTION

The ECM supplies either 5 or 12 volts to power various sensors
or switches. The power is supplied through resistors in the
ECM which are so high in value that a test light will not light
when connected to the circuit. In some cases, even an ordinary
shop voltmeter will not give an accurate reading because its
resistance is too low. Therefore, a digital voltmeter with at least
10 megohms input impedance is required to ensure accurate
voltage readings. Tool 5-8840-2392-0 meets this requirement.
The ECM controls output circuits such as the injectors, IAC,
etc., by controlling the ground or the power feed circuit through
transistors or through either of the following two devices:
• Output Driver Module (ODM)
• Quad Driver Module (QDM)

ECM COMPONENTS

The ECM is designed to maintain exhaust emission levels to
government mandated standards while providing excellent
driveability and fuel efficiency. The ECM monitors numerous
engine and vehicle functions via electronic sensors such as the
throttle position sensor (TPS), heated oxygen sensor (HO2S) if
applicable, and vehicle speed sensor (VSS). The ECM also
controls certain engine operations through the following:
• Fuel injector control
• Ignition control module
• Evaporative emission (EVAP) purge
• A/C clutch control

ECM VOLTAGE DESCRIPTION

The ECM supplies a buffered voltage to various switches and
sensors. It can do this because resistors in the ECM which are
so high in value that a test light may not illuminate when
connected to the circuit. An ordinary shop voltmeter may not
give an accurate reading because the voltmeter input
impedance is too low. Use a 10-megohm input impedance
digital voltmeter (such as 5-8840-2392-0) to assure accurate
voltage readings.
The input/output devices in the ECM include analog-to-digital
converters, signal buffers, counters, and special drivers. The
ECM controls most components with electronic switches which
complete a ground circuit when turned  "ON."  These switches
are arranged in groups of 4, called a quad driver module
(QDM), which can independently control up to 4 output.