SECTION 3 - CHASSIS & TURNTABLE
3120740
– JLG Lift –
3-97
INTAKE AIR TEMPERATURE (IAT) SENSOR
The intake air temperature (IAT) sensor is a thermistor which
changes its resistance based on the temperature of air enter-
ing the engine. Low temperature produces a high resistance of
100,000 ohms at -40°C (-40°F). High temperature causes a low
resistance of 70 ohms at 130°C (266°F). The ECM supplies a 5-
volt signal to the sensor through a resistor in the ECM and
monitors the signal voltage. The signal voltage will be high
when the incoming air is cold and 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. An
IBM PC-compatible computer with diagnostic soft-ware can
be used to display 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 engine com-
partment 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 35 or DTC
45.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The manifold absolute pressure (MAP) sensor responds to
changes in intake manifold pressure (vacuum). The MAP sen-
sor 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 engine control purposes.
• Barometric pressure (BARO)
If the ECM detects a voltage that is significantly lower than the
estimated MAP value for 2 or more consecutive seconds, DTC
14 will be set. A signal voltage significantly higher than the
estimated MAP value for 2 or more consecutive seconds will
set DTC 24.
ENGINE CONTROL MODULE (ECM)
The ECM controls the following:
• Fuel metering system
• Ignition timing
• On-board diagnostics for engine functions
The ECM constantly observes the information from various
sensors. The ECM controls the systems that affect engine per-
formance. The ECM performs the diagnostic function of the
system. It can recognize operational problems, alert the opera-
tor through the Malfunction Indicator Lamp (MIL), and store
diagnostic trouble codes (DTCs). DTCs identify the problem
areas to aid the technician in making repairs.
The ECM supplies either 5 or 12 volts to power various sensors
or switches. The power is supplied through resistances in the
ECM which are so huh 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 meg ohms input impedance is required to ensure accurate
voltage readings. The ECM controls output circuits such as the
fuel injectors, electronic governor, etc., by control ling the
ground or the power feed circuit through transistors or other
solid state devices.
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