JLG 450A_AJ Series II Service Manual - Manual (Page 150)

SECTION 3 - CHASSIS & TURNTABLE
3-100 – JLG Lift – 3121180
Intake Air Temperature (IAT) Sensor
The intake air temperature (IAT)
sensor is a thermistor which
changes resistance based on
temperature of air entering the
engine. Low temperature pro-
d u c e s a h i g h r e s i s t a n c e o f
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 and
monitors signal voltage. Signal
voltage is high when incoming air
is cold and low when incoming
air is hot. The ECM calculates
incoming air temperature by measuring voltage. The IAT
sensor signal adjusts spark timing according to incoming
air density. An IBM PC-compatible computer with diagnos-
tic software can display air temperature entering the
engine. Temperature should read close to ambient air tem-
perature when engine is cold, and rise as engine compart-
ment temperature increases. If engine has not been run for
several hours (overnight), IAT sensor temperature and
engine coolant temperature should read close to each
other. An IAT sensor circuit failure sets DTC 35 or DTC 45.
Manifold Absolute Pressure (MAP) Sensor
The Manifold Absolute Pressure (MAP) sensor responds
to changes in intake manifold pressure (vacuum). MAP
sensor signal voltage to ECM varies from below 2 volts at
idle (high vacuum) to above 4 volts with ignition ON,
engine not running, or at wide-open throttle (low vacuum).
The MAP sensor determines:
• Engine vacuum level for engine control purposes.
• Barometric pressure (BARO)
If the ECM detects a voltage 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 monitors information from various
sensors and controls systems that affect engine perfor-
mance. The ECM performs system diagnostics. It can rec-
ognize operational problems, alert operator through
Malfunction Indicator Lamp (MIL), and store diagnostic
trouble codes (DTCs). DTCs identify problem areas to aid
in making repairs.
The ECM supplies 5 or 12 volts to power various sensors
or switches. Power is supplied through resistances in the
ECM which are so high in value 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 volt-
meter with at least 10 meg ohms input impedance is
required to ensure accurate voltage readings. The ECM
controls output circuits such as fuel injectors, electronic
governor, etc., by controlling ground or power feed cir-
cuits through transistors or other solid state devices.
The ECM is designed to maintain exhaust emission levels
to government mandated standards while providing excel-
lent operation and fuel efficiency. The ECM monitors
numerous engine functions via electronic sensors such as
the throttle position (TP) sensor and heated oxygen sen-
sor (HO2S).