JLG 600A_AJ Service Manual - Manual (Page 124)

SECTION 3 - CHASSIS & TURNTABLE
3-74 – JLG Lift – 3121201
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 100,000 ohms at -40°C (-40°F). High temper-
ature 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 volt-
age. The signal voltage will be high when the incoming air
is cold and low when the incoming air is hot. By measur-
ing the voltage, the ECM calculates the incoming air tem-
perature. 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 compartment 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 sensor signal voltage to the ECM varies from below 2
volts at idle (high vacuum) to above 4 volts with the igni-
tion 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 sec-
onds, DTC 14 will be set. A signal voltage significantly
higher than the estimated MAP value for 2 or more con-
secutive 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 vari-
ous sensors. The ECM controls the systems that affect
engine performance. The ECM performs the diagnostic
function of the system. It can recognize operational prob-
lems, alert the operator 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 resis-
tances 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. There-
fore, a digital voltmeter with at least 10 meg ohms input
impedance is required to ensure accurate voltage read-
ings. 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.