JLG 601S ANSI Service Manual - Manual (Page 120)

SECTION 3 - CHASSIS & TURNTABLE
3-80 – JLG Lift – 3120746
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 Engine Module supplies a 5-volt signal to the
sensor through a resistor in the Engine Module and moni-
tors 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 Engine Module
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 tempera-
ture 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 cool-
ant 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 Engine Module 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 Engine Module detects a voltage that is significantly
lower than the estimated MAP value for 2 or more consec-
utive seconds, DTC 14 will be set. A signal voltage signifi-
cantly higher than the estimated MAP value for 2 or more
consecutive seconds will set DTC 24.
ENGINE CONTROL MODULE (ECM)/ENGINE
PERFORMANCE MODULE (EPM)
The engine will be controlled by one of two different
Engine Modules. The Ford EFI engine was originally
equipped with an ECM. The EPM was developed to
replace the ECM and provide enhanced performance and
durability. To see the physical difference between the ECM
and EPM, refer to Figure 3-47., ECM/EPM Identification.
The Engine Module controls the following:
• Fuel metering system
• Ignition timing
• On-board diagnostics for engine functions
The Engine Module constantly observes the information
from various sensors. The Engine Module controls the
systems that affect engine performance. The Engine Mod-
ule performs the diagnostic function of the system. It can
recognize operational problems, alert the operator
through the Malfunction Indicator Lamp (MIL), and store
diagnostic trouble codes (DTCs). DTCs identify the prob-
lem areas to aid the technician in making repairs.
The Engine Module supplies either 5 or 12 volts to power
various sensors or switches. The power is supplied
through resistances in the Engine Module which are so
low in value that a test light will not light when connected
to the circuit. In some cases, even an ordinary shop volt-
meter will not give an accurate reading because its resis-
tance is too low. Therefore, a digital voltmeter with at least
10 meg ohms input impedance is required to ensure
accurate voltage readings. The Engine Module controls
output circuits such as the fuel injectors, electronic gover-
nor, etc., by control ling the ground or the power feed cir-
cuit through transistors or other solid state devices.