SECTION 3 - CHASSIS & TURNTABLE
3-106
– JLG Lift –
3120740
FUEL PUMP ELECTRICAL CIRCUIT
When the key is first turned "ON," the ECM energizes the fuel
pump relay for two seconds to build up the fuel pressure
quickly. If the engine is not started within two seconds, the
ECM shuts the fuel pump off and waits until the engine is
cranked. When the engine is cranked and crankshaft position
signal has been detected by the SECM, the ECM supplies 12
volts to the fuel pump relay to energize the electric fuel pump.
An inoperative fuel pump will cause a "no-start" condition. A
fuel pump which does not provide enough pressure will result
in poor performance.
FUEL RAIL
The fuel rail is mounted to the top of the engine and distrib-
utes fuel to the individual injectors. Fuel is delivered to the fuel
inlet tube of the fuel rail by the fuel lines.
ELECTRONIC GOVERNOR AND THROTTLE BODY
In the 2.5L EFI industrial engine, throttle control is achieved by
using an electronic governor which is controlled by the engine
control module (ECM).
The electronic governor consists of a throttle body, an elec-
tronically-actuated throttle plate, and a built-in throttle posi-
tion (TP) sensor. There are two pigtails that exit the governor
body. The 3-wire pigtail connects the TP sensor to the ECM.
Refer to "Throttle Position (TP) Sensor" for more information.
The 2-wire pigtail carries the throttle signal from the ECM to
the governor. Desired engine speeds are stored in the configu-
ration program for each specific application, and can be
changed with the ECM calibration software. When an engine
speed is selected with the toggle switch, the ECM sends the
appropriate signal to the governor. This is a pulse-width mod-
ulated (PWM) signal which cannot be read with conventional
diagnostic tools such as a voltmeter. A 12-volt signal is pulsed
on and off at a high rate of speed. The width of the "on" pulse
determines the amount of throttle opening. The ECM sends a
signal with the appropriate pulse width to the governor based
on the operator's choice of switch settings.
The electronic governor also acts as an idle air control (IAC)
valve. Changes in engine load are detected by the ECM by
comparing manifold absolute pressure (MAP) with throttle
position. When the ECM detects a change in engine load, it
can adjust idle speed by changing the PWM signal to the gov-
ernor.
OPEN LOOP AND CLOSED LOOP OPERATION
The ECM will operate in the following two modes:
• Open loop
• Closed loop
When the engine is first started, the system is in "open loop"
operation. In open loop, the ECM ignores the signal from the
heated oxygen sensor (HO2S). it uses a pre-programmed rou-
tine to calculate the air/fuel ratio based on inputs from the TP,
ECT, and MAP sensors.
The system remains in open loop until the following condi-
tions are met:
• The HO2S has a varying voltage output showing that it is
hot enough to operate properly (this depends on tempera-
ture).
• The ECT has reached 160°F (71°C).
• Seven minutes has elapsed since starting the engine.
After these conditions are met, the engine is said to be operat-
ing in "closed loop." In closed loop, The ECM continuously
adjusts the air/fuel ratio by responding to signals from the
HO2S (except at wide-open throttle). When the HO2S reports a
lean condition (low sensor signal voltage), the ECM responds
by increasing the "on" time of the fuel injectors, thus enriching
the mixture. When the HO2S reports a rich condition (high
sensor signal Voltages the ECM responds by reducing the "on"
time of the fuel injectors, thus leaning out the mixture.
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