JLG 660SJ Service Manual - Manual (Page 128)

SECTION 3 - CHASSIS & TURNTABLE
3-84 – JLG Lift – 3121298
Heated Exhaust Gas Oxygen Sensor
There are two Heated Exhaust Gas Oxygen Sensors (HEGO).
The first HEGO is mounted in the exhaust system downstream
of the engine. It is used to measure the amount of oxygen
present in the exhaust stream and communicate that to the
ECM via an electrical signal. The amount of oxygen present in
the exhaust stream indicates whether the fuel/air ratio is too
rich or too lean. If the HEGO sensor signal indicates exhaust
stream is too rich, the ECM will decrease or lean the fuel mix-
ture during engine operation. If mixture is too lean the ECM
will richen the mixture. The ECM continuously monitors the
HEGO sensor output. If a rich or lean condition is present for an
extended period of time, and the ECM cannot correct the con-
dition, the ECM sets a diagnostic code and turns on the MIL
light in control box.
The second HEGO is mounted in the exhaust system after the
muffler. It measures the amount of oxygen in the exhaust sys-
tem after the catalyst treatment has been completed in the
muffler. If the ECM detects that the catalytic action in the muf-
fler is not sufficient and fuel correction cannot correct the mal-
function the MIL light is illuminated in the control box and a
DTC code will stored in the computer.
THE HEATED EXHAUST GAS OXYGEN SENSOR IS AN EMISSION CONTROL
DEVICE. IF THE HEGO FAILS TO OPERATE, REPLACE IT WITH AN OEM REPLACE-
MENT PART. THE HEGO SENSOR IS SENSITIVE TO SILICONE OR SILICONE BASED
PRODUCTS AND CAN BECOME CONTAMINATED. AVOID USING SILICONE SEAL-
ERS OR HOSES TREATED WITH SILICONE LUBRICANTS IN THE AIR STREAM OR
FUEL LINES.
Gasoline Multi Point Fuel Injection System (MPFI)
The primary components of the Gasoline Multi Point Fuel
Injection (MPFI) fuel system are the fuel tank, electric fuel
pump, fuel pressure and temperature sensor manifold, fuel fil-
ter, and fuel rail.
Gasoline Fuel Pump
Gasoline is stored as a liquid in the fuel tank and in drawn into
the fuel system by an electric fuel pump. The fuel pump
receives a signal from the ECM to prime the fuel system for
approximately two seconds before start. Priming the fuel sys-
tem provides for a quicker start when engine begins to crank.
Gasoline Pressure And Temperature Sensor
Manifold
This engine is equipped with a fuel injector rail that does not
have a pressure regulator or a return circuit to the fuel tank.
Fuel pressure for this engine is regulated by the engine’s ECM.
The ECM receive fuel pressure and temperature feedback from
the gasoline fuel sensor manifold and uses this information to
control the ground side of the fuel pump. Fuel pressure is regu-
lated by the ECM pulse width modulating (PWM) the fuel
pump. The fuel pressure and temperature sensor manifold has
a return or "bleed" circuit that connects back to the fuel tank.
This circuit is used to bleed off any vapor that develops in the
line and return a small amount of fuel to the tank.
Fuel comes from the fuel tank and passes through the fuel
pump. Fuel exits the fuel pump, passes through the filter and
then enters the fuel pressure and temperature manifold
assembly. Fuel flows through the feed circuit and is delivered
to the fuel injector rail. Fuel that enters the bleed circuits
through the bypass valve in the manifold is returned to the
fuel tank.
Figure 3-102. Heated Exhaust Gas Oxygen Sensor
Figure 3-103. Gasoline Fuel Pressure and Temperature
Manifold Assembly