JLG 660SJ Service Manual - Manual (Page 159)

SECTION 3 - CHASSIS & TURNTABLE
3121202 – JLG Lift – 3-109
3.21 GM ENGINE LPG FUEL SYSTEM
DIAGNOSIS
Fuel System Description
To maintain fuel and emission control on the LPG fuel sys-
tem the Engine Control Units (ECM) relies on numerous
engine sensor and output data from the Electronic Pres-
sure Regulator (EPR). The ECM will then determine the
target fuel calibration and command the EPR to reposition
the voice coil to the proper position which, subsequently
reposition the secondary lever in the pressure regulator to
maintain proper control. The EPR and ECM will continue
to communicate back and forth during normal operation.
In the event that the EPR fails to communicate or the Com-
munications Area Network (CAN) cable fails to transmit
data the regulator will operate in an open loop configura-
tion. As the air valve vacuum in the mixer venturi is com-
municated to the secondary chamber of the regulator the
secondary diaphragm will be drawn in a downwards
motion. This downward motion will cause the secondary
lever to open thus allowing more fuel to enter the mixer.
In the (LPR) the fuel is vaporized and the pressure
reduced in two stages. The first stage reduces the pres-
sure to approximately 1.0 to 3.0 psi (6.8 to 20.6 kPa). The
second stage reduces the pressure to approximately neg-
ative 1.5" of water column.
The fuel is then drawn from the secondary chamber of the
LPR by the vacuum generated by air flowing through the
mixer. This vacuum signal is also used to generate lift for
the mixer air valve. This vacuum signal is most commonly
referred to as air valve vacuum. In the mixer, the fuel mixes
with the air entering the engine. This air/ fuel mixture is
then drawn into the engine for combustion.
Diagnostic Aids
This procedure is intended to diagnose a vehicle operat-
ing on LPG. If the vehicle will not continue to run on LPG,
refer to Hard Start for preliminary checks. Before proceed-
ing with this procedure, verify that the vehicle has a suffi-
cient quantity of fuel and that liquid fuel is being delivered
to the LPR. Also, ensure that the manual shut off valve on
the LPG tank is fully opened and that the excess flow valve
has not been activated.
Tools Required:
• 7/16 Open end wrench (for test port plugs)
• DVOM (GM J 39200, Fluke 88 or equivalent).
• 12 volt test light
Diagnostic Scan Tool
• Diagnostic Display tool.
Pressure Gauges
• IMPCO ITK-2 Test kit
• Water Column Gauge / Manometer (GM 7333-6 or
equivalent).
• 0-10 PSI Gauge
Test Description
The numbers below refer to step numbers on the diagnos-
tic table.
5. This step determines if the LPR requires replacement
6. This step determines if the problems are in the mechan-
ical side of the Pressure Regulator or the Electronic Voice
Coil
10. This step determines if the Mixer requires replacement
14. This step determines if the Lock Off requires replace-
ment
17. This step determines if the Fuel Filter requires replace-
ment.
Figure 3-89. EPR Assembly