JLG 450A_AJ Series II Service Manual - Manual (Page 182)

SECTION 3 - CHASSIS & TURNTABLE
3-132 – JLG Lift – 3121180
3.20 GM ENGINE LPG FUEL SYSTEM
DIAGNOSIS
Fuel System Description
The LPG fuel system Engine Control Unit (ECM) relies on
engine sensors and output data from the Electronic Pres-
sure Regulator (EPR) to maintain fuel and emission con-
trol. The ECM determines target fuel calibration and
commands the EPR to reposition the voice coil to the
proper position which, subsequently reposition the sec-
ondary lever in the pressure regulator to maintain proper
control. The EPR and ECM continue to communicate dur-
ing normal operation.
If the EPR fails to communicate or the Communications
Area Network (CAN) cable fails to transmit data, the regu-
lator will operate in an open loop configuration. As the air
valve vacuum in the mixer venturi is communicated to the
secondary chamber of the regulator, the secondary dia-
phragm will be drawn in a downwards motion. This down-
ward motion will cause the secondary lever to open and
allow more fuel to enter the mixer.
In the Low Pressure Regulator (LPR) fuel is vaporized and
pressure reduced in two stages. The first stage reduces
pressure to approximately 1.0 to 3.0 psi (6.8 to 20.6 kPa).
The second stage reduces pressure to approximately
negative 1.5" of water column.
Fuel is then drawn from the secondary chamber of the
LPR by vacuum generated from 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, fuel mixes
with air entering the engine. This air/ fuel mixture is then
drawn into the engine for combustion.
Diagnostic Aids
This procedure is for diagnosing a vehicle operating on
LPG. If the vehicle will not continue to run on LPG, refer to
Hard Start for preliminary checks. Before proceeding with
this procedure, verify vehicle has sufficient quantity of fuel
and liquid fuel is being delivered to the LPR. Ensure LPG
tank manual shut off valve is fully opened and 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
Numbers below refer to step numbers on the diagnostic
table.
5. This step determines if LPR requires replacement
6. This step determines if problems are in mechanical side
of Pressure Regulator or Electronic Voice Coil
10. This step determines if Mixer requires replacement
14. This step determines if Lock Off requires replacement
17. This step determines if Fuel Filter requires replacement.
Figure 3-91. EPR Assembly