JLG 680S Service Manual - Manual (Page 170)

SECTION 3 - CHASSIS & TURNTABLE
3-122 – JLG Lift – 3121234
Low Pressure Regulator (LPR)
The LPR is a combination vaporizer, pressure regulating
device. The LPR is a negative pressure, two stage regula-
tor that is normally closed when the engine is not running.
When the engine is cranking or running, a partial vacuum
is created in the fuel line which connects the regulator to
the mixer. This partial vacuum opens the regulator permit-
ting fuel to flow to the mixer.
Propane fuel enters the primary port of the LPR and
passes through the primary jet and into the primary/
exchanger chamber. As the propane passes through the
heat exchanger the fuel expands and creates pressure
inside the chamber. The pressure rises as the fuel
expands when the pressure rises above 1.5 psi (10.34
kpa), sufficient pressure is exerted on the primary dia-
phragm to cause the diaphragm plate to pivot and press
against the primary valve pin thus closing off the flow of
fuel. This action causes the flow of fuel into the regulator
to be regulated.
When the engine is cranking, sufficient vacuum will be
introduced into the secondary chamber from the mixer
drawing the secondary diaphragm down onto the spring
loaded lever and opening the secondary valve allowing
vaporized fuel to pass to the mixer. This mechanical action
in conjunction with the EPR reactions causes the down-
ward action on the secondary lever causing it to open
wider allowing more fuel to flow to the mixer.
THE VOICE COIL SECTION OF THE EPR ASSEMBLY IS AN EMIS-
SIONS CONTROL DEVICE AND CANNOT BE REBUILT. IF THE
COIL ASSEMBLY FAILS TO OPERATE PROPERLY, REPLACE IT
WITH AN OEM REPLACEMENT PART ONLY.
Air Fuel Mixer
The air valve mixer is an air-fuel metering device and is
completely self-contained. The mixer is an air valve
design, utilizing a relatively constant pressure drop to
draw fuel into the mixer from cranking to full load. The
mixer is mounted in the air stream ahead of the throttle
control device.
When the engine begins to crank, it draws in air with the
air valve covering the inlet, negative pressure begins to
build. This negative pressure signal is communicated to
the top of the air valve chamber through 4 vacuum ports in
the air valve assembly. A pressure/force imbalance begins
to build across the air valve diaphragm between the air
valve vacuum chamber and the atmospheric pressure
below the diaphragm. The air valve vacuum spring is cali-
brated to generate from 4.0 inches (101.6 mm) of water
column at start to as high as 14.0 inches (355.60 mm) of
water column at full throttle. The vacuum being created is
referred to as Air Valve Vacuum (AVV). As the air valve
vacuum reaches 4.0 inches (101.6mm) of water column,
the air valve begins to lift against the air valve spring. The
amount of AVV generated is a direct result of the throttle
position. At low engine speed the air valve vacuum is low
and the air valve position is low thus creating a small ven-
turi for the fuel to flow. As the engine speed increase the
AVV increases and the air valve is lifted higher thus creat-
ing a much larger venturi. This air valve vacuum is com-
municated from the mixer venture to the LPR secondary
chamber via the low pressure fuel supply hose. As the
AVV increases in the secondary chamber the secondary
diaphragm is drawn further down forcing the secondary
valve lever to open wider.
Figure 3-78. Low Pressure Regulators
Figure 3-79. Air Fuel Mixer