BENDIX LOW PRESSURE INDICATORS User manual

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Bendix®LP-2™& LP-3™Low Pressure Indicators
SD-06-1600
DESCRIPTION
The low pressure indicator is a safety device designed to
giveanautomaticwarningtothedriverwheneverairpressure
intheairbrakesystemisbelowthesafeminimumfornormal
vehicleoperation. It is usually used tooperatean electrical
buzzeror warninglight,or both,whichare audibleorvisible
tothe driver.
Two styles of low pressure indicators are currently
manufactured.
The LP-2™low pressure indicator, which is the older style
and consists of a die cast body with a spring loaded
diaphragm clamped between the body and the Bakelite
cover.
The LP-3™low pressure indicator is the newer style,
consisting of a die cast body, nylon cover and employs a
spring loaded o-ring diaphragm and piston. The LP-3™
indicator is available with either one terminal or two. The
singleterminal unit utilizes ametallicgasket between body
andcasetogroundthelowercontractstrip.Thetwoterminal
unit utilizes a phenolic insulating gasket to isolate both
terminalsfrom thevehicleframe.
TheelectricalcontactsprovidedinboththeLP-2™andLP-3™
indicators remain closed by spring force until the air brake
systempressure below the diaphragm is above thesetting
(force) of the low pressure indicator spring. The setting of
theindicator and piece number is markedona label on the
valvebody. Ifa labelis not present,then thevehiclemanual
should be consulted for the proper setting. The nominal
settingof the indicatoris 60 psi;however, pressuresettings
mayvary dependingupon thevehicle.
FIGURE 1 - LP-2™LOW PRESSURE INDICATOR FIGURE 2 - LP-3™LOW PRESSURE INDICATOR
COVER
DIAPHRAGM
BODY
SPRING
CONTACT
POINTS
COVER
SPRING
CONTACT
POINTS
PISTON
BODY
O-RING
DIAPHRAGM

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OPERATION
To describe the operation, we shall assume that the low
pressureindicator isset for 60psi. Whenair pressure atthe
supply port and under the diaphragm is above 60 psi, the
electrical contacts remain open because the force exerted
by air pressure underneath the diaphragm overcomes the
forceexerted by thespring above thediaphragm.
Whenairpressure belowthediaphragmdropsbelow 60psi,
the spring exerts a force which is greater than the force
exertedbytheairpressurebelowthediaphragm.Thiscauses
the diaphragm (and the piston in the LP-3™indicator) to
move and allow the electrical contacts to close. This
completes or closes the electrical circuit to the warning
device,warning thedriverof lowairpressure inthesystem.
PREVENTIVE MAINTENANCE
Important: Review the Bendix Warranty Policy before
performinganyintrusivemaintenanceprocedures.Awarranty
maybe voidedifintrusive maintenanceis performed during
thewarranty period.
No two vehicles operate under identical conditions; as a
result, maintenance intervals may vary. Experience is a
valuableguideindeterminingthebestmaintenanceinterval
for air brake system components. At a minimum, the low
pressure indicator should be inspected every 6 months or
1500 operating hours, whichever comes first, for proper
operation. Should the low pressure indicator not meet the
elements of the operational tests noted in this document,
furtherinvestigationandserviceofthevalvemayberequired.
TESTING FOR SERVICEABILITY
OPERATING TEST
1. If possible, determine the setting of the low pressure
indicatorbyreferringtothelabelonthevalveorthevehicle
manual.
2. Operationof thelow pressureindicator maybe checked
withignitionswitch“on”byreducingthesystempressure
and observing that low pressure warning occurs when
system pressure drops below the setting of the low
pressure indicator. The contacts will be closed when
thewarningdeviceoperates.Ifthesettingofthe indicator
is unknown, the contacts should close between
approximately 70 psi and 50 psi.
LEAKAGE TEST
1. With air pressure present at the supply port, coat the
indicatorwith soap solution. No leakage permitted.
REMOVING
1. Block the wheels. Otherwise, secure the vehicle with
otherthan service brakes.
2. Theignition switch should be in the “off” position.
3. Drain the air from the system.
4. Disconnecttheelectricalconnectionsatthelowpressure
indicator.
5. Disconnect the air line and mounting bolts or unscrew
the Indicator from the fitting and remove.
INSTALLING
1. Installin a convenientlocationfor servicing.
2. Connectto areservoirpressurelineatahigh pointin the
systemfor adequatedrainage.
3. If installing an LP-2™indicator, use a supply line of 1/4
O.D.minimum.
4. Connectthe Indicatorterminalsinserieswith theignition
switch and the warning device.
DISASSEMBLY
NOTE: It is generally recommendedthatthe low pressure
indicator, if faulty, be replaced with a new unit;
however,service partsare available;andif repairs
arenecessary,the following will apply:
LP-2™indicator.Unscrew the cover retainerfromthe body.
Removecoverandremovespringanddiaphragmassembly.
LP-3™indicator. Remove cover screws, lockwashers.
Remove cover, contact disc, spring, and shim(s). (Note:
Shimsmayormaynotbepresent.)Removecontactplate,
gasket,piston, and o-ring diaphragm.
CLEANING AND INSPECTION
1. Clean all metal parts in mineral spirits and dry them
completely.
2. Inspect all parts for excessive wear or deterioration.
Checkthe valve spring forcracksor corrosion.
3. Inspectcontact points.If contact pointsare notseverely
pitted, they can be dressed with a fine file.
Replace all parts that were discarded and any parts not
foundtobeserviceableduringinspection,usingonlygenuine
Bendixreplacement parts.
ASSEMBLY
NOTE: When using pipe thread sealant during assembly
andinstallation, take particular care to prevent the
sealant from entering the valve itself. Apply the
sealantbeginningwith thesecondthread backfrom
theend.
LP-2™INDICATOR
1. Placeand positionthe diaphragmassembly inthe body.
Position the spring so that it rests on the upper
diaphragmfollower.
2. Placecoveroverthediaphragmandscrewcoverretainer
to the body and tighten securely. (Torque to 110-130
inchpounds.)

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LP-3™INDICATOR
1. Lubricate bore of body and both sides of the o-ring
diaphragm with silicone lubricant BW-650-M (Bendix
pieceno. 291126).
2. Installo-ringdiaphragm in body.(Note: o-ringportion of
diaphragmshould face supplyport.)
3. Install piston in body. Flat side of piston should face
o-ringdiaphragm.
4. Install gasket. (Always use a phenolic gasket in a two
terminal switch and a metallic gasket in the single
terminal.)
5. Position contact plate over fingers of piston. Contact
plate should rest on face of gasket.
6. Ifshim(s) are used, place shim(s) incover.
7. Placespring in cover.
8. Place contact point so that it rests on spring.
9. Install cover on body, using machine screws, making
certain that the contact plate is in position over fingers
of piston, and arm of contact plate is positioned so that
it will fit in groove of cover.
10. Tightenscrews securely.(Torqueto20-30inchpounds).
TEST OF REBUILT LOW PRESSURE INDICATOR
Afterrebuilding, performthe leakageand operatingtestsas
outlinedin section “Testing forServiceability.”
WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID
PERSONAL INJURYOR DEATH:
When working on or around a vehicle, the following
general precautions should be observed at all times.
1. Park the vehicle on a level surface, apply the
parking brakes, and always block the wheels.
Always wear safety glasses.
2. Stop the engine and remove ignition key when
working under or around the vehicle. When
working in the engine compartment, the engine
should be shut off and the ignition key should be
removed. Where circumstances require that the
enginebe in operation, EXTREME CAUTIONshould
be used to prevent personal injury resulting from
contact with moving, rotating, leaking, heated or
electrically charged components.
3. Do not attempt to install, remove, disassemble or
assemble a component until you have read and
thoroughly understand the recommended
procedures. Useonly theproper toolsand observe
all precautions pertaining to use of those tools.
4. If the work is being performed on the vehicle’s air
brake system, or any auxiliary pressurized air
systems,makecertain to draintheair pressure from
all reservoirs before beginning ANY work on the
vehicle. If the vehicle is equipped with an AD-IS®
air dryer system or a dryer reservoir module, be
sure to drain the purge reservoir.
5. Following the vehicle manufacturer’s
recommendedprocedures, deactivate theelectrical
system in a manner that safely removes all
electrical power from the vehicle.
6. Never exceed manufacturer’s recommended
pressures.
7. Never connect or disconnect a hose or line
containing pressure; it may whip. Never remove a
component or plug unless you are certain all
system pressure has been depleted.
8. Use only genuine Bendix®replacement parts,
components and kits. Replacement hardware,
tubing, hose, fittings, etc. must be of equivalent
size, type and strength as original equipment and
be designed specifically for such applications and
systems.
9. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
not attempt repairs requiring machining or welding
unless specifically stated and approved by the
vehicle and component manufacturer.
10. Prior to returning the vehicle to service, make
certain all components and systems are restored
to their proper operating condition.
11. For vehicles with Antilock Traction Control (ATC),
the ATC function must be disabled (ATC indicator
lampshould be ON) prior to performing any vehicle
maintenance where one or more wheels on a drive
axle are lifted off the ground and moving.

4BW1447 © 2006 Bendix Commercial Vehicle Systems LLC All rights reserved. 7/2006 Printed in U.S.A.