Bendix Commercial Vehicle Systems LP-3 User Manual

Bendix® LP-2™ & LP-3™ Low Pressure Indicators

COVER

SPRING

SD-06-1600

COVER

SPRING

DIAPHRAGM

BODY

FIGURE 1 - LP-2™ LOW PRESSURE INDICAT O R FIGURE 2 - LP-3™ LOW PRESSURE INDICAT O R

DESCRIPTION

The low pressure indicator is a safety device designed to
give an automatic warning to the driver whenever air pressure
in the air brake system is below the safe minimum for normal
vehicle operation. It is usually used to operate an electrical
buzzer or warning light, or both, which are audible or visible
to the driver.

Two styles of low pressure indicators are currently
manufactured.

CONTACT

POINTS

O-RING

DIAPHRAGM

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
single terminal unit utilizes a metallic gasket between body
and case to ground the lower contract strip. The two terminal
unit utilizes a phenolic insulating gasket to isolate both
terminals from the vehicle frame.

The electrical contacts provided in both the LP-2™ and LP-3
indicators remain closed by spring force until the air brake
system pressure below the diaphragm is above the setting

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.

(force) of the low pressure indicator spring. The setting of
the indicator and piece number is marked on a label on the
valve body. If a label is not present, then the vehicle manual
should be consulted for the proper setting. The nominal
setting of the indicator is 60 psi; however , pressure settings
may vary depending upon the vehicle.

CONTACT

POINTS

PISTON

BODY

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1

OPERATION

To describe the operation, we shall assume that the low
pressure indicator is set for 60 psi. When air pressure at the
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
force exerted by the spring above the diaphragm.

When air pressure below the diaphragm drops below 60 psi,
the spring exerts a force which is greater than the force
exerted by the air pressure below the diaphragm. This causes

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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 the driver of low air pressure in the system.

PREVENTIVE MAINTENANCE

Important: Review the Bendix Warranty Policy before

performing any intrusive maintenance procedures. A warranty
may be voided if intrusive maintenance is performed during
the warranty period.

5. Disconnect the air line and mounting bolts or unscrew
the Indicator from the fitting and remove.

INSTALLING

1. Install in a convenient location for servicing.

2. Connect to a reservoir pressure line at a high point in the
system for adequate drainage.

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3. If installing an LP-2

indicator, use a supply line of 1/4

O.D. minimum.

4. Connect the Indicator terminals in series with the ignition
switch and the warning device.

DISASSEMBLY

NOTE: It is generally recommended that the low pressure

indicator, if faulty, be replaced with a new unit;
however, service p arts are available; and if repairs
are necessary , the following will apply:

LP-2™ indicator. Unscrew the cover ret ainer from the body .
Remove cover and remove spring and diaphragm assembly .

No two vehicles operate under identical conditions; as a
result, maintenance intervals may vary. Experience is a
valuable guide in determining the best maintenance interval
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,
further investigation and service of the valve may be required.

TESTING FOR SERVICEABILITY

OPERA TING TEST

1. If possible, determine the setting of the low pressure
indicator by referring to the label on the valve or the vehicle
manual.

2. Operation of the low pressure indicator may be checked
with ignition switch “on” by reducing the system pressure
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
the warning device operates. If the setting of the 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
indicator with soap solution. No leakage permitted.

REMOVING

1. Block the wheels. Otherwise, secure the vehicle with
other than service brakes.

2. The ignition switch should be in the “off” position.

3. Drain the air from the system.

4. Disconnect the electrical connections at the low pressure
indicator.

LP-3™ indicator. Remove cover screws, lockwashers.
Remove cover, contact disc, spring, and shim(s). (Note:
Shims may or may not be present.) Remove contact plate,
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.
Check the valve spring for cracks or corrosion.

3. Inspect contact points. If contact points are not severely
pitted, they can be dressed with a fine file.

Replace all parts that were discarded and any parts not
found to be serviceable during inspection, using only genuine
Bendix replacement parts.

ASSEMBLY

NOTE: When using pipe thread sealant during assembly

and installation, take particular care to prevent the
sealant from entering the valve itself. Apply the
sealant beginning with the second thread back from
the end.

LP-2™ INDICA TOR

1. Place and position the diaphragm assembly in the body.
Position the spring so that it rests on the upper
diaphragm follower.

2. Place cover over the diaphragm and screw cover retainer
to the body and tighten securely. (Torque to 110-130
inch pounds.)

2