Troubleshooting guide

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Governor — Controls the unloading of the air compressor
to maintain a system air pressure between predetermined
minimum (cut-in) and maximum (cut-out) levels (usually,
between 110-130 psi).
GVWR — Gross Vehicle Weight Rating. The total weight
capacity of a single vehicle, as determined by axle ratings.
Hand valve — See Trailer control valve.
HSA — Hill Start Assist. Interface between the transmission
and braking system to help the driver prevent the vehicle from
rolling downhill when moving up a steep incline from a stationary
position.
Hold-off spring — A spring within a relay valve or quick
release valve that's designed to increase the point at which the
valve reaches initial delivery. See Crack pressure.
Hysteresis — the lag in response between brake actuation
and functioning.
Inversion valve — Most commonly referred to as the valve
used on straight trucks, towing trucks and buses to exhaust air
from the spring-applied parking brake to provide supplemental
rear braking in the event of a rear axle service system failure.
Generically, an inversion valve is any pilot-operated three-way
valve — that is normally open from supply to delivery — that
closes the supply to delivery and exhausts the delivery to
atmosphere with a positive control signal.
J1587 — The Society of Automotive Engineers (SAE) heavy duty
standard diagnostic data link.
Jackknife — Out-of-control articulation of a tractor-trailer
typically resulting from lock-up or spinning of tractor drive axle(s).
Typically, the greatest risk of a jackknife is on slippery roads when
the vehicle is towing an empty, or lightly-laden, trailer.
LAS — Lateral Acceleration Sensor.
Low pressure indicator — A pressure-sensitive electrical
switch or pressure transducer that operates an in-cab buzzer
and warning light when air pressure falls below a predetermined
level. FMVSS 121 stipulates that the low pressure warning
occur before the service circuit pressure reaches 60 psi.
MIR — Modifi ed Independent Regulation. A Bendix-patented
method of controlling the opposite sides of a steer axle during
ABS operation. The MIR is designed to help reduce steering
wheel pull during an ABS event on road surfaces with poor
traction (or areas of poor traction, e.g. asphalt road surfaces
with patches of ice).
Parking brake — See Spring brake.
Parking brake priority — A type of trailer spring brake control
valve which prioritizes the release of the trailer parking brake
on initial charging to minimize the potential for brake drag.
PLC — Power Line Carrier. The serial communication protocol
used to communicate with the trailer over the blue, full time
power wire.
PMV — Pressure Modulator Valve. An air valve which is used
to vent, or block, air to the brake chambers to limit, or reduce,
brake torque.
Pneumatic balance — This is achieved when individual air
chambers receive the same pressure within recommended
tolerances. Industry groups, including Bendix, recommend
having the pressure at each axle be within 2 psi for ascending
pressures of 10-40 psi.
Pneumatic timing balance — This is achieved when all
chambers of a combination vehicle apply the service brakes
within a given period of time, so that each brake in the system
will do its fair share of the work. In a combination vehicle, lack
of proper timing may occur because tractor brakes tend to
apply faster than the trailer brakes. See Trailer push.
Pressure differential — The difference in air pressure
between any two points within a braking system.
Pumping the brakes — A slower series of heavy brake
applications ("snubbing") used in an attempt to prevent
foundation brake overheating and resultant fade, on long
downgrades.
Push rod — A rod that extends out from a brake chamber, that
connects to the arm of a slack adjuster, using a clevis pin.
QR — Quick-Release valve. Valve that allows faster release
of air from the brake chamber after a brake application. To
balance the system, quick release valves have hold off springs
that produce higher crack pressures (when the valves open).
Ratio valve — Reduces the delivery (output) pressure
proportionally to the supply (input) pressure. Most ratio valves
have a feature which is required for North American highway
applications that will progressively decrease the ratio (to one-
to-one ratio) thus providing full pressure delivery with higher
brake application pressures.
RDU — Remote Diagnostic Unit. A hand-held diagnostic tool
used on tractors and trucks.
Relay valve — Increases the application speed of the service
brake. Installed near brakes with larger air chambers (type 24
or 30). The treadle valve activates the relay valve with an air
signal. The relay valve then connects its supply port to its delivery
ports. Equal length air hose must connect the delivery ports of
the relay valve to the brake chambers.
Retarder Relay — A relay which is used to disable a retarder
when ABS is triggered.
Glossary, continued