BENDIX TCH-013-010 Fascicule

Chuff Test
Most Bendix

®

 ABS ECUs are capable of

automatically performing a check of the ABS
modulators when ignition power is applied.  This
check (referred to as a "chuff" test) is a diagnostic
enhancement designed to assist maintenance
technicians in verifying proper modulator wiring
and installation when servicing the ABS.  This
bulletin concerns the potential for contamination
in ABS modulator valves to limit or prevent the
audible “chuff” test sequence at vehicle start up.

How Chuff Test Works
When ignition power is applied, each modulator
solenoid is briefly energized.  If the air system is
fully charged and the service brake pedal is
depressed during ignition, the modulator creates
a single, sharp audible “chuff” of air pressure. The
modulators are energized in a certain pattern, as
follows: right front, left front, right rear, left rear.
This test is performed only when the vehicle is
stationary (the ECU will not perform a chuff test if
the vehicle is in motion). Refer to the service
literature for the specific ECU (available for
download at www.bendix.com).

Purpose of the Chuff Test
The chuff test is a diagnostic enhancement
designed to assist the maintenance technician in
verifying proper modulator wiring and installation
when servicing the ABS.  This diagnostic check
is not required to be conducted by vehicle
operators at every ignition cycle.  Rather, the ABS
indicator lamp on the vehicle dash is designed to
give the ABS status to the vehicle operator.
Vehicle operators should observe the ABS
indicator lamp at start up and during vehicle
operation.  See “ABS Dash Indicator Operation”
below.

ABS Braking Control Operates Differently
Compared to the Chuff Test
ABS control is performed in a "closed-loop"
manner where the ECU controls the vehicle
modulators while closely monitoring the wheel
speed.  Based on the recovery of the wheel
speed, the ECU will deliver shorter or longer
modulator control signals as necessary.   By
contrast, the signals to the modulators during a
chuff test are single, small pulses  — much shorter
than the signals sent during a typical ABS event.

Effect of Contamination on the Chuff Test
The presence of contamination in the modulator
valve can, in some cases, prevent the test pulse
of air from creating an audible “chuff” during the
chuff test, however the modulator valve may still
be fully functional and capable of performing ABS
functionality.  In all cases, there is no effect on the
normal service braking of the vehicle.
Bendix ABS ECUs monitor the operation of the
modulator valves and when a problem is detected,
the ECU will illuminate the ABS indicator lamp on
the dash during operation.  The status of the ABS
indicator lamp, therefore, and not the lack of an
audible chuff at start up, should be used as your
guide to the ABS system status.
The vehicle will still have normal service braking
and it still can be driven, even when the ABS
indicator lamp is illuminated, although without the
benefits of full ABS.  The vehicle should be
serviced as soon as possible by a qualified
mechanic to restore full ABS functionality.

General Information: ABS Dash Indicator
Operation
The ABS dash indicator lamp gives the vehicle
operator an indication of ABS status.  At each
vehicle ignition, the ABS dash indicator lamp