HF and QR De-energized De-energized the modulator delivery port and flows to the
service brake chambers.

Antilock Exhaust

HF and QR

Energized

Energized

Air from the brake chambers exhausts
through the modulator exhaust port.

The modulator prevents supply air from
passing through the modulator and, at the
De-energized
same time, prevents air from the brake
chambers from exhausting.

Antilock Hold

HF and QR

Antilock
"Reapply"

The modulator re-applies air to the brakes in
HF and QR De-energized De-energized the same manner it did during a non-antilock
event.

Energized

Figure 6 - Bendix® M-40QR™ & M-40HF™ Modulators: Operational Modes
4

ANTILOCK APPLY MODE

PREVENTIVE MAINTENANCE

If a service brake application is made and the antilock
system detects an impending wheel lockup, the antilock
controller will make a controlled brake application using
the modulator.

GENERAL

In order to control the brake application, the coils of the two
solenoid valves contained in the modulator are energized,
or de-energized, in a pre-programmed sequence by the
antilock controller. When a solenoid coil is energized,
and – depending on whether the exhaust or hold solenoid
is energized – it either opens or closes, thereby causing
the exhaust or reapplication of air pressure to the brake
actuator. The solenoids in the modulator are controlled
independently by the Electronic Control Unit (ECU).

Perform the tests and inspections presented at the
prescribed intervals. If the modulator fails to function as
described, or if the leakage is excessive, replace it with a
new Bendix® brand unit, available at any authorized parts
outlet.

ANTILOCK EXHAUST MODE
When wheel lock is detected or imminent, the antilock
controller energizes the supply and exhaust solenoids in
the modulator.
Further delivery of air to the brake chamber is prevented,
and air is allowed to exhaust through the exhaust port.

ANTILOCK HOLD MODE
The antilock controller will place the modulator in the hold
position when information from the wheel speed sensors
shows that the correct wheel speed (braking force) has
been attained. The antilock controller will also place
the modulator in the hold position – prior to entering the
reapply mode – when it detects recovery from a locked
wheel condition.
In this mode of operation, the exhaust passage is sealed,
holding the air pressure in the brake chamber at that level.
At the same time, no application air is permitted to flow to
the brake chamber.
The modulator can enter both the antilock exhaust or
reapply mode from the antilock hold mode depending on
the needs of the antilock controller.

ANTILOCK “REAPPLY” MODE
If the antilock controller senses that wheel speed has
increased sufficiently enough to allow re-application of
braking pressure, without further wheel lock-up, it permits
the supply air to re-apply air to the brakes in the same
manner it did during a non-antilock event.

Read and follow the General Safety Guidelines on
page 2 of this document.

EVERY MONTH, 10,000 MILES, OR
350 OPERATING HOURS
1. Remove any accumulated contaminates and visually
inspect the exterior for excessive corrosion and physical
damage.
2. Inspect all air hoses and wire harnesses connected to
the modulator for signs of wear or physical damage.
Replace as necessary.
3. Test air line fittings for leakage and tighten or replace
as necessary.
4. Perform the routine OPERATION & LEAKAGE TESTS
described below.

OPERATION & LEAKAGE TESTS
OPERATION TEST
This test uses the Chuff Test (described on page 3).
To properly test the function of the modulator will require
two (2) service technicians.
1. Park the vehicle on a level surface and block or chock
the wheels. Release the parking brakes and build the
air system to governor cut-out.
2. Turn the engine ignition key to the OFF position then
make and hold a full brake application.
3. With the brake application held and one (1) service
technician posted at one (1) of the modulators, turn
the vehicle ignition key to the ON position. One or
two short bursts of air pressure should be noted at the
modulator exhaust. Repeat the test for each modulator
on the vehicle. If at least a single burst of exhaust is
not noted or the exhaust of air is prolonged and not
short, sharp, and well defined, perform the Electrical
Tests.

5

LEAKAGE TEST
1. Park the vehicle on a level surface and block or chock
the wheels. Release the parking brakes and build the
air system to full pressure.
2. Turn the engine OFF and make 4 or 5 brake applications
and note that the service brakes apply and release
promptly.
3 1 2
1

2

3

3. Build system pressure to governor cut-out and turn the
engine OFF.
Bendix® M-40HF™
Modulator

7. The leakage at the exhaust port of most Bendix
components, including the M-40™ modulators, should not
exceed a one-inch bubble in three seconds. If leakage
at the modulator is determined to exceed the maximum
limits, replace the modulator.

ELECTRICAL TESTS

Optional Adapter

2

1

Bayonet
Terminal Function
Name
1
Exhaust
2
Source
3
Hold

1
2

3

3

Twist-Lock Connector
(Bayonet Connector)
41
Packard
Terminal Function
Name
41
Source
42
Hold
43
Exhaust

3

1

2

1

2

3

42

43

Packard Electric
Connector
(280 Series)
Figure 7 - Exterior Views and Electrical Connections
6

5. Begin timing pressure loss for two minutes while
watching the dash gauges for a pressure drop. The
leakage rate for the service reservoirs should not exceed four (4) psi within two minutes for a single vehicle,
six (6) psi within two minutes for a tractor/trailer, and
eight (8) psi within two minutes for a tractor with two
trailers.
6. If either circuit exceeds the leakage values above, apply
a soap solution to the exhaust port of the modulators
and any other components in the respective circuit,
then apply the service brakes.

Twist-Lock
Connector
(Bayonet
Connector)

Bendix® M-40QR™
Modulator

4. After determining the pressure loss with the brakes
released (2 psi/minute allowed), make and hold a
full service brake application. Allow the pressure to
stabilize for one minute.

1. Before testing the solenoid assembly of a suspect
modulator, its location on the vehicle should be
confirmed using the troubleshooting procedure for the
specific antilock controller in use. (See the Service Data
Sheet available on the document library on bendix.com.)
2. Proceed to the modulator in question and inspect its
wiring connector. Disconnect the connector and test the
resistance between the pins on the modulator. Refer
to Figure 7.
A. HOLD to SOURCE (41-42):
3 1 2
12V systems: Read 4.8 to 5.6 Ohms.
24V systems: Read 13.9 to 16.3 Ohms.
B.

EXHAUST to SOURCE (41-43):
12V systems: Read 4.8 to 5.6 Ohms.
24V systems: Read 13.9 to 16.3 Ohms.

C.

Individually test the resistance of each pin to
vehicle ground and note there is NO continuity.

If the resistance readings are as shown, the wire
harness leading to the modulator may require repair or
replacement. Before attempting repair or replacement
of the wire harness, refer to the test procedures
specified for the antilock controller in use for possible
further testing that may be required to substantiate the
wire harness problem. If the resistance values are not
as stated, replace the modulator.

MODULATOR REMOVAL

TECHNICAL INFORMATION

1. Locate the modulator that will be replaced and clean
the exterior.
2. Identify and mark or label all air hoses and their
respective connections on the valve to facilitate ease
of installation.
3. Disconnect both air lines and the electrical connector.

Mode

Modulator Type
Supply Port (from brake, relay, or
quick release valve): 1/2" NPT

Port
Options

Delivery Port (brake actuator):
• 1/2" NPT
• Push-to-connect for 1/2" tubing

4. Remove the modulator from the vehicle.
5. Remove any air hose fittings present. These fittings
will be re-used in the replacement modulator.

MODULATOR INSTALLATION
1. Install all previously-removed air hose fittings, making
certain thread-sealing material does not enter the
valve.
2. Install the assembled valve on the vehicle.
3. Reconnect both air lines to the valve using the
identification marks made during step 5 of Modulator
Removal.

Solenoid
Voltage

12 Volts DC Nominal, optional 24 Volt
available

Weight

1.35 pounds

Maximum
Operating
Pressure

150 psi gauge

-40°F (40°C) to 185°F.
Operating
Temperature [A short-term operating temperature
of 230°F (110°C) for up to 10% of the
Range
valve’s lifetime is acceptable.]

4. Reconnect the electrical harness to the modulator.

Pressure
Differential

1 psi maximum (supply to delivery)

5. After installing the valve, test all air fittings for excessive
leakage and tighten as needed.

Mounting
Hole Sizes

0.33" diameter through body

Figure 8 - Bendix® M-40QR™ & M-40HF™ Modulators

Bendix®
M-40HF™
Modulator

Bendix®
M-40QR™
Modulator

Packard
Terminal
Name

Bayonet
Terminal
Name

Function

41

2

Source

42

3

Hold

43

1

Exhaust

Valve
Port

Function

1

Supply

2

Delivery

3

Exhaust

Figure 9 - Bendix M-40QR & M-40HF Modulators DIN Symbol

7

GLOSSARY
See the Service Data Sheet for the vehicle’s Electronic
Control Unit for more information.

J1939 - A high speed data link used for communications between
the ABS ECU engine, transmission, and retarders.

ABS - Antilock Brake System.

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.

ABS Event - Impending wheel lock situation that causes the
ABS controller to activate the modulator valve(s).
ABS Indicator Lamp - An amber lamp which indicates the
operating status of an antilock system. When the indicator lamp
is on, ABS is disabled and the vehicle reverts to normal brake
operation.
ATC - Automatic Traction Control. An additional ABS function
in which engine torque is controlled and brakes are applied
differentially to enhance vehicle traction.
ATC / ESP Lamp - A lamp that indicates when stability functions,
including traction control, roll stability program, or yaw control
are operating.

QR - Quick Release. Quick release valves allow faster release
of air from the brake chamber after a brake application.
RSP - Roll Stability Program. An all-axle ABS solution that helps
reduce vehicle speed by applying all vehicle brakes as needed,
reducing the tendency to roll over.
SAS - Steering Angle Sensor.
TCV - Traction Control Valve.
YC - Yaw Control. Helps stabilize rotational dynamics of vehicle.
YRS - Yaw Rate Sensor.

ECU - Electronic Control Unit.
ESC / ESP® - Electronic Stability Program. Full stability system,
offering both roll and yaw stability to help provide loss of control
and rollover mitigation.
Diagnostic Trouble Code (DTC) - A condition that interferes with
the generation or transmission of response or control signals in
the vehicle's ABS system that could lead to the functionality of
the ABS system becoming inoperable in whole or in part.

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SD-13-4958 Rev. 001 © 2018 Bendix Commercial Vehicle Systems LLC, a member of the Knorr-Bremse Group • 10/18 • All Rights Reserved

8