TM-9-2320-272-23-2
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; FIELD MAINTENANCE MANUAL; VOLUME 2 OF 5
SEPTEMBER 2012
TM-9-2320-272-23-2 - Page 56 of 1417
FIELD MAINTENANCE
ANTILOCK BRAKE SYSTEM (ABS) ELECTRONIC DIAGNOSTIC TROUBLESHOOTING
INTRODUCTION
1.
The Antilock Brake System (ABS) is designed to prevent rear wheel lockup by automatically regulating brake
pressure on rear wheels, as needed, for maximum adhesion. The ABS improves handling and controllability
during emergency stops and normal braking in rain, snow, ice, or sand.
2.
The ABS consists of two wheel-end tone rings and wheel speed sensors located on the rear-rear axle hubs,
two modulator valves, and an Electronic Control Unit (ECU). The ABS system interfaces with the vehicle’s
electrical and pneumatic systems. The controlling unit of the entire system is the ECU. The ECU receives
signals from the wheel speed sensors and instructs the appropriate modulator valves to adjust brake
pressure as necessary.
3.
This chapter contains troubleshooting information and tests for locating and correcting malfunctions which
may develop in the ABS on vehicles covered in this manual. Each symptom or malfunction given for an
individual component or system is followed by step(s) you should take to determine the cause and corrective
action needed to remedy the problem.
4.
The following is a list of definitions used in the troubleshooting procedures:
a.
Clear Mode – Clears faults from memory of the ECU.
b.
Electronic Control Unit (ECU) – Receives signals from wheel sensors on rear-rear axle hubs and
activates the appropriate response.
c.
Fault – A malfunction detected and/or stored in the memory of the ECU.
d.
Active Fault – A condition that currently exists in the system, even when battery switch has been
turned OFF, then ON, and vehicle driven in excess of 5 mph. The ABS warning lamp will come on
and remain on. Active faults must be repaired before they can be cleared from ECU memory.
e.
Stored Fault – An active fault that has not been cleared from ECU memory, or intermittent fault that
has occurred, but no longer exists. The ABS warning lamp will come on and stay on after vehicle
has been driven in excess of 5 mph and go off after battery switch has been turned OFF.
5.
Disconnect power from ABS system by placing battery switch and start switch in the OFF position and
disconnecting battery ground cable before testing for shorted or open leads or making any repairs.
6.
Most ABS problems are related to:
a.
Cut, corroded or abraded leads.
b.
Corroded connectors or terminals.
c.
Connectors not latched or seated correctly to mating connectors.
7.
Diagnostic Code Descriptions.
a.
Code 00, 07, 8.8, and C1. Indicates that the system is working ok. Diagnostic tester will indicate
code 00 with vehicle moving and code 07 when vehicle is stationary indicating that there are no
active faults. Code 8.8 is a self diagnostic test. Code C1 indicates that the ABS is a 2S/2M system
(two sensors with modulator valves).
b.
Code 03 or 04. Indicates that a wheel sensor or wheel sensor lead has a short or open circuit.
Disconnect wheel sensor lead from wiring harness lead and measure the resistance between the
two pins of wheel sensor lead. The multimeter reading should be between 980 and 2350 Ohms.
Replace wheel sensor if not within limits. To check wiring harness lead, disconnect ABS main wiring
harness connector from ECU and check for short or open circuit between sensor lead pins 1 and
10 (figure 1) for 2A right rear wheel sensor and pins 2 and 11 for 2B left rear wheel sensor. Also
check for damaged connector pins. Replace or repair as necessary.
TM 9-2320-272-23-2
0154
0154-1
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