System Description; Abs/Auto-Matic Traction Control/Torqtrac 4; Abs/Tt4 Diagnostics; System Operation - Hummer H1 Service Manual

7-34 Brake System
ANTI-LOCK BRAKE SYSTEM (ABS) AND AUTO-
MATIC TRACTION CONTROL, TORQTRAC 4
™
(TT4 )

System Description

The ABS/TT4 is a hydraulic add-on system consisting of:
• Hydraulic modulator and pump assembly
• Electronic control unit
• Four wheel speed sensors
• ABS and TT4 warning lights
The add-on ABS/TT4 system is a four channel brake pressure
modulation system fitted additional to the standard brake sys-
tem. TT4 uses ABS and standard brake components to auto-
matically control traction at the wheels when accelerating on
low friction surfaces.

System Operation

ABS/TT4 system warning lights located in the status center of
the instrument panel illuminate when there is a problem with
the system. The lamps also illuminate during the initial key-on
for a system self check and bulb test sequence. After key-on
the ABS lamp will shine for approximately three seconds then
go out and remain out unless there is a fault in the system or the
ignition is cycled. The TT4 lamp will shine for three seconds
then go out until there is a problem with the system
(Figure 7-44). The TT4 lamp will flash during an overheat pe-
riod due to extended continuous use. Whenever a lamp is lit
due to a failure in the system, ABS and TT4 will be shut down
partially or wholly, depending on the nature of the fault.
ignition
ABS
on
Warning
Lamp
T T4
T T4 Info
Lamp
Figure 7-44: ABS/TT4 Warning Light Sequence.
4-1-00
_____________________________________________________________
™
3 sec
3 sec
8-S07-002
Anti-lock braking operates by continually sensing the speed of
each wheel. Wheel speed information is transmitted to an Elec-
tronic Control Unit (ECU) which evaluates the information to
determine when any wheel is about to lock. If locking com-
mences, the ECU transmits a control signal to the appropriate
solenoid valve in the modulator controlling hydraulic pressure
to the brake, thus, brake pressure is adjusted to maintain wheel
rotation.
The TorqTrac 4 system uses many of the same components as
the ABS to control wheel spin when accelerating the vehicle on
low friction surfaces. Information from the wheel speed sen-
sors is evaluated by the ECU which determines if a wheel is
loosing traction and spinning. The ECU actuates the modulator
pump to provide hydraulic pressure which is modulated by the
solenoid valves and sent to the brake of the spinning wheel.
This has the effect of transferring the torque to the non-spin-
ning wheel on the higher friction surface. The system has an
overheat protection feature which disables TT4 after 60 sec-
onds of continuous use. This will be indicated by a blinking
TT4 lamp. TT4 will automatically return to normal operation
after a 60 second cool down period.

ABS/TT4 DIAGNOSTICS

Fault Diagnostic Procedure

Faults are stored in the ECU memory in code form. The infor-
mation can be retrieved by initiating and reading a series of
flash and pause sequences (blink codes) on the ABS warning
light or with the Wabco Diagnostic Controller.
Use of the blink code procedure will determine the location of
the fault prior to performing physical wiring and component
tests, thus reducing diagnostic time.
NOTE: An attempt to initiate the blink code diagnostic may
result in a solid "on" ABS warning lamp without progressing
to the start phase of the diagnostic. If the above condition is
present, remove the ABS Warning Lamp Relay from the exte-
rior fuse box and perform the blink code diagnostic (See Figure
7-59). After repairs are completed, replace the warning lamp
relay in the socket.
Blink Code Procedure
To initiate the blink code procedure, connect the blink code
switch jumper J–44237 to the Diagnostic Link Connector
(DLC) of the vehicle. The jumper allows you to short pin 4 to
pin 15 of the DLC.
1. Switch on the ignition, the ABS warning light will illumi-
nate and extinguish if there are no active faults.
2. Five seconds after connecting the switch jumper, the ABS
warning light will extinguish, indicating the start of the
blink code cycle.
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