1991 GEO Tracker Brake Service (ONLINE REPAIR MANUALS)

The Rear Wheel Anti-Lock (RWAL) system, Fig. , reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. Pressure applied to the rear wheel brakes is regulated by a control valve assembly located under the master cylinder. The control valve consists of a dump valve which releases hydraulic pressure into an accumulator, and an isolation valve which holds rear brake pressure. Operation of the dump valve and isolation valve is controlled by an Electronic Control Unit (ECU) which is mounted next to the master cylinder.

The ECU receives ...

The height sensing proportioning valve must be adjusted whenever the valve and/or linkage is disassembled or removed for service. In addition, adjustment should be checked if excessive front wheel lock-up is experienced with a lower than desired brake application rate when vehicle is at or near maximum GVWR.

• Raise and support vehicle leaving wheels on, and allow axle to hang free (no load condition).
• Remove lever retaining nut and disconnect lever from valve shaft, Fig. .
• Rotate valve shaft to permit installation of correct adjustment gauge, Fig. ,...

MASTER CYLINDER

Some vehicles use a conventional tandem master cylinder with two hydraulic pistons operating in line. The primary piston controls the front brake system and the secondary piston controls the rear brake system.

Other models use a master cylinder that operates the same as the conventional master cylinder, but also incorporates a quick take-up feature in the rear chamber to reduce excessive pedal travel which may result from increased fluid displacement required to move the low drag caliper piston out against the rotor.

The quick take-up uses a spring-loaded ball...

EHCU MODULE

The EHCU module is not serviceable and must be replaced as a unit. Refer to Fig. when replacing the unit.

• Bleed brake booster by depressing brake pedal several times.
• Remove washer tank attaching bolts, then position aside.
• Remove EHCU electrical connectors.
• Disconnect module brake lines. Use caution not to get brake fluid on paint or electrical connections.
• Remove upper to lower bracket assembly attaching bolts.
• Remove EHCU module and upper bracket assembly from vehicle.
• Remove upper mounting bracket to EHCU module attaching bolts.
• Reverse procedure to install, noting...

SERVICE BRAKES

These brakes have self-adjusting shoe mechanisms that assure correct lining-to-drum clearances at all times. The automatic adjusters operate only when the brakes are applied as the vehicle is moving rearward or when the car comes to an uphill stop.

Although the brakes are self-adjusting, an initial adjustment is necessary after the brake shoes have been relined or replaced, or when the length of the adjusting screw has been changed during some other service operation. Refer to "Installation" for this procedure.

Frequent usage of an automatic transmission forward range to h...

WIRING DIAGRAMS

Refer to "Wiring Diagrams" in "Vehicle Systems."

READING TROUBLE CODES

Trouble codes are accessed by connecting a jumper wire between terminals A and H of the underdash ALDL connector and observing the flashes of the brake warning lamp. Note the following when performing the Diagnostic Circuit Check to access trouble codes.

• When the terminals are jumped, count the number of short flashes starting from the initial long flash and including the long flash in the count.
• The count sequence will repeat as long as the ALDL connector terminals are jumped with the ignition on.
• ...