Brakes - Judder (vibration) Diagnosis and Repair
NO.: TSB-97-35-004DATE: September, 1997
MODEL: See below
SUBJECT:
BRAKE JUDDER DIAGNOSES AND REPAIR
BACKGROUND
Brake judder is evidenced by brake pedal pulsation and steering wheel vibration when the service brakes are applied. If the brake disc is uneven, the friction force varies when the brakes are applied. Varying friction force creates vibration that transmits through the brake hydraulic circuit to the brake pedal and the vehicle body.
If there is run-out on the disc, a portion of its surface is constantly rubbing against the brake pad. If the vehicle is driven in this condition for long enough, the disc surface becomes uneven. This is known as thickness variation. Disc run-out may be caused by disc deformation or axle hub run-out. Axle hub run-out may be due to a worn axle bearing or hub deformation.
Disc thickness variation may be caused by incorrect adjustment of the brake stop lamp switch or booster push rod. If either the switch or the push rod is out of adjustment, the brake pads are in constant contact with the disc. Thickness variation can also be cause by rust formation (when the vehicle is stationary for long periods of time). If the rust cannot be removed, the disc must be replaced.
If the disc meets minimum thickness specifications, run-out and thickness variation can be brought within specifications by using an on-car brake lathe. An on-car brake lathe, unlike an off-car lathe, will "fine-tune" the disc to the hub and compensate for any hub variations. MMSA recommends the Accu-Tum Model 8750 on-car brake lathe.
For each vehicle repair to eliminate brake judder, use a copy of the brake inspection charts in this bulletin to record your measurements.
AFFECTED VEHICLES
All disc brake models
Stop Lamp Switch Clearance Check
1. Measure the clearance between the stop lamp switch outer case and the brake pedal.
Standard Value: 0.5 - 1.0 mm
(0.020 - 0.039 in.)
2. lithe measured clearance is not within the standard value range, loosen the lock nut to adjust the clearance to be within the standard value range. Then tighten the lock nut.
Front Brake Drag Torque Check
1. Remove the wheel. Secure the brake disc to the hub with the wheel nuts.
2. Place the transmission in Neutral.
3. Release the parking brake.
4. Start the engine and depress the brake pedal. Hold the pedal down for five seconds.
5. Turn the engine off and release the brake pedal.
6. Rotate the brake disc forward ten revolutions.
7. Install a torque wrench to the front hub assembly as shown. Measure the hub torque (A).
NOTE:
On some models, it may be necessary to use a spring scale as described in Group 35, BASIC BRAKE SYSTEM in the service manual.
8. Remove the brake caliper. Measure the hub torque again (B).
9. Subtract the measurement taken in Step 8 from the measurement taken in Step 7. (A - B).
Standard Value: 4 Nm (35 in-lb.) or less
Brake Booster Push Rod Check
CLEARANCE BETWEEN BRAKE BOOSTER PUSH ROD AND PRIMARY PISTON ADJUSTMENT
1. Set the special tool to the master cylinder.
2. Make sure that the shaft of the special tool makes a light contact with the master cylinder piston.
3. Turn the wing bolt to secure the shaft.
4. Using a hand vacuum pump, apply vacuum [-66.7 kPa (about 20 in. Hg)] to the brake booster.
5. Position the special tool slightly off-center of the brake booster push rod as shown.
6. Slide the special tool toward the center of the brake booster. Verify that the projection of the shaft comes into contact with the end of the push rod of the brake booster as shown.
If the push rod contacts any portion of the shaft other than its projection, shorten the push rod.
If the push rod does not contact the shaft, lengthen the push rod.
Disc Thickness and Thickness Variation Checks
NOTE:
Check disc thickness variation when the brakes are cold.
1. Remove the brake caliper and brake pad.
2. Use a soft cloth to remove dirt from the disc.
3. Using a waterproof marker, draw a circle about 10 mm from the outer edge of the disc. Then mark eight equal divisions on the circle.
4. Using a micrometer, measure the disc thickness at the marks made in Step 3.
5. If any of the measurements is under the minimum thickness specification listed in the table on pages 9 and 10 of this bulletin, replace the disc.
6. If the thickness measurements vary more than 0.01 mm (0.0004 in.), resurface the rotor, using an on-car brake lathe such as Accu-Tum Model 8750. Follow the brake lathe manufacturer's instructions.
Wheel Bearing Freeplay Check
1. Remove the brake disc.
2. Place a dial indicator near the center of the axle hub. Measure the freeplay in the bearing shaft direction.
Standard Value: 0.05 mm (0.002 in.)
3. If the freeplay is greater than the standard value, continue with Step 4.
4. Loosen the lock nut.
5. Tighten the lock nut to the specified torque. Refer to the appropriate service manual for lock nut torque specification.
6. Measure the wheel bearing freeplay again. (Repeat Steps 2 and 3.)
Standard Value: 0.05 mm (0.002 in.)
7. If the freeplay is greater than the standard value, replace the wheel bearing.
Brake Disc Run-Out Check
1. Inspect the brake disc surface for grooves, cracks, and rust. Clean the disc of all removable debris.
2. Position a dial gauge approximately 10 mm (0.40 in.) from the outer edge of the disc and zero the gauge. Measure the run-out. Run-out limit is 0.03 mm (0.0012 in.). The gauge reading should not exceed this limit.
3. If the run-out reading exceeds 0.03 mm (0.0012 in.), resurface the disc using an on-car brake lathe such as Accu-Tum Model 8750. Follow the brake lathe manufacturer's instructions. On all-wheel-drive vehicles, the propeller shaft must be disconnected from the differential carrier.
4. If resurfacing would make the disc thinner than the minimum thickness specified in the table in this bulletin, replace the disc.
Brake Disc Specifications
Accu-Tum quick mounting plate hole identification is shown later in this bulletin. Universal Red Plates are provided in the Accu-Tum brake lathe kit. Mount types are shown at the end of this bulletin.
WARRANTY INFORMATION
Standard warranty procedures apply.