Correction
CORRECTION
Test Drive
Before any diagnosis is performed, it is essential that the vehicle be road tested to determine the exact nature of the problem and to get a feel for the vehicle vibration characteristics. The road test is used to determine whether brake pulsation is present or whether the vibration is due to some other cause (tire and wheel vibration/balance, bent or out-of-round wheel rims, drivetrain, etc.).
When a brake pulsation condition is confirmed, a road test will determine whether the front or rear brakes are the cause.
If vibration/roughness is determined to be from a source other than brakes, perform necessary repairs.
Brakes should be tested while driving on a dry, clean and reasonably smooth road surface. Rear brake pulsation is more easily isolated. When rear brake pulsation has been eliminated or repaired, a full brake system test can be used to diagnose front brake pulsation. Begin by testing the rear brakes as follows:
Rear Brake Pulsation Test
1. Accelerate to approximately 20 mph.
2. Place gear selector in neutral.
3. While holding the parking brake release in the return position, lightly apply the parking brake to stop the vehicle. (Failure to hold the parking brake release in the return position could result in rear brake "lock").
4. OBSERVE: Does the parking brake pedal pulsate? Is there noticeable vibration in the vehicle (different than road induced)?
5. If either of these conditions is present, refinish or replace drums/rotors as required.
If no rear brake pulsation is indicated, test the front brakes as follows:
Front Brake Pulsation Test (Full Brake System)
1. Perform 20 to Zero mph brake test:
a. Accelerate vehicle to 20 mph.
b. Place selector in neutral.
c. Apply brakes lightly to stop the vehicle. If pulsation is felt, service front brakes as described in this bulletin. If no pulsation is present, continue to Step 2.
2. Perform 45 to 50 mph brake test:
a. Accelerate vehicle to 50 mph.
b. Place selector in neutral.
c. Apply brakes using medium pedal pressure. If pulsation is felt, service front brakes as described in this bulletin. If no pulsation is present, continue to Step 3.
3. Heat up the brake system by performing 10 stops from approximately 30 mph using medium pedal pressure. Then repeat 20 to Zero mph test in Step 1.
a. If pulsation is felt, service front brakes as described in this bulletin.
b. If no pulsation is present, check tires and drivetrain components for other possible causes of customer complaint.
ROTOR REFINISHING
Rotor refinishing should only be performed when rotor thickness variation (non-parallelism) is present or when the rotor surface has scoring grooves deeper than .015 inches. Replacement rotors, taken from stock, should be mounted and checked for lateral run-out, and if run-out exists, then the rotor must be refinished on the vehicle.
When rotors are refinished, a correct surface finish is important for satisfactory braking. Following a light finish-cutting operation of between .001 and .002 inches, correct surface should be obtained by rubbing with 120 grit emery cloth. Surface finish may be checked by running a fingernail over the surface of a new rotor front stock or one from a known good vehicle. For best results, cutting equipment should be maintained in correct working condition. If an acceptable finish surface cannot be obtained, contact Kent-Moore Automotive Division for assistance.
On-Vehicle Rotor
The recommended procedure for refinishing brake rotors is through the use of on-vehicle rotor turning equipment (Kent-Moore J 37704). This method is particularly effective because it can compensate for stacked tolerances that cause rotor/bearing system runout and help prevent a return of the condition. On-vehicle rotor turning equipment is available through Kent-Moore, Part Number J 37704. Full setup and operating instructions are provided with the equipment. Contact Kent-Moore for ordering information.
We believe this source and their equipment to be reliable. There may be additional manufacturers of such equipment. General Motors does not endorse, indicate any preference for, or assume any responsibility for product equipment from these firms, or for any such items which may be available from other sources.
Lateral Runout Check
A lateral runout check is an integral part of the on-car refinishing procedure. If off-vehicle rotor refinishing is used, the following lateral runout check should be performed:
1. Remove wheel, invert wheel nuts and reinstall them on the studs. Tighten nuts to 20 lbs.ft. following the tightening sequetice shown in Figure 1.
2. Fasten a dial indicator to the caliper so that the indicator button contacts the rotor approximately 13 mm (0.500 in.) from the outer edge.
3. Zero the dial indicator needle.
4. Rotate the rotor one complete revolution while observing runout indicated on the dial. Total indicated runout (T.I.R.) should not exceed 0.08 mm (0.003 in.).
On front wheel drive vehicles, lateral rotor runout due to stacked tolerances can often be reduced by rotating (indexing) the rotor one or two bolt holes in relation to the hub. If excessive runout cannot be corrected by indexing the rotor, check the hub and bearing assembly for looseness.
NOTICE:
Whenever the brake rotor has been separated from the wheel bearing flange, clean any rust or foreign material from the mating surfaces of the wheel bearing flange and rotor. Failure to do this may result in increased lateral runout of the rotor and brake pulsation.
5. If lateral rotor runout exceeds the amount specified in Step 4, refinish the rotor on the vehicle.
Rotor Thickness Variation Check
Variations in rotor thickness can be checked by measuring the thickness of the rotor at four or more points around the surface of the rotor. Use a micrometer calibrated in one tenth increments per .001 in. All measuring points must be the same distance in from the edge of the rotor.
A rotor that varies in thickness by more than 0.013 mm (0.0005 in.) can cause pedal pulsation and/or front end vibration during brake applications. Rotors with 0.013 mm or higher thickness variations should be refinished or replaced as necessary.