BRAKE PULSATION AND ON CAR BRAKE ROTOR TURNING
VEHICLES AFFECTED: 1980-1989 ALL CARS AND LIGHT DUTY TRUCKS SERIES
This bulletin cancels and replaces bulletin P.S.B. 88-5-118 of which copies should be discarded.
CONDITION
Some vehicles may exhibit a pulsating or vibrating condition (roughness) when the brakes are applied. This bulletin specifically addresses pulsation resulting from disc brake rotor thickness variation. It should be recognized that vibration perceived by the customer may be induced by rear brakes or may also be due to vibration from other components such as drivetrain, tires and/or road surface conditions.
CAUSES
One possible cause of rear drum brake pulsation is an out-of-round condition (radial runout) in a rear drum. This may occur during manufacture; however, a brake drum may also be distorted if pried off, struck with a hammer or dropped during routine brake inspection or service.
Causes of rear brake pulsation on vehicles equipped with four-wheel disc brakes are essentially the same as those described for front brake pulsation.
FRONT BRAKE PULSATIONS
One possible cause of front brake pulsation is variation in the thickness of the disc brake rotors. This condition generally appears after the vehicle has accumulated some mileage. Rotor thickness variation is the result of a combination of conditions that cause the rotor to wear while the vehicle is in motion and the brakes are not applied. All of the following three elements must be present:
1. Rotor/Bearing System Runout 2. Caliper Drag 3. Abrasive Lining Material
1. Rotor/Bearinq System Runout
For uneven wear to occur, rotor/bearing system runout must be great enough to produce high spots on the rotor. Lateral runout on brake rotors may be caused by tolerance "stack-up" and by distortion due to over tightening wheel nuts or tightening the wheel nuts in the wrong sequence.
(To prevent distortion of the hub and rotor assembly, wheel nuts should be tightened in two steps using a suitable torque wrench and following the alternate nut tightening sequence shown in Figure 1. Applying torque in two steps helps reduce distortion by providing a gradual and even distribution of clamp load over the surface of the hub.)
2. Caliper Drag
Disc brake pads (inboard and/or outboard) must drag sufficiently to contact high spots on the rotor when brakes are not applied. Caliper drag is usually caused by interference between the caliper and the bracket stops on the steering knuckle or by binding of the caliper mounting bolt sleeve bushings.
(Caliper to bracket stop clearance should be checked with calipers in position and the mounting bolts torqued to specification. Clearance, measured at the two locations shown in Figure 2, should be within specifications for the vehicle tested. If clearance is too great, the caliper may move and cause a clunking noise when brakes are applied. If there is not enough clearance, the caliper may drag on the knuckle. Caliper bracket stops can be filed to adjust clearance.)
When servicing calipers, caliper mounting bolts should be cleaned and inspected for rust corrosion. Always replace corroded mounting bolts; do not attempt to polish away corrosion. To prevent binding, the mounting bolt sleeve bushings should be coated with silicone lubricant, Part Number 992887 or equivalent. Refer to Figure 3. A light application of silicone lubricant should also be applied to the caliper bracket stops.
1989 cars are equipped with a new caliper mounting bolt and sleeve bushing assembly that reduces the possibility of binding. New parts are available for service of past model front wheel drive cars as follows:
Carline Part Number ------- ----------- C-H-A/Hvy 18016163 J-N-L-A/Light 18016164
3. Abrasive Lining Material
For uneven rotor wear to occur, shoe lining material must be abrasive enough to cause wear while rubbing against rotor high spots. Some types of brake lining material are more abrasive than others. The more abrasive the material, the more quickly wear will occur when rotor high spots contact the lining.
Product engineering selects the type of brake lining material to be used based on the friction coefficient requirements of a particular vehicle. Brake linings and other components should be replaced only with the parts recommended in the parts catalogue or the latest service bulletin.
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 32 Km/h (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 32 to Zero Km/h brake test
a. Accelerate vehicle to 32 Km/h (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 72 to 80 Km/h brake test:
a. Accelerate vehicle to 80 Km/H (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 50 Km/h using medium pedal pressure. Then repeat 32, to Zero Km/h 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 .025 mm and .05mm (.001"-002") 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 from 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 Finishing
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 ronout 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 or 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 27 N.m (20 ft.lbs.) following the tightening sequence shown in Figure 1.
2. Fasten a dial indicator to the caliper so that the indicator button contacts the rotor approximately 13mm (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 0.08mm (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 fore'lgn 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. All measuring points must be the same distance in from the edge of the rotor.
A rotor that varies in thickness by more than 0.013mm (0.005 in.) can cause pedal pulsation and/or front end vibration during brake applications. Rotors with 0.013mm or higher thickness variations should be refinished or replaced as necessary. 208b/12-17
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
