EXPERTEC TSB (38-11)

Publication date: 1988-06-06
Reference number: 38-11

EXPERTEC TSB

TECHNICAL SERVICE BULLETIN

Reference Number(s): 38-11, Date of Issue: 07/10/88

BRAKE PULSATION AND DISC - BRAKE ROTOR SURFACE REFINISH

Model(s): 1985-88 Delta 88, Ninety-Eight, Cutlass Supreme

Classic, Custom Cruiser, Calais

1986-88 Toronado

1982-1988 Firenza, Cutlass Ciera

Bulletin No.: Oldsmobile 38-11

Date: 07/10/1988

BULLETIN ILLUSTRATION LIST

Fig 1: Graphic

Fig 2: Graphic

Fig 3: Graphic

SERVICE INFORMATION

On today's vehicles the primary reason for brake pulsation is variation in the thickness of the rotor, which generally occurs after some mileage. The causes of rotor thickness variation tend to occur while the vehicle is in motion and the brake pedal is in the unapplied position. Three events must occur simultaneously:

The rotor/bearing system runout must be out of specification enough to produce high spots on the rotor face in relation to the caliper.
There must be a condition of caliper drag which causes the inboard and/or outboard shoe to rub on the rotor high spots.
The lining material must be sufficiently abrasive to cause wear on the high spots, and therefore, to develop rotor thickness variation after accumulated mileage.

Following is a discussion of each of these events along with information about the conditions which can contribute to each:

SYSTEM RUNOUT

Excessive system runout can exist as a result of the stack up of the tolerances of a rotor and a bearing which have been manufactured at one end or the other of their respective design specifications, or if either the rotor or bearing are out of specification. The manufacturing process controls which determine the amount of rotor or bearing runout are continuously being improved to reduce the amount of system and/or component runout. During manufacturing of the brake rotor, tolerances of the braking surfaces for flatness, thickness variation and lateral runout are held very close. Maintaining these close tolerances on the rotor's braking surface is necessary to prevent brake roughness. A lateral runout check of the rotor must be performed to determine the extent of runout.

Lateral Runout Check

Remove the wheel and tire. Invert the wheel nuts and reinstall them on the studs to the specified torque. Be sure to follow the alternating nut torquing sequence to ensure proper rotor alignment.
Fasten a dial indicator to the caliper so the dial indicator button contacts the rotor about 13 m (0.500 in.) from the outer edge.
Zero the dial indicator.
Move the rotor one complete revolution and observe total indicated runout (TIR). Lateral runout cannot exceed 0.08 mm (0.003 in.). On front-wheel-drive vehicles, excessive lateral runout of the rotor can often be improved by indexing the rotor on the hub one or two bolt positions from the original position. If the lateral runout CANNOT be corrected by indexing the rotor, check the hub and bearing assembly for looseness.
NOTE: Whenever the brake rotor has been separated from the wheel bearing flange, remove any rust or foreign material from the mating surfaces of the wheel bearing flange and rotor. Failure to do this may result in excessive laterial runout of the rotor, causing brake pedal pulsation.
If lateral runout exceeds that specified in Step 4, refinish the rotor to specifications, reinstall rotor, and remeasure lateral runout.

Wheel Bolt Torque

Overtorqued wheel nuts or torquing wheel nuts without alternating the tightening sequence, especially on aluminum wheels, can also create a rotor runout condition. Refer to Fig 1 .

The use of a torque wrench when tightening wheel nuts is necessary to prevent distortion of the rotor, hub or wheel. The wheel nuts should be tightened in two steps to assure an even clamp load.

CALIPER DRAG

Caliper drag is usually caused by interference between the caliper to knuckle interface or a caliper bolt binding on the caliper bolt bushing. Caliper-to-knuckle clearance is important. Clearance should be checked with the caliper in position and the caliper mounting bolts properly torqued. If execssive clearance exists, a clunking noise may occur on each brake apply. Too little clearance can cause caliper drag. The desired clearance is .005"-.012" at each caliper-to-knuckle bracket interface, or a total of .010"

.024" The caliper bracket stops can be filed to obtain proper clearance. Refer to Fig 2 .

When servicing the caliper, the caliper bolts should be cleaned and the space between the caliper bolt bushings should be filled with silicone grease such as part no. 1052863 or equivalent. This will prevent the caliper dragging or binding on the mounting bolts. Refer to Fig 3 .

LINING MATERIAL ABRASIVENESS

The degree of abrasiveness of the brake lining material is detemined by the manufacturer. A highly abrasive material will tend to create a rotor out-of-parallel condition if system runout and caliper drag are present. The properties and characteristics of brake linings we specified by Product Engineering. Some brake lining designs may be more aggressive when stopping, i.e., higher friction coefficient, while others are less aggressive but may be very durable and long lasting. The linings for any specific vehicle are selected on the basis of friction, wear, etc., to optimize front and rear braking for the specific vehicle.

The brake lining used should be whatever is recommended by the manufacturer in the parts catalog or in the latest service bulletin. Brake pedal pulsation can also occur in the rear on drum brakes if there is radial runout in the drum and/or axle flange, or bearing.

Thickness Variation Check

If a brake pedal pulsation condition is felt, a thickness variation check should be performed on the rotor. Thickness variation can be checked by mearsuring the thickness of the rotor at four or more points around the circumference of the rotor. Use a micrometer calibrated in one tenth increments per .001 inch. All measurements must be made at the same distance in, from the edge of the rotor. A rotor that varies by more than 0.013 mm (0.0005 in.) can cause pedal pulsation and/or front-end vibration during brake applications. A rotor that does not meet these specifications should be refinished or replaced as necessary.

Rotor Refinish Recommendations

Rotor refinish should only be performed because of non-parallelism (thickness variation), runout, or deep scoring gooves deeper than .015 in. When replacing a rotor with a new rotor from stock, it should NOT require refinishing unless one of the conditions mentioned above is present. Also, routine replacement of disc brake linings does NOT require rotor refinishing. When rotor refinishing is required the following recommended procedures should be used to obtain the proper micro-inch surface finish which is required for new rotors (10 to 50 micro-inches with non-directional swirl pattern). When refinishing a rotor for a pulsation condition, it is recommended that at least 0.010 in. be removed from the inbond and out-board side of the rotor surface to ensure removal of the high and low spots on the face of the rotor.

Fig 4: ROTOR ROUGH CUT/FINISH CUT TABLE

The following is also important

The brake lathe must be in good working order and have capability to produce the intended surface finish.
Excessive spindle speed or too deep a cut can result in a rough finish.
Cutting tools must be sharp.
Adpaters must be clean and free of nicks. (As an example, the 1988 Cutlass Supreme requires the use of an adapter because of its two- piece composite rotor design. Refer to Fig. 10.
The rotor surface contacting the arbor and/or the adapter must be free of debris and coarse surface corrosion.
If the dealership's rotor turning equipment cannot produce a finish cut equivalent to a new rotor's surface, finish cuts may be further improved and made non-directional by dressing the rotor surface with a sanding disc power tool.

Light scoring of the rotor surfaces not exceeding 0.38 mm (0.015 in.) in depth, which may result from normal use, will not affect brake operation. To become familiar with the required surface finish, drag a fingernail over the surface of a new rotor from the parts stock or one on a new vehicle. If your brake equipment cannot produce this smooth a finish, when correctly used, contact the equipment manufacturer for corrective instructions. Following these refinishing specifications and recommendations will help to hold the rotor surface finish to the specified range. Controlling the braking surface finish of the rotor is necessary to provide adequate early stopping ability and to avoid pulls and erratic performance and will aid in extending lining life.