Suburban 3/4 Ton 4WD V8-305 5.0L (1986)
Technical Service Bulletin # 883125
Date: 880801
Brakes - Pulsations and Rotor Surface Refinish
Number:
88-312-5
Section:
5
Date:
AUGUST, 1988
Subject BRAKE PULSATIONS AND DISC BRAKE
ROTOR SURFACE REFINISH
Model and Year 1982-88 CHEVROLET MODELS
TO:
ALL CHEVROLET DEALERS
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:
1.
The rotor/bearing system runout must be out of specification enough to produce high spots on the rotor face in relation to the caliper.
2.
There must be a condition of caliper drag which causes the inboard and/or outboard shoe to rub on the rotor high spots.
3.
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 Information
SYSTEM RUNOUT:
Excessive system runout can exist as a result of the stack up of the tolerances of a rotor and a bearing which has 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:
1.
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.
2.
Fasten a dial indicator to the caliper so the dial indictor button contacts the rotor about 13 mm (0.500 inch) from the outer edge.
3.
Zero the dial indicator.
4.
Move the rotor one complete revolution and observe total indicated runout (T.I.R.). Lateral runout cannot exceed 0.08 mm (0.003-inch).
^
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.
NOTICE:
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 lateral runout of the rotor, causing brake
pedal pulsation.
5.
If lateral runout exceeds that specified in Step 4, refinish the rotor to specifications, reinstall rotor, and remeasure lateral runout.
Wheel Bolt Torque, Caliper and Lining Material
