Brakes, Front - Pulsation Diagnosis, Correction, Repair
File In Section: 5 - BrakesBulletin No.: 43-50-09
Date: December, 1994
Subject:
Front Brake Pulsation - "New" Diagnosis, Correction and Prevention
Models:
1995 and Prior Year Passenger Cars
Preventing Brake Pulsation with Uniform Wheel Nut Torque and Unrestricted Pad Movement
Extensive testing has proven that the leading contributor to brake pulsation is non-uniform wheel nut torque. Because of the importance of this information, the following steps are being taken:
^ Issued service bulletin on front brake pulsation.
^ Video on diagnosing and fixing front brake. pulsation issued with bulletin.
^ Essential tools shipped to dealer.
Condition
Vehicle pulsates with light brake apply. (Refer to the following Pulsation Test Drive Procedure to properly diagnose customer concerns.)
Important:
A vibrating/pulsating brake pedal is normal with a stop where ABS is activated.
Cause
Front brake pulsation is caused by thickness variation of the rotor braking surfaces. The two leading causes of thickness variation are uneven wear and/or corrosion of the rotor.
Uneven wear accounts for 60% of all pulsation concerns. Rotor corrosion accounts for the rest of pulsation concerns. Rotor corrosion can be affected by the geographic area you live in and/or by the amount of time a vehicle sits without being driven.
The most significant contributors to thickness variation (uneven wear), which causes pulsation, are:
1. Non-uniform wheel nut torque (causes distortion of the braking surface).
2. Pad slide force restriction combined with # 1 (Non-uniform wheel torque).
a. Caliper to knuckle/shoe to knuckle interface
b. Slide bushing lube
c. Corroded slide bushing (pin binding)
Important:
Both rotor distortion and pad slide force restriction must be present to lead to thickness variation.
Correction
The only way to eliminate thickness variation is to replace or properly turn the rotor. Be sure to check the minimum thickness before attempting to turn a rotor. Refer to shop manual for proper rotor turning procedures.
Prevention
To prevent/minimize thickness variation/uneven wear from recurring, follow this procedure:
Important:
This should be done every time disc brakes are serviced.
1. Clean the caliper to knuckle or shoe to knuckle interface and lubricate with a thin film of silicone grease GM P/N 18010909 (or equivalent).
2. Clean the attaching/slide system and lubricate all moving surfaces with silicone grease. Do not use any abrasive cleaners on the pins. Remove the rubber bushings and clean the bores and bushings before lubricating.
3. Clean all rotor, bearing, and wheel mounting surfaces of any corrosion and/or dirt.
4. Use a torque stick on an impact wrench, or a torque wrench to consistently and uniformly fasten the wheel to the specified torque for the vehicle. The star pattern must be followed.
Important:
Torque sticks must be used any time an impact wrench is used to tighten wheel nuts.
Summary
Non-uniform wheel nut torque and improper tightening sequence are the leading causes of rotor distortion which leads directly to front brake pulsation when combined with pad slide force restriction.
Pulsation Test Drive Procedures
Caution:
Verify braking ability before test and obey laws while performing the road test, test the brakes at low speed to be sure the vehicle stops properly. Also, be sure to perform the road test in a safe area where traffic laws can be observed to protect both public and personal safety.
Coast Down Vibration Test:
1. Accelerate to approximately 40-50 mph (70-SO kph).
2. Place transmission in neutral.
3. Allow vehicle to coast down to 20 mph (35 kph) (do not use brakes).
4. Observe: Any tire/wheel imbalance or vehicle vibration, (other than road surface induced).
5. Correct or proceed to next section if none present.
Rear Brake Pulsation Test:
1. Accelerate to approximately 10-15 mph (16-25 kph).
2. Place transmission in neutral.
3. On vehicles equipped with a park brake release, hold release in return position and lightly apply the park brake to slow the vehicle.
Notice:
Failure to hold the release in the return position could result in rear brake lock up.
4. Observe: Does the park brake pedal pulsate or is there a noticeable vibration - other than road induced?
5. Correct or proceed to next section if OK.
Vehicles with Push to Release Park Brake
1. Accelerate to approximately 10-15 mph (16-25 kph).
2. Place transmission in neutral.
3. On vehicles so equipped, gently apply the park brake until you can feel the car slowing down due to the park brake action. Let vehicle come to a stop before pushing the park brake pedal to release it.
4. Observe: Does the park brake pedal pulsate or is there a noticeable vibration - other than road induced?
5. Correct or proceed to next section if OK.
Front Brake Pulsation Test:
1. Perform 20 mph (35 kph) brake test.
a. Accelerate to 20 mph (35 kph).
b. Place transmission in neutral.
c. Lightly apply brakes to stop the vehicle.
d. Optional - If pulsation is present, turn vehicle 180. and repeat stop. If pulsation is still present in the opposite direction, it is only on 1 (one) rotor. If pulsation is greatly reduced or gone, it is on both rotors (the thickness variation on each rotor will go in and out of phase with each other after turning and reduce and/or cancel the effects on braking pulsation.
e. Correct or proceed to Step 2 if OK.
2. Perform 45-50 mph (75-80 kph) brake test.
a. Accelerate to 50 mph (80 kph).
b. Place transmission in neutral.
c. Apply brakes using medium pressure.
d. Optional - If pulsates, do 180° turn test in 1-d above.
e. Correct or proceed to Step 3 it OK.
3. Heat up brake system by making 10 stops from 30 mph (50 kph) using medium pressure.
a. Repeat step 1 - 20 mph (35 kph) brake test.
b. Optional - If pulsates, do 180° turn test in 1-d above.
c. Correct or if no pulsation found, further diagnostics required to determine cause(s) of customer concern.
Parts Information
Parts are currently available from GMSPO.
Important:
Kent-Moore torque stick tool numbers and the divisions listed below completes the division torque stick essential tool requirement for servicing GM vehicles. All other divisions received torque stick tools in prior years essential tool packages. Additional torque sticks can be ordered through Kent-Moore Tools at 1-800-345-2233.
Tool Information