Brake Pulsation

BRAKE PULSATION

Brake pulsation is caused by brake rotor thickness variation. Brake rotor thickness variation causes the piston in the brake caliper, when applied, to "pump" in and out of the caliper housing. The "pumping" effect is transmitted hydraulically to the brake pedal. Brake pulsation concerns may result from two basic conditions:

1. Thickness Variation Pulsation is Caused by Lateral Run Out (LRO). LRO on a brake corner assembly is virtually undetectable unless measured (with a dial indicator after the brake service) and will not be detected as brake pulsation during an after brake service test drive. If the brake corner is assembled with excessive LRO (greater than 0.050 mm (0.002 in), thickness variation will develop on the brake rotor over time and miles. Excessive LRO will cause the brake pads to wear the brake rotors unevenly, which causes rotor thickness variation. Pulsation that is the result of excessive Lateral Run Out usually develops in 4,800-16,000 km (3,000-10,000 mi). The more excessive the LRO, the faster the pulsation will develop. LRO can also be induced when uneven torque is applied to wheel nuts (lug nuts). Improper wheel tightening after tire rotation, spare tire usage, brake inspection, etc. can be the cause of brake pulsation. Again, it usually takes 4,800-16,000 km (3,000-10,000 mi) AFTER the service event for the condition to develop. The customer does not usually make the connection between the service event and the awareness of the pulsation. The proper usage of torque wrenches and/or torque sticks (torque limiting sockets) will greatly reduce or eliminate the pulsation conditions after wheel service events. The improper use of impact wrenches on wheel nuts greatly increases the likelihood of pulsation after wheel service.

The following are examples of pulsation conditions and reimbursement recommendations:

- If the customer noticed the condition between 4,800-16,000 km (3,000-10,000 mi) and it gradually got worse, normally the repair would be covered. The customer may tolerate the condition until it becomes very apparent.

- If a GM dealer performed a prior brake service, consider paying for the repair and then strongly reinforce proper brake lathe maintenance.
- If the customer had the brake service done outside of a GM dealership, normally GM would not offer any assistance.

- If a customer indicated they had wheel service, ask who performed the service. Then;

- If a GM dealer performed the service, consider paying for the repair and then strongly reinforce the use of torque sticks at the dealer. Two common size torque sticks cover 90% of all GM products. Each technician needs to use torque sticks properly every time the wheel nuts are tightened.

- If the customer had the wheel service done outside of a GM dealership, normally GM would not offer any assistance.

2. Thickness Variation Pulsation Caused by Brake Rotor Corrosion - Rotor corrosion is another form of thickness variation, which can cause a pulsation concern and can be addressed as follows:

- Cosmetic Corrosion:

In most instances rotor corrosion is cosmetic and refinishing the rotor is unnecessary.

- Corrosion - Pulsation Caused by Thickness Variation (Lot Rot / Low Miles - 0-321 km (0-200 mi):

At times more extensive corrosion can cause pulsation due to thickness variation. This usually happens when the vehicle is parked for long periods of time in humid type conditions and the braking surface area under the pads corrodes at a different rate compared to the rest of the braking surface area. Cleaning up of braking surfaces (burnishing) can be accomplished by 10 - 15 moderate stops from 56- 64 km/h (35 - 40 mph) with cooling time between stops. If multiple moderate braking stops do not correct this condition, follow the "Brake Rotor Clean-Up Procedure" below.

- Corrosion - Pulsation Caused by Thickness Variation (without rotor flaking / higher mileage - 3,200-8,000 km (2,000-5,000 mi):

In some cases, more extensive corrosion that is not cleaned up by the brake pad over time and miles can cause the same type of pulsation complaint due to thickness variation. In these cases, the rotor surface is usually darker instead of shiny and a brake pad foot print can be seen against the darker surface. This darker surface is usually due to build-up, on the rotor material surface, caused by a combination of corrosion, pad material and heat. To correct this condition, follow the "Brake Rotor Clean-up Procedure" below.

- Corrosion - Pulsation Caused by Thickness Variation (with rotor flaking / higher mileage - 8,000 + km (5,000 + miles) :

At times, more extensive corrosion over time and miles can cause pulsation due to thickness variation (flaking). This flaking is usually a build up, mostly on the rotor material surface, caused by a combination of corrosion, pad material and heat. When rotor measurements are taken, the low areas are usually close to the original rotor thickness (new rotor) measurement and the high areas usually measure more than the original rotor thickness (new rotor) measurement (depending on mileage and normal wear). To correct this condition, follow the "Brake Rotor Clean-up Procedure" described below.

Important
In some flaking instances, cleaning-up this type of corrosion may require more rotor material to be removed then desired. Customer consideration should be taken in these situations and handled on a case by case basis, depending on the amount/percentage of rotor life remaining and the vehicle's warranty time and miles.