XK-8 Convertible (X100) V8-4.0L (AJ27) (2001)
Brake Caliper: All Technical Service Bulletins
Brakes - Judder/Shudder/Pulsation
V8 XJ Series/XK
1997 MY-ON XK Range
1998 MY-ON V8 XJ Series
Brake Disc Vibration - Diagnosis - General Information
This bulletin provides general background information to assist in resolving a customer complaint of brake vibration or judder.
The following points should be observed diagnosing a customer complaint.
GENERAL INFORMATION REGARDING BRAKE VIBRATION
Brake vibration can be described as judder, shudder, pulsing etc., all are terms for the vibration of the vehicle and the vibration, transmitted as a
pulsation of the hydraulic fluid in the brake line, which occur when the brakes are applied.
The vibration of the vehicle is felt through the seat and the steering wheel; the pulsing of the fluid is felt through the brake pedal.
Brake vibration occurs mainly on the front brakes.
Partial corrosion of the disc surface eventually causing variation in the brake disc thickness, or an alternating surface finish giving varying
Variation in the brake disc thickness on an otherwise true running disc will cause a pumping of the fluid as the applied pads and pistons follow
the disc, moving moving both pistons the same time, alternately expanding and contracting the caliper fluid volume causing pressure pulses in
the line to the brake pedal.
The variation of the brake disc thickness can be caused by corrosion of the area of the disc that is not covered by the pads when the vehicle is
parked, causing a partial deterioration of the disc surface.
Corrosion of the disc is accelerated by the addition of salt to the road surface in winter, which is than sprayed onto the disc by splash, or by
exposing the vehicle to salt ladden air in a marine environment
Brake disc protective bags must be left in place during PDI and removed just prior to customer delivery. This will ensure that corrosion will
not form on stored vehicles.
A continuous variation in the position of the brake disc face relative to the brake pad. i.e. the disc plane is not perpendicular to the hub center line.
This fault can be caused by, a distortion of the disc, or, the incorrect assembly of the disc to the hub, and is commonly known as "run-out".
Brake disc run-out causes pumping of the fluid, by the inner and outer pad/piston assemblies following the disc in parallel, shifting from side to
side, and so pumping fluid across the caliper causing pressure variations in the line.
The brake disc will normally run between the retraced pads and any wear that occurs through partial contact will be distributed evenly on the
disc surface. Run-out on the other hand, will cause the disc to collide with the pads as it revolves, the two points of collision being 180° apart
and on opposite sides of the disc: the results of this condition will be a pattern of wear causing variations in the disc thickness.
When the brakes are applied, this wear will cause pulsation of the pistons and the fluid; the severity of this wear will depend on the mileage.
Disc run-out can be caused by wear, as above, hub bearing adjustment damage and also by foreign matter included between the hub and the