Since I drive an '03 Z24 Cavalier 117 km daily
round trip and had a pulsation condition which started from
a few thousand kms from new, I'll share some of my approach
and diagnostic considerations as they would apply.
I'll approach this as if it is a problem vehicle, even
though it may seem to be routine. There are many questions I
might and factors to consider as possible contributors to
why one vehicle's brake may pulsate more than another of the
same model. The object is to find the root cause of a
braking concern and rectify it eliminate or at least reduce
the frequency at which such a condition occurs when a fix
isn't possible with off the shelf parts and careful
procedures.
A thorough write up and explanation may be in order and
whenever possible road test with the customer driving to
observe their driving habits/style.
Is the vehicle equipped with a manual transaxle or
automatic? Some vehicle have notable differences in braking
use over the same driven distance, even though the model
line is notable for periodic pulsation concerns. As far as
the typical statement "they all do that," it doesn't cut it
as far as I'm concerned. With more careful technique than an
assembly line robot, pulsation can be reduced or eliminated
in some vehicle models which are notably more prone to
pulsation concerns.
How frequently is the vehicle driven? Parked inside a
garage, carport or outside in the elements. Short trips,
long trips?
ABS/ETC or not? Mine has it, while my daughter's '05 does
not and the foundation brakes feel and operate very
differently between these two variants of the same model
line. Time to noticeable pulsation on her manual transaxle 5
speed was almost 60,000 km (40,000mi approx)and pad life was
similar.
Is the noise while braking related to braking, such as
excessive movement of the brake calipers knocking against
the caliper mounting brackets slides of the knuckle or pad
vibration squeal issues? Do we just assume that this is
brake related? No, I'd prefer a road test with the customer
and also on my own afterwards, to get some plan of attack
and specific areas that I would be inspecting.
Does the vehicle exhibit any or the same level of pulsation
at other speeds and is it affected by weather such as rain?
Is it consistent, intermittent or increases over milege
driven?
Inspect tires and wheels for correct size, construction,
condition and pressure at all four corners. Jounce the
vehicle to cursory inspect the suspension and visually
inspect condition when hoisted.
Verify wheel hub bearing play is in specs before removing
the wheels. With the vehicle lifted and the suspension
supported with wheels removed and lug nuts reinstalled,
inspect rotor surface condition. Poorly refinished rotors
with chatter marks can cause noisy and poor braking surface
contact with the pads.
Has anti-seize compounds been used on the lugs stud threads
or surfaces which can result in over torquing of the lugs
and rotor distortion?
Are the brake pads in good condition, correct for the
application, not rusted or separated from the backing
material and is wear and condition of the front brake
components reasonably even side to side?
Gently retract the caliper pistons slightly to allow
verification of whether the calipers slide freely on their
mounting pins and bushings and between the stering knuckle
pads. Is there a corrosion buildup at the knuckle pads or
excessive clearance?
Inspect front brake flex hose condition and routing for
twists or damage during a previous installation. Remove and
support calipers from the knuckles. Inspect caliper pistons,
boots, pins and bushings, contact surfaces.
Inspect the rotor braking surfaces for surface cracks, "lot
rot" or pad "halo." Inspect rotor vents for plugging with
rust and road debris. Measure rotor run out with a dial
indiactor and thickness variation at multiple points around
the rotor. Measure and compare rotor material thickness and
compare with machine to and discard dimensions and thickness
variation specs. Index all wheels and brake components
relative to their corner and mounting location on each axle
hub flange as a reference point.
With the suspension still supported under the control arms,
inspect for CV shaft excessive runout or angularity/binding
concerns caused by accident damage or wear. If excessive
movement of the shaft is present, is it due to cradle shift,
motor and/or transaxle mount failure or? On vehicles with
separate front suspension cradles, is the cradle properly
aligned?
Inspect the steering tie rod ends and ball joints condition.
Make sure that upper suspension strut mounts allow free
rotation of the strut.
Inspect the condition and adjustment of the rear brakes and
master cylinder fluid level, condition and operation. Do the
brakes release immediately when foot pressure on the pedal
is released? Do the rear brakes contribute to braking or are
the front brakes doing almost all of the work? Given my
first impression of the braking condition, is brake pedal
travel typicla of what I would expect? Is the vacuum booster
functioning correctly?
Is there any brake lead or pull during braking? Are there
any signs of braking imbalance from rotor colouration,
obvious metallurgical differences in the rotors, quality
pads of the correct type installed? In the case of a pullIs
rotor temperature and braking pressure even and in specs?
Does the vehicle have OEM rotors or aftermarket? How long
since previous brake services and repairs were completed,
wheels last removed?
John, here's how my own vehicle played out. Each morning I
drive 40 miles to work at 4:10 am. The temperature
regardless of the time of the year is reasonably cool at
that time of the day. Depending on traffic, I may apply the
brakes as little as 7 or 8 times or as many as 20 times
during the trip, which takes me at highways speeds for the
first 20 miles, in town for 10 miles and freeway for the
last 10 miles.
Brake pulsation on this 4T40e equipped car became noticeable
by 19,000kms. I do not generally brake aggressively. I brake
gently each morning for the first couple of stops to remove
any light surface rust and try to ensure to minimize parking
the veehicle without wet brakes.
When the pulsation first became evident, it was at lighter
pedal pressures at 30 mph, seemed fine at higher speeds
where more pressure was required. At each stop during the
journey on dry days, the pulsation became more noticeable,
reagrdless of the braking pressure applied.
Inspecting the vehicle thoroughly revealed .001" thickness
variation and 0.001-0.0015" rotor run out with the vehicle
hoisted. Run out was the same with the wheels installed and
hub bearing play was nil. Brake pads were in almost new
condition, nicely bedded in, rotor surfaces clean and in
excellent condition and vents clear. The brakes were applied
and released several times, rear brakes adjusted. I
qualified the shop off car brake lathe to less than 0.0005"
run out and cleaned the rotor hat inside and out and
resurfaced the brake rotors using new cutting tips,
requiring approximatey 0.004" maximum material removal only
in total and the best finish possible was obtained. The
rotors were sanded for 60 seconds per side for a
non-directional finish with either 120 or 150 grit aluminum
oxide abrasive paper, whichever was handy. The rotors were
washed with soap and water, rinsed throroughly and dried
with a lint free cloth. The rotors were measured and
compared thickness was 0.001"
The hub rotor mounting flanges were cleaned although there
was no rust scale build up at this time. With the suspension
supported, flange runout was well within specs as was lug
stud circle run out. With the rotors remounted, they were
indexed on the hubs to the point where assembled run out was
less than 0.001"
In this case I resurfaced the rotors intentionally after
careful lathe set up, so that I could accurately determine
the total assembly run out. I could have used the ProCut on
car lathe, but I was on a mission to inspect the dimensions
and run out of each component. Quality off car machining can
be effective with proper cleaning, preparation and lathe
qualification.
The caliper slides had no lube at contact points, as they
never did from new. The pins and bushings required moderate
pressure allowing the calipers to slide. I removed the pins,
washed them, removed the rubber bushings carefully and
cleaned the bushinng bores in the calipers of light
corosion. The pads were cleaned and refitted. Everything was
lubricated, reassembled and carefully torqued.
The brakes were burnished lightly and treated gently. Within
a few thousand kilometres the concern was back as bad as
ever. Reduced caliper and pad drag from careful service had
increased the daily driven distance before the condition
became evident each day, but at its height the pulsation was
absolutely identical to how it was previously. Inspecting
the rotors and components revealed less run out and
thickness variation than before and everythign else checked
out. I suspect that one of the brake rotors was warping, due
to some metallurgical issue where heat was not being
dissipated evenly. As in other situations there can be a
"memory" effect when this type of condition exists. When
driving in wet weather there was never ever any more than
minimal pulsation with this set of rotors. On dry days it
would shake notably enough to make you nod your head!
I decided to install a set of aftermarket rotors for
comparison. I did no other special preparation other than
minimal cleaning and rotor indexing for minimal run out. I
replaced the pads with new OEM pads ($125 my cost) since
there was approximately 25% remaining and I didn't wish to
work on my vehicle frequently. The vehicle had covered
62,000km (approx 40,000 miles). It has now covered almost
120,000 kms and in the last 3,000 has developed a very
slight pulsation which has not worsened. From what I can see
at a glance, there is still a decent percentage of brake pad
thickness remaining, but I will inspect more closely at the
next LOF interval in a couple of weeks.
In this case, one rotor was definitely warping in my
opinion. On some vehicles it can take as much as 0.008" run
out caused by thickenss variation before pulsation becomes
evident, while another may exhibit pulsation with 0.0025"
runout due to excessive thickness variation.
You will probably recall the Chev A body Celebrity
pulsations dure to metallurgical disposition in the rotors
many years ago.
When evaluating brake pulsation, we must take into account
more than just the braking system. The attitide of the
vehicle and any sudden shift such as an engine moving on its
mounts can cause CV axles to bind and cause pulsation. Some
tires with construction concerns may cause a pulsation when
the carcass distorts under braking loads.
The should we or shouldn't resurface rotors debate is
endless. Sometimes replacment rotors are not expensive and
can be high quality or cheap junk. On some vehicles rotor
resurfacing is justifiable and works well, on others it is a
poor option. On car or off car lathe? I've used both
extensively with great results. Both require careful
cleaning, preparation and set up an both can provide results
well within manufacturer or my preferred tolerances.
If we choose not to resurface a brake rotor because the
surface finish is in excellent condition with no run out,
thickness variation or construction concerns, the concern
exists that the use of a different friction material to the
one being replaced, may cause less than satisfactory
braking. In reality it is rarely the case where there is no
rust buildup around the rotor inner and outer edges which
might interfere with a brake pad of slightly different
shape. Still, there's nothing worse to a consumer than
experiencing a brake pulsation shortly after a brake
service, which the vehicle never experienced before. Unless
we are careful, it is all too easy to create problems.
There's a whole host of things that I haven't even touched
on here, but the bottom line is that the tecnician must have
diagnostic and service expertise and never consider any
brake repair as "routine."
I'm sure that with input from every one here we could
compose a text book on braking system service and identify
some of the more unusual diagnostic findings that have
resulted from or been caused by "routine brake service or
repair."
