theoretically, but because it is a fleet car, for anything I take it
to one of two "approved" shops. If they say it has to go to the
dealer, *then* I take it to the dealer.
I really wanted to get my local wrench (near my house) with the
program so I could just drop it off there for maintenance/repair, but
when I tried to get them on board I inadvertantly caused all sorts of
kerfuffle and pissed off my boss, so I won't do that again. There's
some things that are worth making a big deal over, and there's times
you just grin and say "why yes, of course, I'll come in on Saturday
just so I can get my car serviced at the approved shop." Not that I'm
bitter or anything. Well, OK, I am, but aside from little PITA stuff
like this, I'm legitimately happy to still have a good job these
days. The pay's OK and even if it's a PITA to deal with, having a car
with gas and maintenance paid for is quite nice - certainly if I were
providing my own transportation my old 944 wouldn't cut it, not
without a major overhaul first, not with the amount and type (mostly
city, with interspersed blasts of high-speed freeway) driving that I
have to do. Even if my workload is temporarily doubled because
another employee decided that this wasn't the right place for her.
Yes, I am checking Usenet from my desk at work, while waiting for
files to download. *sigh* I have no life... (must remember to call
Follow up: I finally got a chance to take the car back today, and the
mechanic went for a ride with me and didn't even get the car up to
speed, he just got up to maybe 30 MPH and rode the brakes a little,
felt a slight pulsing, said "mmm-hmmm," and turned around and went
back to the garage. This time they turned the REAR rotors. He said
that Impalas were susceptible to rotor warpage, he had one himself and
had had the same problem. I'm still trying to figure out how a warped
rear rotor(s) could cause the steering wheel to shake but he claimed
that you have to turn all four. (I guess this was a different guy
than worked on the car last time.) I picked the car up this PM but
have not had a chance to try it at speed yet, but the slight pulsing
at low speed is gone.
Now I have another question, how does the parking brake work on the
newer Impalas, is that a little drum brake inside the rotor hat like a
corvette or my old 944, or does the cable just pull the caliper shut
like an old VW with rear discs? I'm wondering if setting the parking
brake causes rear rotor warpage if the latter, in which case I will be
more careful in the future about braking hard when I know I'm going to
park soon (I can't avoid using the parking brake, my driveway slopes
quite a bit.)
Correct me if I'm wrong, but I believe this is the first mention of a
pulsing that can be felt in the brake pedal at low speed? You are now
asking if this could have been caused by something you have done, but
previously you said the car had the problem when you first got it.
You didn't actually tell us what they did to fix the problem with the
rear brakes. It sounded like you maybe don't know for sure. You did say
the mechanic said they should have done all 4 rotors at the time of the
first visit but that was before he took it apart and did the work. Did
they tell you what they did? I agree the second guy is definitely more
competent. He probably test drove the car after doing the work. The
first guy obviously didn't.
My wild guess would be that the origins of the problem are sticking
rear calipers due to sitting. that might well have something to do with
the parking brake that may have rusted up from sitting. believe you
said it sat for 4 months over the winter.
I would also guess you are going to find the brakes now do a lot better
job of stopping the vehicle at highway speeds.
Having a pulse in the brake hydraulics that is felt in the pedal thaqt
is caused by one wheel is going to carry to the other 3 wheels also. So
there is no mystery about how it got to the front wheels.
I didn't mention it because it is very faint, inoffensive and hard to
notice unless you're deliberately looking for it... if that were the
only problem I wouldn't have taken it in. The steering wheel shaking
violently when braking from high speed... yeah, that's a problem.
No, I'm just asking if the parking brake design is such that it clamps
the pads to the rotors, *in which case* in the future I'll take care to
try to not have to use the parking brake immediately after having put a
lot of heat in the brakes. Yes, the car did exhibit these symptoms when
I first got it and it's only gotten worse over the few thousand miles I
have driven it, but it was previously used by other drivers before I got
it. I'm certainly not taking responsibility for "causing" the issue
even if parking brake use is the actual cause because I would consider
applying the parking brake after stopping the car to be normal operation
and the car should be able to accept it.
They resurfaced the rear rotors.
Yes. on the first visit they resurfaced the front rotors. On the
second visit they resurfaced the rear rotors. That's apparently it,
other than on the first visit they also balanced/rotated the tires at my
request, and also changed the oil.
Not sure exactly how long it sat, but yes, the last previous user of the
car left the company last year, so it did sit for several months, some
of them such "winter" as we get here. My original thought was uneven
rust on the rotors (I've seen cars that didn't move *at all* for a long
time develop rust on the rotors, but when you roll it forward you find
that the area under where the pads sat isn't rusted at all) I figured
it'd wipe off after a few hundred miles, but it didn't. Nobody
mentioned anything about a stuck caliper, so if that is in fact the root
cause, either it's unstuck itself or the problem is going to come back :(
Along the same lines, *while* the car was in the shop, I ended up having
to unexpectedly make a speed run to a job site (apparently the easiest
way to ensure that you have to leave the office is to schedule for your
car to be in the shop...) and ended up taking another ex-employee's car
that's waiting to be reassigned (same year/model, maybe 20K more miles,
sitting for 2-3 weeks) and I did in fact hear evidence of rust on the
rotors for the first few stops, but by the time I got back to the office
(maybe 70 miles later?) it was braking fine. Please don't ask why we
have so many ex-employees, it seems to be only one position that it's
hard to get someone to fill long-term, and I can't explain it either.
I try not to have to test the ultimate stopping power, but they *feel* a
lot better now :) Yes, I did take the highway home from work and tried
some light braking from speed, and I didn't see any of the steering
wheel shake that I experienced before.
it just seems odd to me that the predominant symptom was a shake in the
steering wheel. You'd think that if there were hydraulic effects that
carried to the other wheels I would notice non-smooth braking, but that
it would be equal on all wheels, so no steering effects. Or, in other
words, the steering wheel shaking would point to a fault with the
*front* brakes, not the rears. Not so, I guess.
replace "roosters" with "cox" to reply.
I wasn't interested who to blame. I was saying the picture you painted
wasn't as clear as it could have been that's all. I had assumed there
wasn't any pulsing in the pedal since you hadn't said anything about
that . I don't think using your parking brake will warp the rotors even
if they are hot. But it might contribute to the rear brakes getting
stuck if you park it for months.
The braking surfaces should be rust free in five minutes if the calipers
were working as they should. The rust on the braking surface never seems
to be as deep as the rust on other surfaces. Don't know exactly why that
is but the rust on the braking surfaces is usually very thin. Often if
you hear scraping from rust it is on the surfaces next to where the pads
contact that have blossomed with rust and it is rubbing against the
sides of the pads.
If the caliper can't slide then it puts a tremendous uneven bending
load on the rotor. It doesn't have to be stuck very long to damage the
rotor. If it had stayed stuck you wouldn't have been driving it for
months before getting it fixed. Once it starts I have never seen a brake
rotor get better on it's own - it's all down hill from there.
The point was if the problem was tires it wouldn't reduce braking power
until you brake to the point where the tires start to let loose of the
pavement. Lousy tires aren't going to make a noticeable difference when
I think it was reasonable to assume it was from the front because that
would be more likely on most cars, but they should have test drove it
afterwards. If the pulse is still there then obviously it is coming from
the back. And because the pulsing will carry to the other wheels it
could lead to uneven wear in the other rotors after a while. So when it
has been happening for a long time it probably is a good idea to do
turn all of them.
Right, but I'm still curious now as to which design it uses... if it is
a caliper parking brake it hypothetically could cause warpage... I'm
still at a loss as to how a rear rotor could warp, seeing as the rear
brakes on a typical car do about jack in normal use.
I agree, I was just hypothesizing that it might cause uneven wear
patterns even after the rust was gone. Have never actually had that
happen though, even on my Scirocco which sat almost as long at one point.
if you're talking about the slides being stuck, I ASSume that that would
have been identified/fixed while turning the rotors... I hope... (this
is why I really prefer working on my own cars myself; I won't do it on a
company car though. My boss would have my head on a platter if I munged
something up, waaaaaaaaaay too much liability.)
well I maybe put 1K miles or so on it in the 10 days between 1st and 2nd
trip to the mech (there's a reason why it took so long to get it in, and
it's that I had lots of places that I *had* to be...) front rotors still
look "just surfaced" so I don't think I managed to mung up the new job.
I'm planning on putting some "personal use" miles on it this weekend
(finally! a weekend without work!) so I ought to know by Monday whether
it's 100% good or not.
It's actually not a bad driving car, now that it's sorted, except for
the awful OEM tires. Pleasantly surprised. The other car I drove had
apparently already had the tires replaced, that one felt even better
despite the higher mileage.
replace "roosters" with "cox" to reply.
Nope, they use a "monoshoe" inside the rotor hat. It's a horseshoe
shaped friction lined assembly that flexes toward the inner drum surface
when actuated, making roughly 270* of contact.
I haven't noticed that scenario play out, but I guess it's possible.
Normally, that would be true. I've seen some rare cases on 4WD vehicles
where apparently the rear vibration is sent through the driveshaft in
the form of uneven rotational speed (shaft minutely speeds up/slows down
in accordance to the rotor thickness variation while braking). Perhaps
if I had test driven your car I might have describe the complaint
differently. Assuming I would have concurred that there was indeed a
brake shudder directly affecting steering oscillation, it's possible
they might have returned your front rotors on-car (without telling you
this time) to cure the last off-car machine intolerance. Otherwise I'll
assume the vibration was just more noticeable in the steering wheel,
though it was a chassis vibration coming from untrue rear rotors.
thanks for satisfying my curiosity. I will no longer worry about using
the parking brake :)
y'know, I think I may have just figured it out... if it's a *diagonal
split* system, that would make sense.
Now whether it is or isn't I'm not sure how to find out without either
a) purchasing the factory shop manual or b) draining one circuit I
dunno, seeing as it has 4-wheel ABS and all the lines enter/leave a big
machined block of aluminum.
replace "roosters" with "cox" to reply.
The RM's system description indicates this is a split diagonal setup,
but for what you are concerned about, that applies only during standard
braking where the EBCM/BPMV tandem doesn't cut off, boost, or cycle pump
pressure to any of the 4 wheels during brake assist, electronic
proportioning, traction control, ABS, or vehicle stability operations.
Good luck on draining the fluid as a reverse engineering measure. You
might need to borrow a capable scan tool to bleed the system afterwards :-)
I'm aware of this, that is why I was speaking in hypotheticals :)
that makes sense, actually. will have to file that away in the "weird
automotive crap that can fake out your butt dyno" section of my brain...
replace "roosters" with "cox" to reply.
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