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Discussion Starter · #1 ·
I have two perhaps unrelated vibration issues in my 2006 S40 T5 AWD M66. The first is a bad vibration when braking that I believe is due to a warped rotor in the front. It happens when braking using the brake pedal at any speed but doesn't happen when using the e-brake to slow down. Since the car uses the rear pads and rotors for the e-brake (unlike my 2004 XC70 which has a mini-drum brake system for the e-brake) I think this is a good test to isolate whether it is the front or rear brakes. Is there a way to determine which side rotor is warped other than actually measuring them, perhaps something that can be done on a road test?

The second is a vibration upon acceleration that gets worse when I turn to the left while accelerating. I believe this points to the right front CV axle as the culprit. I had the right front axle replaced about a year ago when the shop I had doing my clutch and Quaife LSD install told me the inner CV joint was bad. Given the rarity of the AWD M66 axle I could not find a low-mileage OE unit in nearby wrecking yards so I went with a new Carquest replacement from Advance Auto. I've also read varying reports here that the rebuilt OE units available online are not that good anymore so I opted for the $70 (with discount code) lifetime warrantied version in stock locally. This also allowed me to keep my worn OE unit rather than turning it in as a core. Anyway, a year later and I'm having the vibration upon acceleration issue so I replace it under warranty with a new unit. It seemed better upon initial road test but a couple of days later and the vibration is back. I know this is a common problem with aftermarket CV axles so I'm trying to determine my next course of action.

I believe the two vibrations are unrelated but I could be wrong. Can a warped rotor be causing a vibration under acceleration? If not and I focus on the CV axle should I just keep getting warranty replacements until I hit the jackpot and get one without a vibration? Also, being an AWD there are the rear CV axles to consider as well. I don't think it could be a rear CV axle since under normal traction conditions the rear doesn't receive significant power and thus wouldn't be under the load the fronts are under when accelerating. Or could they be causing the vibration under acceleration? Last but not least, since I have the OE front right axle I am considering finding an axle rebuild service to rebuild it. Anyone know a good place to take it for a rebuild in the Boston/Southern New Hampshire area?
 

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Vibration could be many things including a intermittent misfire, motor mounts, or perhaps a cv axle (never really seen that honestly) have you replaced the mounts? They are a wear item and I would definitely check those first. As far as the shaking when braking; classic warped rotor symptom. You’re better off either replacing all 4 rotors along with new pads, or just replacing the front pair as unevenly worn rotors aren’t ideal in my opinion.
 

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+1 with crwhite... more often than not its uneven pad deposits on the rotor from improper bedding. However, I had a bad vibration issue with braking and acceleration a few months after buying cheap rotors. Replaced the rotors first and it eliminated the acceleration vibration as well.

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Discussion Starter · #5 ·
Vibration could be many things including a intermittent misfire, motor mounts, or perhaps a cv axle (never really seen that honestly) have you replaced the mounts? They are a wear item and I would definitely check those first. As far as the shaking when braking; classic warped rotor symptom. You’re better off either replacing all 4 rotors along with new pads, or just replacing the front pair as unevenly worn rotors aren’t ideal in my opinion.
I replaced the mounts within the last 1 1/2 years. Replaced the plugs and coils in the last year and there are no misfire codes so I think we can rule out intermittent misfires.

With regard to the rotors, I think it's important to understand what is really occurring when we think a rotor is "warped" before attempting to diagnose and repair (they are likely not actually warped). These articles provide more insight: https://blog.fcpeuro.com/warped-brake-rotors-vibrating-reality-or-internet-myth, https://www.brakeandfrontend.com/warped-rotors-myth/,
I did not know about this. I certainly know about bedding new pads/rotors but didn't know what was really happening when doing so.

+1 with crwhite... more often than not its uneven pad deposits on the rotor from improper bedding. However, I had a bad vibration issue with braking and acceleration a few months after buying cheap rotors. Replaced the rotors first and it eliminated the acceleration vibration as well.
I've had the rotors on for a few years and the vibration is fairly recent. If it was improper bedding would it have occurred earlier in the rotors' life or is it something that can crop up later? That's interesting about the acceleration vibration being eliminated with the new rotors. I think new rotors/pads will be my next step and then I'll reassess both vibration issues.
 

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I'm a fan of the highway scrub - as in driving down the highway at the speed limit with the brakes applied firmly in 10 second increments. If it gets worse you know it's warp, if it gets better you know it's the deposits. Keep on driving for a few miles to cool them off before you get home and all is well. YMMV of course. And don't do nothing that will get you hurt obviously.

Also - I would get some carpart.com axles your way or go to a picknpull. You can get them rebuilt for about $100 not including shipping and you should be good to go. Significantly cheaper and better quality than non-oem units.
 

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Discussion Starter · #7 ·
I'm a fan of the highway scrub - as in driving down the highway at the speed limit with the brakes applied firmly in 10 second increments. If it gets worse you know it's warp, if it gets better you know it's the deposits. Keep on driving for a few miles to cool them off before you get home and all is well. YMMV of course. And don't do nothing that will get you hurt obviously.

Also - I would get some carpart.com axles your way or go to a picknpull. You can get them rebuilt for about $100 not including shipping and you should be good to go. Significantly cheaper and better quality than non-oem units.
Yes, I read your post in another thread about re-bedding the pads. I haven't had a chance to do a full version of it yet but I did try practicing the technique on the freeway the other night. I'm not sure how firmly to apply the brakes plus it's a little tricky with the smaller brake pedal due to the M66 (as you know.)

Fortunately I still have my OE axle and I just dropped it off at an axle rebuild shop in the area. I also looked on car-part.com out of curiosity and only found three AWD M66 front RH axles in all of New England. So glad I remembered to ask the shop that originally replaced it to give my original back to me!
 

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Discussion Starter · #8 ·
I fixed the braking vibration but am still having the acceleration vibration. My left front rotor had a short section of the rust ring broken off and once I replaced with a new rotor the braking has been smooth as can be.

I installed the rebuilt OE axle but unfortunately still have the vibration upon acceleration. When I replaced the left front rotor I checked the CV axle on that side a bit more thoroughly and found a slight amount of play. I ordered an inexpensive new axle to be used temporarily pending the rebuilding of my OE axle. Not sure I'll go back to the place that did my right front axle but we'll see after I install the new axle and reassess the situation.
 

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So I tackled what I THINK my issue was today with my brakes -
I put brand new rotors on there thinking I had warped my old powerstops - but I have been chasing a front end vibration since I bought those wheels from a dude up in Northern CO. Vibration was still there on my drive to work - NO WAY I bought already bad rotors??!?!?!?! Anyways I tore them down again and thought about something - if my slider pins in the caliper were greaseless and sticking it would mean it would try to move the rotor out a bit when I press the brake, causing some shaking. So today I took everything apart - recompressed the front 2 brake cylinders, cleaned up the pads, regreased the pins and reassembled everything. This time I put the wheel on and tightened each lug nut slowly to stop - then torqued to spec - and then pumped the brakes to put the calipers back in place.

So far so good, but I'll give it a go on the highway tomorrow to confirm. I didn't know that was possible but I guess you learn something new every day.
 
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