Brake Service

I noticed last week that my car was telling me that it need a service in 4600 miles. After checking the onboard computer I see that it's saying the front brake pads will need changing.

I'm left scratching my head on this one, some maybe someone can enlighten me. I've never, ever come across a car that tells the driver of a brake service by miles driven, rather than actual wear. All my other cars have brake sensors and I'm left bewildered that some engineer at BMW thought this was a smart idea.

So, help me out here other than taking off the wheel to inspect the pads (and I'm no expert on judging brake wear), what's the best way to approach this?

I know my dealer will happily change the pads, even if they don't need it. Despite owning a BMW I'm not really a fan of wasting money when it's not needed, does the car also have brake sensors that tell me when the pads actually need changing?

To start yes your vehicle does have brake pad sensors they are on the LF wheel and RR wheel. And how it works it varies from model to model. But on your vehicle they use as smart system that works with your CBS (condition based service). The sensor is a two stage sensor. The DSC (ABS control module) sends voltage through both stages of this sensor and monitors the voltage as well. So when the brakes wear down the sensor wears with it and thus cuts through the stages of the sensor thus altering voltage to the control unit. The control unit then takes these measurements (along with other things like brake pedal pressure sensor data) an uses a equation to calculate the mileage left on the brake pads. Why they went to this system over the old system just has a brake light come on once the pads are worn is beyond me but thats BMW for you always reinventing the wheel. The issue BMW is currently having with there CBS data on some vehicles (depending on production) is the equation is wrong basically saying your brakes are do when they clearly are not. This is corrected by programming the vehicle. I would check your brakes your self very easy plenty of DIY out there about it.

Thanks for the clarification. I agree with you. A simple sensor would have worked better. I'll check it out. I was looking at brake pad replacement DIYs today and it seems pretty easy to do.

It's a piece of cake to change brakes just like any other car, but on one with CBS sensors like the E90s, you HAVE to buy new sensors unlike the old type. Having tried to just reuse the old sensors and reset the system, I learned first-hand when it went berserk. New sensors and another reset cured it.

If you have wheels with enough space between the spokes, one easy way to get a look at the inboard pad is to poke an inspection mirror between the back of the caliper and the wheel, and shine a flashlight on the mirror to reflect the light into the opening on the back side of the caliper, which should get you a gander of the inboard pad. You may be able to see the outboard pad looking directly. If you have wheels that won't accommodate that kind of access, you'll have to jack up the car and pull the wheels. Easy enough to do, but if you don't have a floor jack and use the factory provided jack (assuming BMW still provides one, which seems a bit of a non-sequitur w/ the runflats & no spare setup these days, but anyway...) you'll want to be careful and chock both sides on the tire(s) opposite the end you're jacking. Jacking any car should always be on a hard, level surface - asphalt can be softer than it seems, particularly when it's hot out, so plywood when jacking on asphalt may be advisable. For a jack stand, too.

It's advisable to have a jack stand at least set in place under the car when putting any part of yourself under the car (it's difficult to look at pads directly without sticking your head in the wheel-well, for instance, unless you opt for the mirror/flashlight route.

You need a torque wrench to properly tighten the lug bolts back to spec, there's no way to gauge accurately just by feel using the stock lug wrench that the lug bolts are tightened evenly, and not over-tightened.

http://www.harborfreight.com/telescoping-mirror-7361.html

http://www.sears.com/shc/s/p_10153_12605_00941603000P?keyword=inspection mirror

Searching e90post.com should (presumably) find info on where & how to jack you car using a floor jack or the factory jack. There might be similar info here too, somewhere.

Thanks for the guidance.

I'm fairly competent at doing basic jobs on my car, but brakes are a job I've not yet tackled. I'll have to search to see what "good" and "bad" looks like. I saw one video on Youtube where the guy replaced one set of front pads on his E92 M3 in 5 minutes - that clued me in that the job couldn't be that difficult, so I figure when the brakes are finally ready for replacement I'll give it a shot.

Hey, if I can do it, you can do it. My biggest challenge was trying to get my jack up under the car. Once it was up in the air, very easy.

Good to know. To be honest, the main reason I've never tried to do brakes is the hassles of getting the calipers loose (my reference point is the 911, which is a pain if you don't have the right tool), but the video I mentioned above he just loosened them with the tip of a screwdriver, so it seemed pretty easy.

Yeah, I know. It's funny, as I thought the days of BMW dealers being scammers had passed, but the reality is that BMW warranty service allows them to be nice for 4 years, then after you think you know them they start to behave badly. It's perverse because they should treat us better after the warranty runs out and we start paying the bills.

You seem pretty handy in working on your cars and I totally understand why you've invested the time in gaining the knowledge and tools. I figure I'll order the parts and have them ready for when the time comes.

It is very odd, as my dealer was amazing while my car was under warranty. The last two visits since my car went out of warranty I've not been impressed. We'll see how the next couple of visits go and if the poor service continues I'll be finding an indy mechanic and doing more minor stuff myself.

So THAT'S who that ugly person looking at me in the mirror is. I've been wondering!

I haven't had that be much of an issue, I guess I haven't had rotors get so worn with a lip that getting the piston pushed back in a bit to make some clearance for the pad to then clear the lip has been necessary. I kinda think that you won't have a problem unless you have a pretty good lip on the rotor. Front or rear, I'd think you ought to be able to remove the slide bolts and pull the calipers straight off without having to do anything more to free 'em from the carriers, besides popping off the retaining spring-clips (I usually just pop the clips off with a screwdriver).

With the caliper free from the carrier and secured in some fashion so it's not just hanging by the brake line, I use one of these:

http://www.sears.com/cal-van-tools-...00960625000P?prdNo=7&blockNo=57&blockType=G57

to push the piston back into the caliper (easier to keep the piston-side pad in place so the threader doesn't bottom out in the bottom of the piston), or you can carefully push it back in with some large channel-locks. I keep some old backing plates with all the pad material worn off to use with the brake pad spreader.

Clean & appropriately re-grease the slide bolts, stick the new pads (with new wear sensors, if necessary) in the carrier, reposition the caliper & thread the wear-sensor wire, re-install the guide bolts, torque to spec, re-install the retaining clip, and should be good to go. That ought to be the basics of it anyway, online info & the Bentley Manual for your car should address specifics. Rotor replacement, if needed, is typically pretty straightforward, and certainly easily done while doing pads, but requires removal of the carrier, which is typically not much more than removing and re-torqueing the carrier mounting bolts. I _think_ E9x's may use torx, if so, you might need the right-sized torx bits/sockets. The biggest pain with replacing rotors is if you can't get the little rotor retaining/positioning bolt removed, (strips out easily, particularly if rusted in) and you have to drill it out by drilling most of the way through the bolt-head thickness, and then knock off the rotor with a hammer, mallett, or whatever. Those always go back in well-slathered in never-sieze (only enough for the bolt though), and should be snugged but not over-tightened. Important to seat the hex-bit completely in the retaining-bolt when removing to give the best chance of not stripping it.

Make sure to pump the brakes back up after each caliper is done, so you don't risk over-flowing the brake fluid reservoir when compressing the other brake pistons into their calipers and end up with a paint-unfriendly brake fluid mess.

Thanks for the tips! None of those points came up in the references I've seen so far and it's those little things that make the job hell when you're doing it for the first time.