MP4.2+6.0

Did my rear brakes today. Car has had error codes for several months about rear pad wear and implausible readings (only in VAG COM; no dash panel light on), which I had not been able to clear. However, each pad still had more like 5 mm of life, not < = 3mm as the system had locked in on--see my side question/observation below about that. Parking brake functioned fully, and system periodically recalibrated as it should--the whirring noise stuff after shutdown once in a while. Rotors above min spec too in case it somehow the system looked at total piston travel inclusive of rotor wear. They were correctly set to start at 11mm of net pad thickness with the prior pad change--again, see my side note below about what Bentley says.

I decided to do the pads again early to try to resolve this, since it is about to go in for its final Audi paid 75K service and any CPO fixes before the warranty expires. Warranty covers the hydraulic and electric brake components if something proved wrong there. I had tried to clear the codes several times, which would not take. I also tried to re set the pad thickness to the true measurement (as well as other plug numbers in the permitted range too), but that would not take either. I searched (STFA), but prior posts seemed to dead end and folks just seemed to leave it be if the pads were known good and parking brake worked, even if the dash warning light was on (mine wasn't).

Last time I did the pads, I found the caliper pistons fully retracted when I executed the measuring blocks step via VAG COM per Bentley. Bentley says you are supposed to have to squeeze the piston open, but that wasn't my experience that last time. However, this time it was not open fully executing that same step, but rather backed off just enough to get the caliper loose. Basically it was consistent with the Bentley instructions this go around. Continued through the job, this time squeezing the piston in. I had forgotten the D3's don't have detents in the face of the piston (at least mine don't) like my C5 4.2 and other prior Audi's I've done, so I just turned the proper caliper opening tool disk backwards to present a flat face to the pistons. They retracted easily.

Proceeded with the job and closed the pads to the rotor with VAG-COM per Bentley. Still showed errors and the warning light (which comes on as soon as the pads are moved to the open position) at that point. But, then I successfully entered the new pad thickness--I went for 12mm this time (measured at about 11.8mm; Bentley says round down normally) since it short changed me the last time and I look at the wear periodically the old fashioned way anyway. It held the 12mm reading--I had seen a post from Mister Bentley IIRC (?) that he couldn't get it to hold a 12mm setting. I read it a couple of times too in case it actually wants the combined thickness of BOTH pads, since Bentley specifies a range of 10-23mm. ( Hmmm, what's that about anyway as a side question??? 23mm is IMPOSSIBLE on any car pad I have ever installed unless you count both sides. Is it possible Bentley has it wrong? in the two lines prior it clearly says "measure thinner brake pad without backing plate and enter value...") Wrapped it all up, confirmed no codes in the parking brake module, or ABS either. All good.

My conclusions about the codes persistency: if it runs down to 3mm, it likely can't be simply re set via VAG-COM, which I had tried previously. At a minimum, the brakes need to be re-opened via VAG-COM (as if for service) and then shut again. It could be they affirmatively need that piston screwed in to its baseline position too, since for what ever anomaly it had let me skip that step the last time. I'm also wondering in the back of my mind if by chance Bentley is wrong and it should have the thickness of BOTH pads entered (i.e. approx. 22mm); I have never watched the calculated value over the life of a set of pads to really keep track of its accuracy. In any case, the system has recovered with the new pad change as far as clearing all codes.

Hope the info helps if anyone sees the same issue in the future.

Side note about the Bentley pad thickness instruction: FWIW, I'm dabbling with trying to plug in 22mm still as a combined pad starting thickness value (if that will in fact take in VAG COM) and then watching it over the life of the pads in case Bentley has it wrong or somehow the dealer computer (which Bentley is technically written around) multiplies the input amount by two, and VAG COM doesn't. That could explain why starting from 11mm (instead of 22mm), it concluded my pads were down to 3 mm or less, when they each had close twice that, at least some months ago when I first found the codes.

Further side note about VAG-COM (VCDS): Last time I did the job it was with probably a couple year old S/W version. This time it was current. They now have short cuts built in via a pull down menu to let you just choose to open the pads for a change out. No more needing the measuring blocks number out of Bentley. I actually tried it both ways (old and new), to be sure it did the same thing given the recent codes I had experienced. Net, it seemed to be the same either via the pull down menu or the old manual measuring blocks data entry way.

cclcal

thanks for the detailed write up.
I'm still stuck with the brake pad and parking brake fault codes.
I haven't had a chance to check the right rear, I did check the left rear and I forgot to replug in the parking brake motor.
Maybe that messed everything up.

I'll try entering 22mm as 11mm still causes fault codes.

Thanks.

mishar

I had to disassemble rear caliper in order to change rubber dust boot and I must say that there is nothing that can retract pistons. So my conclusion is that you do not remember well about last time.

MP4.2+6.0

Yes, unplugging the connector mid job seems to be a no-no from the posts. But, logically you have to pull the calipers completely at times for other repair reasons, so you would think there would be a documented procedure to deal with it.

From my experience, try a recycle of the brake system with VAG COM first--as if you are doing the brake pad job and opening then closing them. Then re set pad thickness as you wish. A bit of a mystery exactly what cured mine, but I suspect it is the cycling open and closed that cleared the codes; if not then probably moving the pistons back in manually.

MP4.2+6.0

because I remembered other prior posts saying they don't. I also had to demo saw out my tool from my other Audi (...operator error) during a "what more can go wrong" brake job, so I definitely remember any rear brake tool experiences the past few years in the how not to do it dept! New one I got locally is a much nicer set up though than the old one, and it wasn't too pricy.

I saw this pass that VAG COM has a different measuring blocks pull down choice for repairing the brakes generally rather than changing pads, so I suppose I could have stumbled there even w/ Bentley in hand.

mishar

Never mind software, it can't pull pistons back mechanically. Pistons are grooved longitudinally and star like nut travel along those grooves freely moved by threaded central rod turned by motor. That nut push piston touching the base from inside, but it is not connected to the piston so it can't pool it back.

Software option for disassembling pistons actuates ABS to push them all the way out.

S8code3

Thanks for the detailed info - very interesting (and frustrating) quirk. I can only imagine the time and effort involved in figuring this error code out.

cclcal

So i did a couple recycles of brake pad job opening and closing them to no avail. I did have to manually press the piston all the way back to insert new pads.
But piston seems to be fine. How do you move pistons back manually?

thanks

MP4.2+6.0

When I looked at system again, no code but it was back to 3mm wear level showing in VAG COM (which I had set at 12mm). Next, I set it at 11mm (VAG-COM shows a range up to 14mm, which then doesn't agree w/ Bentley). I have scanned it twice since then over several days, and the 11mm setting is holding fine with no change and no codes.

Tool: a standard brake caliper tool. My latest one is a Lisle; I bought it locally at a foreign car parts specialty store--one of the few remaining near me. Looks like this, except in a gray case: http://www.toolsource.com/universal-...rceid=googleps That's just first link that I saw; didn't try to price shop.

Typically the rear pistons on modern Audi's (with self adjusting rear brakes) have detents on the pistons, and then the tool has various adapters with exposed pegs to lock in to those detents and allow you to screw the piston in easily. Can sometimes be done with cheaper tools (like a basic universal adapter on a socket drive), but those are tricky and can often mar the calipers, or your skin when things slip! The D3 has a pretty smooth face to the caliper pistons, just with slightly raised ridges all around the edge. So, I just turned the right sized adapter from the tool set backwards to show a smooth face (instead of two pegs that could have chewed up the piston face). Turned in very easily; not sure if it actually rotated or just went straight in. On my C5 4.2, it has to use the side with the pegs exposed, and it's clear in that car it has to be rotated while screwing it in. But C5's don't have the electric parking brake complication.

Net, took about 4x the time with VAG-COM as it did with the tool! Seems like needless German complexity/more trouble than it's worth in the design, but it is what it is.

cclcal

thanks for the info.

I attempted to mess with the car but noticed that my battery level was @ 5%. Everything still starts and runs.

No, I did not use a battery maintainer (as it says you should).

well, time for me to go buy a battery maintainer/charger now before I can mess with the brakes again.