S3 (02) rear wheels lock on right/left turns

[thegoal007]

OK , thanks for that , understood. So in other words leave it for the experts !

Anyway, there's a new development and why I didn't think of this before I don't why. Basically we removed the 2 30A fuses just above the battery that power the abs/control/relay unit to see if I still had the problem. Once removed the interior panel shows the 2 lights lit: ABS and ESP , but I STILL HAVE THE PROBLEM. Surely this now defintely points to the rear haldex control unit ?? And I was fairly adamant it was not this unit !

I had an engineer with laptop plugged into the access point in the car and drove around slowly to get good settled readings from the sensors & car and then attempted a carefully slow full lock reverse and full lock foward and the problem presented it self , but it was showing no problems whatsoever on the laptop, but the engineer could defintely feel like the rear was gripping and not letting. He was the one who suggested to remove the fuses to rule the abs/control unit out of the equation.

So does the buck stop there ?

[ed'sA3-1.8THK]

i appreciate your concerns

esp(/abs) function (on models with hydraulic assist steering) addresses wheel rotation rate of change, yawing (steering angle) rate of change and length of time brake pedal depressed (brake system hyd pressure maintained

on your 1st gen S3 haldex is not involved in esp intervention as reaction time for "your" haldex is far slower than "your" esp wheel caliper actuation.

esp/abs needs attention

[ed'sA3-1.8THK]

as a side note re: A3/S3 (8P) new chassis & all other recent chassis servitronic elec assist "power steering" the revised esp(/abs) intervention is more comprehensive and alters steering assist level during emergency intervention when max yawing rate of change is exceeded

this was not possible with the earlier hydraulic steering assist

re: (8L) S3 ESP function - during emergency avoidance, limiting max yawing enables the driver to immediately regain vehicle direction control, otherwise uncontroled yawing will always result in a serious crash, frequently including rollover and partial lifting of vehicle

all (4) individual wheel primary calipers, elec assist steering dampening (yawing reduction) and engine thottle increase or decrease (often thottle "increase" is critically needed now) are the "active" components removed from driver control by esp mappings values being exceeded

wheel rpm rates are equalized, steering rate-of-change brought back to predetermined limits and engine thottle is returned to the correct "opening" for the measured road speed and/or "cruise control" setting

your S3 does include yaw rate-of-change sensor, but not steering assist level esp intervention

"pulling the fuses" will only result in unecessary wheel slippage and absolutely NO front traction control as well as disabling a critical vehicle safety system - your decision should be reconsidered

[thegoal007]

I hear what you saying, but how come the part I am suggested to change has effectively been removed, via the controlling fuses and the problem still persists ?

Are you saying by removing the fuses the unit is still active in some way ? The fuses were only removed to prove the fault still existed and put back straight away.

This is all still very confusing. Is there anyway to isolate the problem ?

I feel like a yo-yo going from school of thought to another !

[ed'sA3-1.8THK]

first, why will wheel caliper apply clamping force, pressing brake pads onto rotor surface?

because brake hydraulic fluid is either extreme slow to return to master cylinder AFTER you release brake pedal or because caliper piston is NOT free to move in caliper bore (thick sludge build-up or rust - iron piston/alum bore surface)

OR esp(/abs) multi solenoid valve/manifold has sticking valve bodies that are not freely retracting, while you are driving/using brakes as required

immediately when you experience rear wheel seizure - burning smell - when safe to do so, move to verge get out of your S3 and examine master cylinder reservoir brake fluid level

is level lower than previously checked before you drove off on test run???

1) bad master cylinder (will manifest ALL the time will non-returning caliper pistons) OR NO brakes at all

2)bad/collapsed brake hose(s) - preventing fluid return

3) esp(/abs) blocking fluid return - each wheel separate metal line - can be 1-2 wheels only and can be 1 front & 1 rear wheel locking/dragging

4) individual wheel calipers having sticking piston(s) - for both rear wheels to have corroded lucas alum calipers, your brake fluid is full of water and corrosion has occured over 3-4 yrs or longer AND esp valve body is also corroded

5) if haldex, you will see unusual tire wear pattern - AND you will experience transient rear wheel steering at highway speeds due to wheel rotation rate variation - this is quite noticeable to most drivers - usually blamed on differing rolling diameter tires / tire belt runout / different manufacturers tires paired together AND there will be noticeable esp intervention while driving - car will "dance to the left and to the right and can be v. disconcerting on wet road

you have pulled fuses and problem remains:

A. have someone sit in car - put car on lift - with ALL wheels blocked - running engine - press brake pedal several times and observe if rear brake pads move onto rotor face and release from rotor face - YOU MUST NOT HAVE Parking Brake Applied!

B. while on lift examine all tires for unusual pattern of tread wear

exercise great care with car on lift / engine running (you MUST have vacuum servo working / parking brake MUST be released and this is potentially v. dangerous...

at the end of the day, the brake fluid IS NOT returning to the reservoir tank on master cylinder...

sticking manifold valves in esp valve body DO NOT need electricity to be problematic...

[thegoal007]

first, why will wheel caliper apply clamping force, pressing brake pads onto rotor surface?

OR esp(/abs) multi solenoid valve/manifold has sticking valve bodies that are not freely retracting, while you are driving/using brakes as required

...

I don't know why the rear seems to grip hard to the surface on full lock ? It may be the brakes or the haldex locking up the rear.
If I am reversing on full lock I don't have to press the brakes at all, I use the throttle to control my movement and yet it locks up and I apply more throttle to get out of the lock and i feel fairly violent jump as the wheels then release a bit just to grip again moments later. As I said normal driving is fine just very sharp corners - even when I am not braking. There must be something more to it then just the esp(/abs) multi solenoid valve/manifold randomly not freely retracting because why is it then not doing it all the time even under normal driving or hard/soft braking in straight line driving ?

All my tyres are perfectly fine across the whole width. I never smell any burning (!). When I am attempting 3 point turns I tend not to push it hard and always attempt to straighten the wheels as much as possible to alleviate the locking of the rear. Basically becasue I know of the problem I am very carefull to try and prevent/minimize it every time.

It is not going to be easy getting it up in lift and carrying out what you suggest.

I have to reiterate that when the rear locks and I use the throttle only to continue with my action the car feels a significantly and fierce jump as it releases. (at which point engineers tend have a scared look on their face and say you are a serious problem with the back end ! especially when the laptop in front of them says no problems !) When I feel there is resistance in my sharp moneuvere I tend to straighten my wheels (obviously the front) in order for the rear move more freeely.

[ed'sA3-1.8THK]

my posted reply was just "lost in space" and I am pressed for time out here

short answer - haldex can not apply rear brakes or cause rear brakes to be applied

replace yaw sensor uder steering wheel first as this can, at full lock, cause esp/abs to apply rear brakes, IF electrical malfunction

than replace esp/abs complete unit

the transient electrical faults will not show on computer scans

this is all quite expensive, perhaps time to change car...

and you likely have accumulated water in brake fluid causing brake system wide corrosion...

[thegoal007]

Hey I appreciate all the information coming my way and I'm sure others are reading with interest - just surprised no one has actually come forward with the very same problem ???

I did find the pre-previous reply a bit hard to understand (I'm no expert, but learning especially from your jhelpful replies.

The short answer you gave about the haldex not applying the brakes was an eye opener, but I do not believe the brakes are being applied in my case. I think an early post from "A3_FSI_2.0" said it quite correctly "...sounds a bit like the rear diff is locking up momentarily".

When I am reversing on full lock and using the throttle only to manoeuvre I get to the point where the rear feels locked, as I then ease of the throttle the car rolls forward and as I accelarate slightly to move back I stop at the same rotational position (it feels as though there is something preventing me from moving further back, but can freely allow the car to move forward. Obviously with a bit more throttle the rear feels as though the diff has jumped out of "that stuck cogged position" - my simple interpretation !

Does that change anything ?

BTW, This car is for keeps. Generally love the car. Had the car for 2 years now and it still feels like we are on our first date !!! Although it doesn't mean I just want to throw my money away changing practically every part in the car to solve this problem.

[A3_FSI_2.0]

I agree its unlikely to be the hadex as this just controls the pressure to the hydraulic clutch that allows various levels of power to the read diff. TBH i'd check the CV joints, diffs tend to whine long before they fail.

[thegoal007]

I will keep an eye out for a complete second hand esp/abs control unit. This might take a while.

PS. My diff is not making any noises ? I hear no noises just feel quite a violent shudder as the tyres release. At the point of locking I can make the car rock backwards and forwards so there is no brakes being applied by the car - it feels like the diff will not do that extra turn unless I push it a bit harder (throttle).