Odd brake drag

[Droo]

Hello, I have a very odd brake drag problem, I'll try to explain as clearly as possible.

Car: Audi A4 B6 2002 AKE 2.5TDI Q automatic.
Problem: FL and RR wheels get noticeably hotter after 60+ km drive, if driven longer it gets so hot the brake pads start to smoke.
How I noticed it: I got the car ~4 months ago, it had new pads, ~4 weeks ago I got pad wear warning on the dash, when I replaced the pads, FL pads were worn to the metal, FR were like new.
Replaced/visited: all brake pads and disks, brake hoses, ABS block (used, same PN), brake fluid, rebuilt FL & RR callipers, lubricated all calliper slider pins with proper lubricant.
Notes to possible problem: The old and the replacement ABS block makes a double click solenoid-like sound on ignition. If I press the brakes before turning ignition on first click is barely heard and second click occurs on brake pedal release (with ignition/engine on). No fault codes. Car is pulling to the right (new tyres, alignment done). Apart from the problem braking feels fine.

Anybody have a clue on why this is happening?.. I think I bought the car with this problem. Can it be some air trapped in ABS block or something? I replaced and bled the brake fluid w/o a pressure bleeder. Can it possibly be something not even associated with brake system like dry CV joint or something?..

[old guy]

That is very strange. Have you checked the ABS module for any DTC's? You really need to start there. Clicky click®

It sorta sounds like the ABS is trying to correct a yaw problem. One thing to check would be the output of the wheel speed sensors while driving in a straight line. They should all be registering the same thing. You can do this by logging into the ABS module. Measuring block 001 shows the output of all four wheel speed sensors at the same time.

You could also check the zero position adjustment of the G85 steering angle sensor to make sure it is centered properly. Checking the ABS module may even show a Yaw Rate Sensor (G202) fault.

Good luck!

[jacobsen]

Wow what an issue to have, I'm curious what the outcome is.

Sent from my SM-G920V using Tapatalk

[Kevin C]

A few ideas come to mind to help trouble shoot.


If I remember right, the Audi uses a split braking system , corner to corner. A restricted line between the master cylinder and the ABS module could act as a check valve. A rubber flex line with a tear in the liner can do this ( the stainless steel braided lines have some type of elastomer liner). If I remember right, there is a couple of them between the master cylinder and the ABS unit.

1: Go for a drive spin, stop shut the engine off and then jack one wheel up and see if it's dragging. Crack a bleeder screw and see if the drag goes away. If it does, you have some type of hydraulic problem. If its doesn't, try the same thing but with the car still running. If you still have drag, it might be something with the ABS system.

It's possible that one piston on your master cylinder is hanging up after you apply the brakes. The pistons are paired, one driving the next and each piston drive two wheels (one pistone has the right front left rear, the other has the left front, right rear). If a piston is hanging, the brakes won't fully release.

If it is the the ABS, but you could try driving with the unit unplugged to help narrow the cause.

[Droo]

That is very strange. Have you checked the ABS module for any DTC's? You really need to start there. Clicky click®

It sorta sounds like the ABS is trying to correct a yaw problem. One thing to check would be the output of the wheel speed sensors while driving in a straight line. They should all be registering the same thing. You can do this by logging into the ABS module. Measuring block 001 shows the output of all four wheel speed sensors at the same time.

You could also check the zero position adjustment of the G85 steering angle sensor to make sure it is centered properly. Checking the ABS module may even show a Yaw Rate Sensor (G202) fault.

Good luck!

I did log 001 but not at fast speeds (50km/h), same speed all around. G85, however, shows -2.5 deg. when driving straight. As I mentioned there are no DTC's in ABS module... Also anybody can clarify about the clicking on ignition?.. I was going through other channels on ABS looking for something like brake pressure per wheel but didn't find anything that would seem like it.

[Droo]

A few ideas come to mind to help trouble shoot.


If I remember right, the Audi uses a split braking system , corner to corner. A restricted line between the master cylinder and the ABS module could act as a check valve. A rubber flex line with a tear in the liner can do this ( the stainless steel braided lines have some type of elastomer liner). If I remember right, there is a couple of them between the master cylinder and the ABS unit.

1: Go for a drive spin, stop shut the engine off and then jack one wheel up and see if it's dragging. Crack a bleeder screw and see if the drag goes away. If it does, you have some type of hydraulic problem. If its doesn't, try the same thing but with the car still running. If you still have drag, it might be something with the ABS system.

It's possible that one piston on your master cylinder is hanging up after you apply the brakes. The pistons are paired, one driving the next and each piston drive two wheels (one pistone has the right front left rear, the other has the left front, right rear). If a piston is hanging, the brakes won't fully release.

I dont think its the ABS, but you could try driving with the unit unplugged.

It might be MBC then now that you explained how it works, the only thing I haven replaced yet and I do actually have a replacement part but I kept postponing the job because I know it's a b*tch to replace... I'll try to find some spare time tomorrow and change it.

[old guy]

It sounds like Kevin has you on the right track.

You definately want to correct that steering angle sensor. Is your steering wheel slightly off center when going straight?

[Droo]

It sounds like Kevin has you on the right track.

You definately want to correct that steering angle sensor. Is your steering wheel slightly off center when going straight?

It might be just a little bit. When I did cruise control retrofit I tried to put it dead on but it's impossible, it's either just a little bit off to the left (like a ~1-3 deg.) or more to the right (~5-7 deg.). When I had alignment done before the retrofit, the wheel got ~30 deg. off to the right though...

[old guy]

The -2.5° steering angle sensor discrepancy can be from one of two things. If the wheel is aimed straight when driving in a straight line you need to zero the sensor.

If the wheel is slightly off center when driving straight you need to make a mechanical (alignment) adjustment. Fortunately it's pretty easy to do. Determine which direction you need to move the steering wheel. Pull the car up on ramps and mark both of the outer tie rods. Loosen the lock nuts and make equal but opposite adjustments on each tie rod. This will keep your overall alignment the same but will move your steering wheel toward center.

For reference I have found that one flat on the lock nut equals about 4.5' (.075°) of adjustment.

[Droo]

Hey guys, I changed the MBC yesterday, was not so difficult as anticipated just time consuming to bleed the brakes. Took my routine drive today of ~70 km, now all wheels seem to be of same temperature, not too hot for the touch but quite warm, I would say ~40 degrees celcius. Might be because today's really hot day compared to the days I monitored the wheel heat before. Besides that I instantly understood it is in fact MBC's fault, when I took the old one out and pressed the shaft in it spat a bit of gunk from the reservour holes, even the part that hides in brake booster had some debris on it, I suspect the car had the battery shelf flooded for some time because there were more clues of it. G85 is probbably not calibrated straight since when I changed the ABS block, I'll recalibrate on straight road today. All that's left now is to wait for cooler days to see whether all the wheels be cold as the non dragging ones before the MBC change or if now all the wheels get warm like today... Although I'm not even sure how hot should the wheels be after a normal drive, I never even checked before on other cars I owned that didn't had such a frustrating issue...

[Droo]

While washing my car today I had another thought on why the other two wheels (FR & RL) used to be significantly cooler and now all wheels are about the same temperature - maybe it's not the FL & RR wheels were dragging but rather those two cold ones were not braking (as much)?.. Because I remember this one time when I had to brake quite hard the car pulled to the left (to the side with "dragging brake"). And the brakes do feel better now.