Scary dangerous ABS locking wheels

[tgray]

Here is what I know.
1. There are no codes shown on VDS and no light on the dash (the light works).
2. ABS seems to funtion properly on dry pavement or gravel.
3. ABS locks up and will NOT release the wheel(s) until car comes to a complete stop when stopping on snow or ice. This condition is consistent over and over. The brake pedal is not sticking.
4. ABS feels like it is pulsing and acting correctly as the car is slowing down, but the car will keep slowing down (skiding a wheel(s)) until it stops.
5. Brakes do not stick when pushed hard and not in a skid condition.
6. Brakes locking condition is only when the ABS pump is activated.
7. With ABS fuse pulled there is never an issue with brakes locking or sticking on.

Here is what I have done.
1. I had changed the original module because of a power failure code but still had the code with the new module. I found the problem was the fuse panel corrosion
2. This new module was locking the wheels just like mentioned above. So I assumed it was a bad used module I purchased. This one would lock up even on grass or gravel just like above statements.
3. I reinstalled the original module and it seemed all good until the first snow encounter and it too is locking up as stated.
4. I have bled the brakes and pedal is firm.

I have searched and found no other returns on this forum but did find someone with the same symptoms on the google search. All of the solutions that were given were wrong for these cars.

 

[compu_85]

I'd look closely at the wheel speed tone rings, and see if they are rusty, broken, or otherwise degraded. It could also be a weak wheel speed sensor.

-J

 

[tgray]

I will inspect those tonight. Thanks for the pointer. The car had been sitting a while before this condition happened.

 

[Genesis]

That sounds like a cross-fed signal from one tone ring to another, which causes the computer to think you have a lock-up condition on one wheel that is persisting.

The way ABS works is that when the brake signal is on (pedal is depressed) the tone ring signals must show the wheels are rotating at the same speed. If they're not the one(s) with the slower signal, or no signal (no rotation at all) are presumed locked.

If you're on snow or ice there will USUALLY be a slightly different rotational rate under either acceleration or braking. This is why the ABS is gated and only comes on when the brakes are engaged. Thus, since this is happening once it engages even if you fully release the brake pedal, I'd check the wiring carefully for a false "brakes on" signal to the ABS system -- that would definitely cause the sort of thing you're seeing.

 

[tgray]

I think I figured it out.
1. The brake pads were rusted tight into their holders on both back wheels.
2. One guide pin for the one caliper was frozen.
3. When normal brakes were applied I did not notice as not enough force was given to the brakes to make them jamb.
4. When the ABS kicked in it literally hammered the frozen parts into the rotor locking the wheels.
5. On ice once a tire breaks free it continues to skid. That is what the ABS is for - stopping the skid by pulsing. Pulsing the brakes does not do much if the brake is frozen and the pulsing would continue to hammer the rusted parts into the rotor and the rust would not release the pad.
6. Once the wheel stopped it could then regain traction and brake loose the stuck parts.
The strange part is the wheels did not appear dragging when even up on a jack. I think in this condition the caliper working like it should would allow the wheels to work themselves free in normal driving and not be noticed.
What a scary ride when your rear brakes don't let loose with the slightest touch at highway speeds.
The weird part is I have logged over 500,000 miles on these cars with many frozen pads over the years and never experienced anything like this.

Thanks for your advice. I now have to wait until the next snow storm as this condition never happened on normal roads.
Moral of the story = don't live in the Midwest where they use salt that rots out these cars every possible way.

 

[tgray]

I spoke too soon. I found some slippery snow and the condition still exists with no change. Not sure if all wheels are locking up. Seems like it is the back ones, but the slightest touch of the brake that causes a wheel to slip engages the ABS and it holds the brake hard until the car stops. There is nothing I can do to undo the brake but wait until the car stops completely. I don't even hear the ABS motor kick in. As soon as the car is at a complete stop the brake releases and all is back to normal freewheeling.
The same condition happens in reverse as well.
The speed tone rings all look in good shape.
The car was working fine for many years. No one has worked on this car but me for many years so nothing can be wired wrong. When I reinstalled the ABS module it did seem to work fine for a while until today.
I spun the rear wheels on the jack and they gave good speed sensor outputs into Vagcom.
Could the ABS motor be sticking? I would think it would throw a code if that was the case.
Could there be an air bubble in the ABS? I can try bleeding some more on another day
I will also try turning off the key while it is in brake lock and see if that releases it.

 

[tgray]

I put the car into a skid, the brakes lock up and when I turn the key off the brakes release back to normal while the car is still moving.
I would think this would point to the ABS system/module and not a mechanical lock.
The car does drive fine with the fuse pulled.

 

[STDOUBT]

Did you check the tone rings?
I once had very similar issues. I had found that a sheet metal screw had made its way behind
that little fence. It eventually stuck its pointy end out one of the holes and tore the face of my
RF ABS sensor. Removed rotor to free the screw, replaced the sensor. Bam. Back to normal.
EDIT: sorry, I re-read your recent posts. Does seem to point to the ABS itself.

 

[tgray]

I am stumped. The crazy part is this same issue was happening with another used module I had installed. As rare as this problem is I can't believe that both behave exactly the same. But when I installed the used module was when I first noticed the problem. Of course it could have been that way a long time. I just never knew until it snowed one day. Maybe there is a corroded wire that talks to the speed sensor or something the engineers didn't think would ever go wrong. I read on one of the threads someone quoted VW in saying if a module goes bad it will not lock up the wheels. This seems be exactly the case. For now I am going to run with one sensor unplugged. This turns the ABS light on and the car functions normal with no ABS. At least I have control if I need to drive it.

 

[Powder Hound]

Good luck on sorting this out. The only other comment I can think of is that if you had an air bubble in the ABS, your pedal wouldn't be firm. You must have bled your system well. Besides, if you had an air bubble in there, it would have the effect of the ABS not working as efficiently. It would then be much less likely to be able to lock up any wheel.

Cheers,

PH

 

[gforce1108]

The only other thing I've seen do something similar is a bad booster. An internal failure will cause the vacuum to pull the pedal down as if you were standing on it. That of course should happen any time the brakes are applied - not just when the ABS kicks in.

Wondering if when ABS kicks in the pedal drops to a point that the booster hits a bad spot? Notice any hissing when this happens (vacuum leak at booster)?

 

[BobnOH]

Silly question, but possibly related- Has the car had regular brake fluid changes?

 

[JB05]

Check the individual cables for rodent damage.

 

[tgray]

I didn't see any indication that there was any rodent wire damage anywhere when changing modules. Not sure of brake fluiid changes before 100000 miles. I know the car did not have regular brake fluid changes but when I changed the modules out 2x it was purged out then pretty good and didn't look bad. The car did seem to work fine for a while right after I put the original module back in as I tried locking it up on some gravel. Now it seems so bad that even on gravel the wheels lock solid. I am going to try to do some bleeding again with the Vagcom when I get a chance and see if that changes anything. Maybe some corrosion happened inside from moisture in the fluid. Everything keeps pointing to the module not working right.

 

[tgray]

I think I figured out the issue thanks to Genesis. He mentioned cross wired wires. I did not touch the wires but I did touch the brake lines. I bet I have a line crossed. That would all make sense. This car is a new beetle and most of the module job is by "feel". With the pre-bent lines they all seemed to line up right but now I wonder. If the ABS is applying brake force to the very wheel it is trying to unlock then it all makes sense why no codes and nothing malfunctioning. This would explain why both modules behave the same. I see the diagram in the Bentley so I will check it out tomorrow.

 

[Genesis]

That would definitely do it.

 

[tgray]

Yes it does mess things up with the lines crossed. I got a better light and saw 2 lines that I thought would not mess up when I did the swap were in the wrong spot. I think the right rear ended up being swapped with the master cylinder feed.
Now all works like it should again. Actually, better with the brake pads cleaned up. Another one to add to the "don't do that stupid again" list.
Thanks for all your comments and ideas. Things seem so easy and simple when the problem is found but ideas and suggestions help narrow things down. Even if ideas sound stupid it is still good to ask the questions because somethings can get overlooked or the questions lead to better questions.
This one really puzzled me with the millions of TDI's out there and no one else had this problem and the exact same problem was found on 2 different modules.
I learned something while digging around on the web about changing brakes on an ABS system. The instruction was to never just squeeze the caliper back into the holder without releasing the bleed valve. The reason is, if there is debris, without opening the bleed valve, it can push it back into the ABS module and ruin it. There is no way to clean it out. I have done this for years and never thought twice about squeezing the piston back where it came from.