Mechanics

AUDI A4 ABS PROBLEM

1996 Audi A4 • 130,000 miles

I recently did a 4 wheel brake job on my daughters car. I test drove and all seem to be fine, stopped good. An hour or so later when my daughter was driving the car home the ABS light and the Brake light came on. I assume the ABS is causing the brake light to come on. What could the cause be?
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Guest
February 24, 2013.



The Anti-Lock Brake Computer detected a problem, turned the system off, set a diagnostic fault code, and turned the yellow light on to tell you. Since the red warning light is on too, chances are there is no problem with the anti-lock system. The computer will turn the system off when there's a problem in the base braking system because it may not be able to function properly when there's a brake fluid problem.

First check the level of the brake fluid. You had to push the pistons into the calipers to make room for the new, thicker pads. Doing that would have pushed a lot of fluid back up into the reservoir, so if the level is low now, there's a leak. Check the parking brake to be sure it's fully released. If it is not, suspect a rear cable is rusted in the partially-applied position. Finally, if no other cause is found, did you ever push the brake pedal more than half way to the floor when working the pistons back out of the calipers to contact the pads? If you did, there's a real good chance the master cylinder was damaged. The lip seals can be torn on the crud and corrosion that build up in the bores where they don't normally travel. That will result in a low and mushy brake pedal or one that slowly sinks to the floor when you hold steady pressure on it. That internal leakage often doesn't show up for a few days after the damage occurs.

When you pump the pedal to run the pistons out of the calipers one side will always get there first and start to build hydraulic pressure before the other side does. Those unequal pressures will cause the pressure-differential valve to shift position and turn on the switch to turn on the red warning light. You may have inadvertently caused that valve to move not quite far enough to turn the light on, but it is now. On Chryslers and GMs that valve is spring-loaded to center itself when you release the pedal. On Fords it is not and they can be real frustrating to get centered again. I'm not sure if yours is spring-loaded or not, but to be safe, never push the brake pedal more than half way to the floor.

Caradiodoc
Feb 25, 2013.
It's all just guessing until the codes (if any) are read. If the scanner can't communicate with the ABS Module, that would also explain why those
lights are on.

Do not disconnect the battery to try and clear codes! : -)

If you can't find the pressure-differential valve. It's OK, doesn't have one, that's what the ABS is for.

Thomas

Tiny
Exovcds
Feb 25, 2013.
How does the driver get alerted to a loss of pressure in half of the hydraulic system?

Caradiodoc
Feb 25, 2013.
It's called pedal feel.

I was gonna say common sense. But.

Tiny
Exovcds
Feb 25, 2013.
Sorry, that doesn't fly with federal regulations. Every car and light truck sold in the U.S. Since the late '60s has been required to have a dual hydraulic braking system with a warning light to warn of a failure of one half of that system. That is the job of the pressure-differential valve and switch. I suspect your mind went to the proportioning valve. That might be not used when the car has anti-lock brakes.

The politicians know we can't be trusted to go by pedal feel or by common sense. You and I might recognize the low pedal, but I can share at least a dozen instances where drivers had to be told and convinced they had a problem. In one case a relative with a horrendous grinding from the brakes on her older Ford said the problem went away and I didn't need to fix it. Of course it did. The pad ground through the rotor and the piston fell out of the caliper, so there was no more noise. She noticed the red warning light on the dash but didn't know what it meant. Do you want to rely on common sense and pedal feel with millions of drivers like her on the road?

Caradiodoc
Feb 25, 2013.
"You and I might recognize the low pedal, but I can share at least a dozen instances where drivers had to be told and convinced they had a problem."

That's why I didn't say "common sense". North American Drivers just don't have it. They need driver training classes like they have in Germany.

It was late last night, maybe I am mistaken. Don't have time to look at a component / wiring diagram. Have to get to work.

Still doubt that is the problem. Loss of ABS Module Communication is common for that vehicle.

Tiny
Exovcds
Feb 25, 2013.
It goes way beyond lack of driver training. Some of us don't know to stop the engine when the oil light comes on. Don't know what the Check Engine light is for. Need I go on?

Caradiodoc
Feb 26, 2013.
Any news about ABS Module Communication?

Caradiodoc,

'96 Jetta non-ABS on the hoist today. Low brake pedal and no warning light on in dash.

No pressure differential switch anywhere in sight ( at master cylinder or elsewhere ). Level sensor tested faulty, so that
is why the brake warning light is not on with low level.

I know you have been in the business longer than me. But just like the battery issue, you have to sometimes go with the
flow and not preach what was once taught or read. Especially when it applies to domestic cars, since it does not always
apply to Imports.

Also,

I changed the DSG Tranny fluid on a 2010 Jetta yesterday. Disconnected and removed battery to get at DSG Filter.
Didn't use a memory saver. None of the systems were affected and had no issue starting the engine after.

Anyways.

I'm quite sure that the above Audi has a Module issue. At least until we get a reply back from the owner and hear
otherwise! : -)

Thomas

Tiny
Exovcds
Mar 1, 2013.
Believe it or not, I am very happy to hear that everything worked. My information comes from an instructor who owns a specialty shop catering to other shops with cars they can't figure out. He incorporates his findings into his class presentations. My community college gave him free use of a large classroom in exchange for allowing me and the other instructor to sit in. The problems he always referred to from disconnecting the battery were on Volkswagens, BMWs, and GM products. I've talked with many GM owners and many mechanics, some of them former students, who all say the same things about GM computers and the need to buy new ones from the dealer. What I have NOT heard from GM owners is that they had to tow their cars to the dealer to have computers unlocked. I don't recall ever hearing about having to do that with any other car brand either. What I DO read about every day is someone replaced the battery and now things don't work. Usually on GMs it seems to be the door locks or power windows. Unfortunately I never get to read about what eventually solved those problems.

One former student works for the local VW dealer who paid the $1200.00 for a year for him to attend those high-level classes, and he nodded in agreement about some of the instructor's VW comments, but I can't remember the specifics. I'm just pleased to know things aren't as bad for owners as I've been led to believe.

Caradiodoc
Mar 1, 2013.
I listed 2 examples that prove the opposite of what you have been taught / have heard (same story by the way that you post each time).

Do your customers a favor and post only first hand experience. Uh, kind of like me! : -) If you have never worked on a VW or Audi, don't post.
You don't see me posting in the GM, Chrysler, Ford etc. Forums!

Sure Dealer Techs get all be best training, as they should! Personally, I have yet to run into anything that is worthy of "OMG, better tow it to the Dealer".

Don't scare your customers. They will take it / tow it to a Garage on their own, once they realize that whatever they are trying to fix is past their comfort zone.

As soon as I can confirm that "a computer reset is involved to get a car running again", I'll be the next in line to preach your gospel. In the mean
time, why scare your customers. Look at a wiring diagram, find the real problem and then post the fix, instead of passing the buck to make a quick buck.

Tiny
Exovcds
Mar 1, 2013.
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