ok so i took my a4 to a axle shop yesterday to have a the front axles replaced. when we got the car back the axles where fine but when i started to drive away from the shop my abs light turned on. then when i was about to come to a stop my brakes started to act funny. when i hit the brake pedal it gets super firm then it does a shaking type thing, then out of no where the brake pedal gets soft or loose. the messed up thing is they didnt call me to pick up the car in tell it was closing time. so by the time i made it back to the shop, the shop was already closed. so i plan on taking it back asap today.

anybody know what they may have done wrong or why its doing it?
other then it being my fault for not doing the axles myself like i should have done! lol
the shop has done other cars of ares and never had a problem

 

The half-shafts have a tone ring (Knotched ring that the wheelspeed sensor reads). Probably not installed correctly or not seated properly.

 

Check the ABS sensor on that front hub, it is most likely pushed out or they damaged it when they installed the new axle. The brakes will act that way because the car thinks 1 wheel is not spinning when the other 3 are.

 

Mike, he stated that the abs light is on, so the computer has already recognized a constant variance in one of the wheelspeed sensors, or complete lack of communication.

So, at this point it will disregard data as the module is in "fault" mode, I'd assume...and not be picking at his brakes too much.

But, odd that the brakes are acting up also. Perhaps you're right and the computer still attempts to use the input it gets, even though it knows it's inaccurate?

 

Odd because my ABS light was on when my front ABS sensor was bad, it read nothing when we looked at it with the vagcom. At that point my car would pull hard to one side when I went to use the brakes and this even caused me to snap the rear output shaft in the transmission when I went to launch the car at the drag strip. So the system is still being used even with the light on. He can always just unplug the ABS harness plug to allow the brakes to work like normal, only thing is the ABS wont kick in.

When I unplugged my ABS harness the car stopped pulling to one side and it went straight when I launched the car hard at the track. Since my car is only used at the drag strip I ended up just leaving the ABS harness unplugged instead of replacing the bad sensor.

 

That is pretty odd. Right now my lights on, getting 0 readout from the left rear, but no abnormal braking. Outside of lack of abs, hrmph.


Trust your experience though, going to definitely unplug my harness to avoid any damage until I replace the half-shaft. Thanks Mike!

 

just like everybody says it's the tone ring. i have the same problem in the rear with the same symptoms. some people say you can get the tone ring separately but i've yet to see where to get it. it could be a bad tone ring or it could be dirty or rusty. most likely dirty in your case.