A3 ABS - Nasty problem, cheap fix.

[DavidTdi]

Something that happened to daughters A3 Tdi brakes this week might help someone who searches on a similar problem.

She reported having to do a hard stop in town on a greasy road and after the ABS had cut in and she was almost stopped the ABS failed and dash lights came on for main warning, ABS, handbrake and traction/stability control. She pulled over when safe and on an engine stop/restart the lights went out and the brakes felt normal. So she brought it here yesterday for a look over.

Taking it out on a wet road to test every time I braked enough to bring the ABS on it only worked for a few seconds then the braking/stability lights all came on and it slewed to a halt without ABS. And every time an engine switch off and restart put the lights out. Braking felt totally normal until the next attempt at an ABS stop when the cycle was repeated.

My Foxwell VW/Audi specific code reader initially didn't show any codes even after a second test trip and rescan. What I then thought was bringing it home without doing the light clearing restart might show codes and it did on a rescan... see image below at the bottom with codes 1130 (Sporadic implausible signal) & 1276 (Hydraulic pump sporadic defective). 

Searching Google hinted at a new ABS pump or controller and I rather wished I had the full VCDS to get more detail.... until I dropped across this guy on Youtube...

 

 

I followed his advice and replaced all the fuses related to the ABS system as he shows at 2min17 and 3min51 plus tightened all the heavy duty terminal nuts as seen at 2min40. My fuses didn't look bad by normal standards but had the same dark marks he shows where they fitted into their contacts. A restart cleared the lights and off to test... damn just the same fault. Resigned myself to putting it into a local specialist and bracing for £600 or more for a new pump.

Then I had a thought that although the dash lights were out it might be holding a troublesome code and sure enough a scan found just the 1130 code remained. Deleted that... restarted no dash warning lights and through several braking cycles full ABS with no faulting and no warning lights.

A right result.

I'm going to pull a theory out of the air that.... Under normal "resting" circumstances the system saw everything as OK. However when the ABS was triggered the extra current draw of the pump could not be met through the imperfect fuse contacts and so forced the system into fault mode.

As an aside the car's owner, my daughter, has never driven a car without ABS and I pondered just how she'd have reacted in a higher speed serious situation with the car slewing all over the place. Similarly with stability and traction control locked out. Don't know if anyone has driven their modern car to the limits of these systems in a safe test scenario but they are a staggering addition to safety compared to "the old days"!

 

[cliffcoggin]

Congratulations on resolving a difficult problem. Would the pump really draw extra current during ABS operation? I assumed those motors were either on or off, and therefore drew a constant current when running.

I guess there is no need to learn cascade braking nowadays with ABS so widespread. I doubt I would be alert enough to use the technique after such a long time.

[DavidTdi]

As far as I understand the pump and solenoids are static until the ABS ECU decides it needs to intervene then they get working frantically. There are two fuse sizes for the ABS... a 10A and  and two 30A under the bonnet. 

I couldn't find the perfect system demo but this shows it from 13min 5secs on...

 

Great stripdown of the pump/solenoid unit here...

 

 

[cliffcoggin]

Thanks for that David, which has taught me something about ABS operation.

Given that your problem only occurred during extreme braking when the ABS became active, I reckon it was probably the current drawn by the solenoid valves that tripped the system, rather than the pump which runs during normal, non ABS braking to maintain servo pressure.