Q5 ABS and ESC Fault - Problem traced but now jammed on a solution!

[FlyingCowboy]

I've hit a complete dead end here - and hoping someone has either gone this route before, or knows how to go this route.

I parked my car at Heathrow Airport long stay car park, went away for 3 days, and when I arrived back on Sunday midday and climbed into my car I was met with

"Stabilisation program and ABS fault - See owners Manual" followed by "Parking Brake malfunction"

The vehicle was perfectly fine 3 days earlier.

I pulled out my ODBII dongle from glove box, plugged it in and ran the codes quick to find it showed ABS electrical fault - front right wheel, and Speed Sensor Error, signal implausible.

First thought was oh ok, the wheel speed sensor has gone duff.

So I ordered a new one and replaced it a few days later, but the fault still exists, with the same error codes.

It was suggested that maybe the battery was reaching end of life and low voltage was causing gremlins, so had the battery tested and although it was ok, it was low on capacity so I replaced it anyway. This of course did nothing for the issue.

Then the hunt began. I swapped sensors around between left rear and front right, still same fault.

it was suggested that the bearing was the issue, but this has recently been replaced on that side, so I had my reservations, especially since the fault states electrical error in circuit - front right wheel. And even if you reset the faults it instantly returns. This doesnt add up to a wheel bearing issue in my head, I would imagine that it would reset and then flag again once vehicle is rolling and not detecting a wheel speed.

Anyway, I went on the mission of metering out each cable, from the wheels to the ABS control module plug. I got hold of the wiring diagram and sat there measuring the sensor resistance first at the ABS plug and then removing the sensor and buzzing out each wire from ABS plus to wheel sensor plug.

I got 5.5 meg Ohms resistance on all of the wheels except front right which showed open circuit....

I got clean buzzes on front left wheel (by measuring pins 22 and 34 on abs plug and inserting a short on the speed sensor plug)
I got clean buzzes on left rear wheel (by measuring pins 20 and 33 on abs plug and inserting a short on the speed sensor plug)
I got clean buzzes on right rear wheel (by measuring between pins 19 and 31 on abs plug and inserting a short on speed sensor plug)
I got open circuits on both wires on front right wheel (by initially measuring between pins 18 and 32 on abs plug with a short on ABS plug, then by measuring directly between pin 18 and pin1 then pin2 and then measuring directly between pin 32 and pin 1 and then pin 2)

This has me completely flumuxed! As I can't understand how both wires from the Wheel Speed Sensor to the ABS control module would be broken all of a sudden.

I removed the wheel arch liner and traced the wires back until they entered the body work through a grommet and they all looked intact, no signs of abrasion or chewing.
Measuring the wheel speed sensor plug to the wheel arch by piercing the insulation with a pin reveals continuity to that point on both wires. But measuring from that point to the plug reveals open circuit. So somewhere between the wheel arch and the plug the cable seems to have been damage miraculously while vehicle was parked!

Anyone seen this before?
Got any pointers?

Any more so, does anyone know where exactly the wiring harness for the wheel speed sensor runs? Because if i can trace it back to the break I can fix it!

[Smac770]

That it was just sitting there for days in public like London and "went bad", I'd expect chewed wiring.

https://audi.7zap.com/en/usa/audi+q5...id-971-971081/

So that's a basis to look at. Audi stopped correctly documenting the wiring harnesses with B8.5, so you can't see the same for a B8.5. 12-16 are the plugs at the front wheel wells, depending on equipment. But these are on a separate harness with the wheel speed sensor, which is then spliced to that harness you see in the diagram.

In the diagram, [50] and [51] are where the harness passes through to the interior harness. The left and center connect to the interior left, the right connects to the interior right. They are bridged by the center tunnel harness.

The left and right [50] and [51] are behind the fender. Such as seen here: youtu.be/C_-MfYokbMc?t=4 You can see the large grommet at [51] in the chassis at the left. The wiring diagrams show it as a straight shot from the 2-pin at the sensor to the 38-pin at the control module. So there will be splices, but no connectors you can test at along the path.


https://audi.7zap.com/en/usa/audi+q5...id-927-927000/
So, front right wheel well harness, item (7), is going to have either just the 2-pin to the wheel speed sensor or also the 4-pin to the level sensor (if 1BL adaptive)

https://audi.7zap.com/en/usa/audi+q5...id-971-971081/
That will splice to engine bay harness right side around where the plugs (12)-(16) are diagrammed, and pass through the grommet behind the fender to the interior harness right at [51]

https://audi.7zap.com/en/usa/audi+q5...id-971-971083/
In the interior right diagram, the engine bay right side comes in at [110], and the interior right passes through to the center tunnel harness at [111]

https://audi.7zap.com/en/usa/audi+q5...id-971-971084/
The center tunnel harness is the upper harness in the diagram, bridging the interior right at [56] to the interior left at [55]. The lower harness is the roof overhead harness, also connected to interior left [55].

https://audi.7zap.com/en/usa/audi+q5...id-971-971082/
And then the interior left, with connections to the engine bay left (with the ABS/ESP connector) and engine bay center, both denoted [95]. And connections to the interior center tunnel and interior overhead roof, both denoted [96].

I suspect these harnesses are not actually separate harnesses spliced together, but just broken up for the parts catalog diagrams. But as you see, it's a long road from the front right wheel speed sensor 2-pin around to the ABS/ESB 38-pin.

[FlyingCowboy]

Awesome thanks for all of this.

And because my vehicle is right hand drive that is all mirrored!! The wiring is switched left to right (why? I have no idea) but the fuse boxes and on opposite side with a left hand drive and a right hand drive and so the wiring seems to be as well.

My right front wheel is a 2 pin plug, the left wheel has the 2 plugs on the 4 core wire.

I will start the tracing process... although I am at the point of paying an auto-electrician to sort this out since I have pretty much isolated the fault!

[Smac770]

Well, some of it is mirrored. But the J104 ABS/ESP unit is on the left side of the engine bay either way. And the front brake pad sensor is on the front left either way. Etc. But the ECM box is on the driver's side, but that's also part of the engine harness, not the engine bay interior harness. So the left and right in the 971081 diagram are the same for LHD or RHD, is what I would expect.

Yeah, another guy, we sorted out it was the LIN bus line from the J393 in the right rear to the alarm in the plenum chamber. He had the run along the whole left side out trying to find the break point. He did eventually just run a new wire and things were good.

[FlyingCowboy]

I'm honestly contemplating doing exactly that.

Just splicing into the cable at the ABS/ASC unit in the engine bay and running a new pair of wires to the front the wheel harness and ignoring the rest of the wiring inside the car.

[Smac770]

It's almost certainly what I would do. Probably work the wire path up as far as you can access with just the wheel well liner out, then splice there to the existing wires that run to the sensor connector. And run that over to the ABS connector. To implement routing back through the interior of the car would just be crazy amounts of work for the return.