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The patient is my 2001 Audi S8 D2 facelift. Car is UK spec, auto transmission. As far as I know, it’s identical to US spec, but right hand drive, so some components are moved around to suit.
It has the Brembo 4 pot brakes on the front and regular floating calipers on the rear.
EBC redstuff pads and Brembo (plain) discs all round which are about 2 months old (so is the brake fluid).
The brakes are strong, but under medium / hard braking I can feel a rapid pulsing in the brake pedal. It feels very similar to what ABS feels like when activated.
I thought it was the ABS triggering and re-seated the sensors at the front as I’d recently done some work in that area. I broke one of the sensors as it was corroded into the aluminium housing, so I’ve driven the car without ABS for the last week waiting for a new sensor which I fitted today.
Thing that confuses me is the pulsing pedal thing did not change throughout. It still happened with the broken sensor and disabled ABS.
It doesn’t feel like a disc / hub out of true (that I have ever felt) as the pulsating seems too rapid compared to the road speed.
I think I might pull the discs and pads tomorrow, clean everything and re-assemble (maybe swap the pads side to side) anyway, but any other ideas for similar symptoms?
Thanks,
Jim.
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Aside from brakes and hub bearings, there’s really nothing that you would feel in the brake pedal itself.
It’s possible that the rotors weren’t machined properly at the factory and you have excessive “runout”.
You can get a runout gauge to test for rotor runoutIf you didn’t scrape clean the surface behind the rotor on the hub face , that can cause pulsation.
Overtorquing the lug nuts can also be a cause.
You might want to double check that everything has been tightened enough, the caliper pins are clean and lubricated and all hardware is present.If you had to panic brake, there may have been enough brake material deposited on a rotor to cause the pedal pulsation.
Likewise, a sticking emergency brake may have over heated the rear brakes and is causing the pulsation.Worn front end components are more typically felt in the steering.
Thanks for your reply.
Had a bit more of a play with it today.
Thing is, when the pulsation happens it’s not really a wub-wub-wub once per wheel revolution thing like a hub or rotor out of true, but more a brrrrrrrrrr – fast pulsing like ABS.Today, I took it for a drive and could readily replicate the problem. I pulled the main electrical connection from the ABS pump under the bonnet. With that disconnected there is never any pulsing. On hard braking from 40mph I could lock it up, and interrestingly the back of the car wanted to overtake the front and got a bit sideways. Maybe it depends on the ABS being connected to do the brake front/rear proportioning properly, or maybe I just have a big V8 over the front wheels and the rears wanted to lock first?
I put the ABS pump connector back and the issue returned. I carried on messing around with test braking for about an hour and I think the problem has improved and may be gone (more general driving required to test fully).
As the car has an auto transmission, you can end up braking down from motorway speed to a set of red traffic lights, then hold the car on the brakes until the light goes green.
My current theory is I may have done this on new rotors & pads and deposited pad material onto the rotor in one spot, creating a very grabby part of the rotor, enough to mildly trigger the ABS – my constant braking today has created a more even layer of pad material on the rotor and improved things.I had previously ruled out the ABS as the problem still occurred when I had a missing wheel speed sensor (ABS disabled), but it seems like the system still tries to operate best it can on 3 sensors because completely disconnecting the ABS pump stopped it.
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