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Hopefully someone can point me in the correct direction, or at least give me an idea about this before taking it to the shop.

The other night I was driving around, streets were pretty snowy around here. I was pulling up to a stop sign and tried to stop, ABS kicked in and I eventually stopped. Seemed pretty normal given the road conditions, but perhaps it felt like the ABS came on way too early. So at the next stop sign, I went to push on the brake pedal but it pretty much went to the floor. The ABS came on again. I tried stopping a few more times and no matter what I did, the pedal went to the floor and the ABS came on every single time.

Some notes:

  • 2005 Mazda 3, 2.3L, 5 speed
  • Tried turning the car off and back on, same problem
  • No ABS, brake or engine light on the dash
  • With the car turned off, pushing on the brake pedal results in a very firm pushback. It doesn't goto the floor
  • Brake fluid reservoir is full
  • I did have the rear pads and rotors replaced about two weeks ago. Brakes have been perfectly fine from then, until last night. Brakes were also fine before I had these done and to my knowledge the shop didn't touch the fluid.

In order to get the car home, I ended up removing one of the ABS fuses. This stopped the ABS from coming on every single time I had to stop, but the brake pedal still goes to the floor.

I've tried to read about this problem, but information seems pretty spotty. I have read that some people's brakes have magically returned to 'normal' after a while...but I'm not holding my breath.

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When you say the ABS came on do you mean the ABS light lit up, or did you have a full brake pedal with a vibrating feel against your foot from the brake pedal? Please add your vehicles year and model. –  Allan Osborne Dec 1 '13 at 19:44
    
Oops, I didn't realize the car make and model wasn't in there. It is a 2005 Mazda 3. When I said the ABS came on, I mean the physical ABS system came on while braking. The ABS light at no point came on. –  canadmos Dec 1 '13 at 19:51
    
I have the exact same brake issue on Mazda 3 after ABS was activated. Brake pedal goes down twice as far as before. Can you elaborate on how this was rectified? –  user9093 Jan 14 at 2:10
    
@AaronHemetsberger - I took the car back to the shop and told them about it. They took a look and sucked some air out of the brake lines. I don't know if that was the actual cause of the ABS events, but it never happened again. –  canadmos Jan 18 at 2:54

4 Answers 4

Turns out that there was a big pocket of air in the system. Sucking it out made everything return back to normal, thankfully.

I also tried to replicate what happened in the first place by driving on some ice and getting the tires to lock up and activate the ABS. I did it a bunch of times and nothing bad happened. So hopefully it is fixed.

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The ABS system is in effect an add-on to the vehicles brake system. You found this to be the case when you removed the fuse and normal braking was present. The ABS system has a module to control the sequence of solonoid operated valves which apply and release the brakes according to the performance of the ABS wheel sensors against a performance map stored in the module. The way forward is a scan on the ABS system to find any indications of a fault. If no codes are stored but the system does not operate correctly the most probable cause is the module. A second but less likely fault is that one of the solonoid vales has become jammed open and allowing a circulaton of the pressure inside the valve body and not to the brakes. Here in the UK we have companies that overhaul and replace IC chips in ABS modules and this usually works out to be very much cheaper then a full ABS replacement unit. First Stop - System scan.

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This is an answer to a variation of the problem:

When ABS activates, the brake pedal goes to the floor but returns to normal height unless ABS is activated again.

This applies to a Saturn S-series (SC, SW, SL) and may apply to other makes and models as well.

The ABS system uses a series of solenoids and valves to redirect fluid flow. A set of valves (solenoid valves/isolation valves) are responsible for isolating the brake pedal from the rest of the brake system during an ABS event. If the ABS unit activates but does NOT isolate the brake pedal, the brake fluid will flow directly into the the accumulator, a spring-loaded brake fluid container for ABS systems. This lets the pedal sink to the floor.

If the brake pedal sinks to the floor only when the ABS unit activates, then your isolation valves may be the problem.

  • You may need to replace the isolation valves (which, on my model, can be removed by removing two torx screws and pulling out the valve, which can be done in about 30 seconds and are easily accessible on the top of the ABS module)
  • It is possible that the wiring between the isolation valve and the ABS controller is faulty.
  • It is possible that the ABS controller itself is faulty.
  • Finally, it may be that the o-rings that isolate the bottom inlet and the top outlet ports on the isolation valve have stretched or deteriorated. This would allow fluid to flow past the valve's body regardless of its open or closed state. Simply replacing these o-rings could solve the problem.

In my case, two new o-rings completely fixed the problem. This, after replacing the solenoid isolation valves and the ABS control unit, which turned out not to be faulty after all. (Note: the ABS system showed no errors or lights. The sinking pedal was the only symptom)

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There is a fault somewhere in the sensors. ABS uses Wheel Speed Sensors to determine when the brakes are at chance for locking up by monitoring the wheel speed. If one of those sensors becomes damaged or malfunctions it may cause the ABS to come on prematurely or even all the time. If no OBD 2 codes exists, physically pull them to look at them. Big, grey plastic with an elbow to it. One may be going bad, which if they have never been replaced is a good guess. ABS diagnostics systems do not usually monitor these for issues, so it could be bad and it would never know. So I would check those :) Most other issues would illuminate the bulb.

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A bad wheel speed sensor would throw a code and trip the ABS light for sure. –  mac Dec 4 '13 at 18:43

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