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Brake Pedal “Surge” - 1991 325ix

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    Brake Pedal “Surge” - 1991 325ix

    After driving for 20 minutes or so, the brake pedal will “surge” and or “pulse” only when the vehicle is at a complete stop. It sounds and feels like the ABS is activated. It does not matter how hard the pedal is pressed. Standing over the open hood, it sounds like the ABS “pump” is activated (front driver side of engine bay).

    The ABS warning light is not activated on the dash. The brake warning light is not on.

    I took the car into local independent BMW specialist. He indicated he could not plug into the ABS to pull codes. He noted the front ABS sensors were making contact with the front tires. He thought this could be the cause of the issue, but strongly cautioned he was guessing. I was quoted $588 to replace both sensors, which seemed high for a guess.

    (1) any thoughts on the exact nature of the issue? Any other diagnostics I can do to help narrow it down?

    (2) how difficult is it to replace the ABS sensors on the front wheels? The Bentley is strangely silent when it comes to ABS.
    1991 325ix Coupe 5 Speed
    1990 325ix Sedan Auto
    2001 M3 Convertible 6 Speed
    +2002 M3 Convertible SMG (RIP)
    1972 Ford Bronco

    Instagram: @steine20

    #2
    Originally posted by Steiner View Post
    He noted the front ABS sensors were making contact with the front tires.
    This needs clarification.

    should be easy to see if it is doing this or not yourself.

    Comment


      #3
      Sounds like crap, it's possible for the wires to touch the tires if they're unplugged and swinging around but the sensors are mounted to the spindle. The parts are probably $150/each or less from a wholesaler and each sensor is held in by one internal hex head bolt and some corrosion. Labor would take what? an hour, tops? in a shop where all of the tools to do the job efficiently will be on hand. On the IX the ABS sensors are mounted opposite the brake caliper on the spindle and look pretty easy to get to. Pull the sensors and clean them off, they might have rust pieces stuck to them, the pulse wheels could be similarly dirty.

      IG @turbovarg
      '91 318is, M20 turbo
      [CoTM: 4-18]
      '94 525iT slicktop, M50B30 + S362SX-E, 600WHP DD or bust
      - updated 1-26

      Comment


        #4
        UPDATE: The pedal surge has been progressively getting worse. When the car is stopped and I have the brake pedal even slightly depressed, the headlights and dash lights will flicker. If I take my foot off the brake, no electrical issues. After driving for a little bit, the brake check light will come on.

        So.... last night I dug into it a little bit. I removed the front ABS sensors and cleaned them both. I also rotated the spin wheels - the both look good and true. I sprayed some Brakleen down in there (not sure if I was supposed to do that).

        I also had a voltage regulator hanging around, so installed that on the alternator (I wasn't sure when the current one had been installed). Checked the battery date (4/18).

        The car started up but still had the headlight flicker at idle with the brake pedal pressed. I drove it for about 10 minutes and the issues seemed to disappear. Towards the end of my drive, the brake pedal started surging again, the brake light check came on, and the lights continued to flicker at idle with the brakes applied.

        After getting home, I removed the tail light inserts and cleaned the connections. I left them off, started the car, and applied the brakes. Was still having the same issues at idle.

        Sounds like I might be chasing an electrical gremlin now. Anyone have any thoughts on any of this?
        1991 325ix Coupe 5 Speed
        1990 325ix Sedan Auto
        2001 M3 Convertible 6 Speed
        +2002 M3 Convertible SMG (RIP)
        1972 Ford Bronco

        Instagram: @steine20

        Comment


          #5
          Originally posted by e30davie View Post
          This needs clarification.

          should be easy to see if it is doing this or not yourself.
          I looked at the front sensors - I did not see any tire or wheel contact.
          1991 325ix Coupe 5 Speed
          1990 325ix Sedan Auto
          2001 M3 Convertible 6 Speed
          +2002 M3 Convertible SMG (RIP)
          1972 Ford Bronco

          Instagram: @steine20

          Comment


            #6
            the sensors don't do anything if the car isn't moving, cleaning them won't do anything.

            swap out your ABS relay.
            cars beep boop

            Comment


              #7
              FYI the relay on the '91 is on the ABS pump itself.
              Build thread

              Bimmerlabs

              Comment


                #8
                Originally posted by nando View Post
                FYI the relay on the '91 is on the ABS pump itself.
                Is the relay on the pump the same kind that would otherwise fit under the dash? Also, if I remove the existing relay and the pedal no longer pulses would that be sufficient to isolate the problem?
                Last edited by Steiner; 01-17-2019, 01:31 PM.
                1991 325ix Coupe 5 Speed
                1990 325ix Sedan Auto
                2001 M3 Convertible 6 Speed
                +2002 M3 Convertible SMG (RIP)
                1972 Ford Bronco

                Instagram: @steine20

                Comment


                  #9
                  yeah, I don't think it's exactly the same but it does the same thing.
                  Build thread

                  Bimmerlabs

                  Comment


                    #10
                    There's a little flat black box between the strut tower and the fuse box. Try unplugging it.
                    AWD > RWD

                    Comment


                      #11
                      ^ The acceleration switch.. yeah good idea. Not sure how it can fail, unless the mercury leaks out or something, but I've heard of that before.
                      Build thread

                      Bimmerlabs

                      Comment


                        #12
                        I would try removing the ABS relay to see if the electrical gremlin disappears.

                        Comment

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