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ABS Problems! Now with more scope!

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    ABS Problems! Now with more scope!

    Hey Guys,

    The ABS light in my E0 is coming on once I start moving. This tells me I have a bad wheel sensor. I tried the trick where I would unplug all but one sensor to identify the problem, but that did not work. The light would always turn on unless I left all 4 sensors unplugged. Instead of spending ~$500 dollars replacing all the sensors I wanted to track down the problem.

    With the help of a friend at an awesome shop, we scoped the wheel sensors and saw this:

    Just as an FYI wire colors correspond with graph colors (The only exception being BLUE = WHITE.

    According to the ETM that means:

    White = RH rear wheel sensor
    Red = LH front wheel sensor
    Green = LH rear wheel sensor
    Yellow = RH front wheel sensor

    Note: All these captures are taken with the sensors plugged into the computer.

    Low Speed.


    Higher Speed.


    After looking over these captures, we determined that the green signal (LH REAR) was much weaker and potentially causing the problem. I swapped out the sensor with a known good used one. I also swapped the half shaft incase the ring was the problem.

    This did not cure the problem so it was back to the shop to have it scoped again.

    Lower Speed.



    Higher Speed.


    This time we noticed that the red and green signal was acting almost like a wave. I am stumped at this point and I would really like to get ABS working.

    Here is what I have done:
    Replaced front wheel bearings (New ABS Rings)
    Checked the wiring to each sensor with an ohm meter (Everything checks out)
    Swapped rear sensors side to side.
    Tried a different ABS computer.

    I also have scopes with the computer unplugged. (Sensor grounded with the scope) I can get more detailed screen shots or other info if you need it.

    Thanks for the assistance!
    Last edited by TheCarousel; 08-18-2015, 10:53 AM.

    #2
    Doh!! I post the same image a few times. I am going to edit the post and provide smaller images.

    Comment


      #3
      I'm not quite sure what to make of that, nor do I know what the signals should look like. It might be illuminating to scope the signals on a car with a working ABS.

      Your experimentation tends to suggest that some signal does reach the ABS module, but it may not be what the ABS module wants to see. I'm with you on not wanting to just buy all new sensors, but at this point swapping in a known good set or replacing them might be the best action.
      The car makes it possible, but the driver makes it happen.
      Jim Levie, Huntsville, AL

      Comment


        #4
        I was thinking the same thing, but I am worried the sensors might not be the problem.


        Originally posted by jlevie View Post
        I'm not quite sure what to make of that, nor do I know what the signals should look like. It might be illuminating to scope the signals on a car with a working ABS.

        Your experimentation tends to suggest that some signal does reach the ABS module, but it may not be what the ABS module wants to see. I'm with you on not wanting to just buy all new sensors, but at this point swapping in a known good set or replacing them might be the best action.

        Comment


          #5
          ...it kind of looks to me like something is humming- there's a lower- frequency wave riding
          on the HF output of the sensors...

          How are all your grounds?

          t
          now, sometimes I just mess with people. It's more entertaining that way. george graves

          Comment


            #6
            I checked them. They all appear to be good. No resistance to ground.

            Originally posted by TobyB View Post
            ...it kind of looks to me like something is humming- there's a lower- frequency wave riding
            on the HF output of the sensors...

            How are all your grounds?

            t

            Comment


              #7
              I replaced the ABS sensors and it did not fix my problem. I have not had an opportunity to scope them yet.

              Grounds also appear to be okay. Any other ideas?

              Comment


                #8
                From what I see, your frequency on the green signal is about 70Hz while the others are 85 Hz, in addition your amplitude is also lower than the rest. I would definitely check the wiring from that sensor to the control unit, you could have a problem in the wiring itself.
                If there is a way you could bypass the wire from the sensor to the ECU just to check if it fixes your signal. If that does it then your wire is damaged somewhere for sure.

                Comment

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