Announcement

Collapse
No announcement yet.

Cruise control disengaging above 65/70 mph

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • Cruise control disengaging above 65/70 mph

    Hi, I have a Roadie Finale with standard 17" wheels and stock tyre sizes.

    I've noticed that
    when the cruise control is activated above 65 or 75 mph, it will engage as expected but automatically disengage once the ECU requires any throttle. i.e. engage cruise above 65 mph, watch boost dial head towards 0 bar, and feel cruise immediately disengage. This problem doesn't occur below 65 mph, and the other features of the stalk remain fully functional at all speeds including the speed limiter function.

    Diagnostic steps followed by my smart specialist, amongst others, have included:

    - Re-code the cruise control
    - R
    eplace reluctors with smartmods reluctors
    - Replace driveshafts with OEM driveshafts and reluctors
    - Re-code the car for 17" wheels
    - Take a live readout whilst driving, which indicated an approx. 3mph difference between front and rear wheels at speeds of 70/80 mph (left/right readings were consistent)
    - Check wheel sensors (all in good condition)
    - No errors logged, although as Evilution noted, errors will only be logged if the car believes there is an error (values outside of the parameters)

    No other problems with the car or odd running; it's only on 22k miles and has had a fair number of sensors and parts changed on a precautionary basis in the year that I've had it. The next step will be to try running the car on a smaller set of standard Roadie wheels, but even if this works, I'd still like a more permanent fix.. any other suggestions?

    Thanks!


  • #2
    If you're running the standard size tyres, there should only be a 1% difference between front & rear, (which equals 1mph @ 100mph).
    Going by this website (http://www.helpfindmea.co.uk/cars/sp...ons-34434.aspx) the sizes are,
    Rear = 225/35R17
    Front = 205/40R17

    This website (http://www.carbibles.com/tyre_bible_pg4.html) has a handy calculator about 2/3 down the page.

    Obviously the error between front & rear will increase with speed, so maybe 3mph is too high a differential for the system.

    Hope this might help.

    Comment


    • #3
      I think the problem is with boosting
      The CC reacts on info, speed increase , breaking, boost
      Maybe check the cycle valve , vacum hoses ,ect

      Comment


      • #4
        I personally thought cruise control operated solely on data from the ESP/ABS systems - i.e. wheel rotation speed.
        I have had the same happen on 2 of our smarts:
        - first was on the Roadster and was caused by knackered reluctor rings < clearly not your problem as you have well & truly replaced them.
        - second (current) one happens on our ForTwo with the winter wheels/tyres fitted. Doesn't happen with standard summer wheels/tyres but every now and then it will disengage with the winter set on.

        I would personally say it's a wheel speed issue - either a faulty ABS sensor, or wrong size / misshapen tyre or misshapen wheel.

        Comment


        • #5
          Thanks folks, it could be an extremely marginal thing; the boost side was checked a year back and no issues.. holds max 1.4 bar without issue but CC seems to disengage due to marginal speed gain rather than light throttle. Maybe I should try it downhill.. Will also try different wheels and report back.. Just wondering, do winter wheels prevent your CC working at all, or just above certain speeds?

          Comment


          • #6
            The winter wheels on our 450 don't prevent the CC from working at any speed - I can set it at 100mph and it will be fine. But every now and then (an intermittent thing) it will just disengage without warning. You can reset it and then it will carry on.

            I put it down to them being a different rolling radius to the normal summer tyres the car runs - as they are significantly different sizes.

            The winter wheels/tyres are: 175/55/15 (R) - 145/55/15 (F)
            The summer wheels/tyres are: 205/45/16 (R) - 175/50/16 (F)

            Comment


            • #7
              it happened to me, finally the problem was the brake sensor at pedal. it didnīt work fine and sometimes send a brake signal disabling control cruise.

              Comment

              Ad Widget

              Collapse
              Working...
              X