Advice needed - drive-by-wire throttle behaving strangely

Discussion in 'Mercedes-Benz' started by smart-za, Jul 27, 2010.

  1. smart-za

    smart-za Well-Known Member

    Hi everyone,

    I need some advice from someone who knows about ECUs - in short, my smart has started lifting off the throttle under acceleration, with no foot movement from me. This is an intermittent problem - it occurred on Friday morning and again this morning on my way to work.

    Some background:
    The smart has a drive-by-wire throttle, as I understand it. There is no mechanical link between the accelerator pedal and the throttle.
    It's also got a manumatic gearbox. In other words, the gearbox innards are mechanical, but it is controlled by the computer. There's no clutch pedal or mechanical gear lever.
    For the last year or so, I've been trying to shift up early to keep the revs down. This has helped my FE.

    So, normal behaviour for the smart in auto mode is as follows: the car pulls away quite slowly while the turbo winds up. The turbo kicks in, and if I don't back off on the accelerator pedal, the car leaps forward and then grabs the next gear. There is a pause while the (slow) gearbox finds its feet, and then the story repeats: slow for a second, lunge forward, change gear, repeat.

    This morning, the behaviour was more like: pull away slowly, turbo kicks in and the car starts to accelerate more strongly, then the torque drops off while the revs continue to rise, and the car refuses to change gear until I press harder on the accelerator and the engine is screaming. It feels like it needs a lot more accelerator input for the same amount of throttle opening.

    My question is this: what sensor or gizmo might be causing this problem? I'm having the car serviced next month, and I need to know what to tell the workshop to look for. Because the problem is intermittent, I can't be sure that they'll be able to confirm it for themselves.

    Thanks in advance for any assistance.

  2. smart-za

    smart-za Well-Known Member

    I think the cause of the problem is a faulty MAP sensor - the problem has been rumbling away in the background, but today I encountered an especially extreme version of it, and it gave me more clues to research.

    While climbing a long hill on the highway today, the smart had the problem described above (less torque for the same amount of accelerator input). I pressed harder to compensate, and then the same thing happened again but worse. It felt like the turbocharger had failed, and I was now driving a 600cc normally aspirated car (way slow).

    After some research on other smart sites, I now think that the car had gone into safe mode, and was holding open its wastegate to prevent the turbo from spooling up (thus limiting the strain on the motor). This seems to be caused by the MAP sensor over-reading.

    I disconnected the battery for a few minutes to get the car out of safe mode, and it goes like normal again (except that the radio is locked and I don't know the code).

    So it'll be off to the dealer next week for a new MAP sensor and possibly resetting of the radio.

    Hope this fixes the problem.

  3. Blackbelt

    Blackbelt Well-Known Member

    Sounds like you may have discovered the issue. While at the dealer, ask them to make sure you have the latest tranny management software. They can reflash it for you in a few minutes. smart is constantly upgrading their software to improve shifting, at least on the 451's.

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