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My mom was driving a 2001 E320 with a passenger, and had parked on a slight upward incline. She stepped out of the vehicle to speak to someone, and as she returned to get her purse (but before she entered the vehicle), the passenger stepped out of the vehicle. At this point, her and two other saw the vehicle start to roll backwards very slowly (which coincided with the gear selector shown to be in D after it had stopped by bumping a vehicle at a low speed at the end of the driveway).

Three things to note here:

  1. The car was parked before my mom left the vehicle (with the engine on), and the car did not move until after she had left.
  2. The passenger was sitting in the passenger’s seat, so pressing the brake and changing the gears seems highly unlikely from any angle I look at it.
  3. The car was parked on an upward incline, hence the slow roll backwards in Drive, and not an accelerated roll backwards if the car was in Reverse.

Is there anything that could have caused the Mercedes to shift from Park to Drive (seemingly on its own), without pressing the brake? I’m wondering if an intermittent failure of the shift interlock, and/or a failure/slippage of the gear shift mechanism could be at play here. I’d greatly appreciate some insight on this.

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    Welcome to Motor Vehicle Maintenance & Repair! Oct 17, 2023 at 21:26
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    If it was accidentally shifted from park to drive the shifter would have to travel through reverse and neutral to get to drive. Doesn't seem likely, but if it did, then you may have a bad brake switch. To test, start the car and check to see if the brake lights are illuminated without the brake pedal depressed. If they are off apply the brakes and let off on them several times. If the lights don't turn off every time you let off the brake pedal the switch needs adjusted or replaced.
    – Jupiter
    Oct 17, 2023 at 22:08
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    Moral of the story: always apply the parking brake when stopping. Make it muscle memory when shifting into P to apply the parking brake. It's a (poorly-enforced) legal requirement in many places, and it saves your park pawl.
    – user71659
    Oct 18, 2023 at 5:37
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    That test won't work on an E320 of that year @Jupiter. The brake switch has 2 separate sensors on it, one activates the brake lights, the other feeds braking information into the computer. The brake lights could work fine with the computer getting no data. If that was the case it would throw a traction control code.
    – GdD
    Oct 18, 2023 at 8:22
  • @GdD. Thanks for the clarification. With that model year a mistakenly assumed it would be a basic switch.
    – Jupiter
    Oct 18, 2023 at 11:12

1 Answer 1

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I can't think of anything that would cause the transmission to switch from park to drive, or between any gear selection, on its own. I won't say it's impossible, but it would be a very novel event.

It is possible that the gear shift interlock is not working, meaning you can change from park to any other mode without pressing the brake. You can test this by trying it out with the car running, just make sure to do it in a clear spot. If you can change from park to drive without pressing the brake then the most likely scenario is that the passenger accidentally shifted the car when getting out, maybe there was a bag or a strap which got tangled.

If the gear interlock is working then the most likely explanation is that your mother simply forgot to put it in park when she got out of the car. Statistically you are most likely to get in an accident close to your home or destination, many people are thinking about what they are going to do when they get there. From your description there were some distractions around so what is most likely is that it's a simple lapse of attention. I'm not dissing your mother, it could happen to anyone.

If the interlock isn't working and you can change from P to D or R without braking then check the gear lever for debris. It's been known for coins or other thin materials to slide down into the mechanism.

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    +1 for the distraction leading to forgetting to put the car in Park. I've been hit from behind at low speeds in stop-and-go traffic twice (no damage, no injuries) and in both cases, the driver got out of their car to apologize and check for damage, and their car rolled forward in Drive and hit me again! Amazing that people forget the basics when they're distracted, but it happens.
    – MTA
    Oct 18, 2023 at 13:24
  • Ouch @MTA! Talk about adding insult to injury!
    – GdD
    Oct 18, 2023 at 14:36
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    I agree, there may be a possibility of driver error, assuming a running vehicle was shifted into Park position before stepping out when it was still in D. In a moment of distraction, any driver can inadvertently exit their vehicle and forget to shift into Park position and engage the parking brake. There are plenty of truck drivers using dash cams recording angry drivers attempting to confront truckers, only to find their car still engaged in D, accelerating away with the idiot driver chasing after it. Youtube.
    – F Dryer
    Oct 18, 2023 at 14:44

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