Anatomy of a mirror
In physics, impulse is force times time. If that impulse were irresistible, then momentum has been transferred and we have some work to do. In criminal law irresistible impulse is a defense for a criminal act known to be wrong but which, through temporarily diminished capacity, would have been committed even with a policeman at the perpetrator’s elbow.
Physics and law collided at the exact spot of the left-hand side view mirror of my Ford Transit Connect van sometime on Wednesday night. An unknown driver transferred some momentum from their vehicle into the mirror of my van. The net change in momentum is the impulse the mirror resisted only briefly before its cover was ripped off and it lay limp. That the unknown vehicle transferred only a portion of its momentum and then kept going without notifying me or the police is the point at which a crash became a crime.
When does a crime become a mystery? Is it enough that I don’t know who hit the van? No. That’s barely a curiosity. Nobody really cares. Parking a car costs money. Pay for a protected space upfront or park on the street and pay as you go in bodywork. The mystery is that the mirrors were sticking out far enough to get hit in the first place.
You see, the Ford folds the mirrors whenever the vehicle is locked. For the last year or so, the car has been occasionally unlocking itself and unfolding the mirrors. These are the irresistible impulses that are the real mystery.
The most plausible explanation, or at least the most ready, is that the unlock button is being pressed on the factory key/fob while in a pocket. This is such a common thing that many cars automatically re-lock doors if a door is not opened within 30 seconds of being unlocked by fob. The next most likely scenario is that a defect in one of the fobs presses the button. Moving down the list, there is a possibility that the car is spontanelously unlocking based on a signal from a defective unlock button inside the car. Go further and we get into malign events which are even less likely. The car itself has no memory of these events. None, anyway, that it wishes to share with me at present.
Let see what we can learn. The first quick hypothesis: an unlock event initiated from within the car by the unlock button can be distinguished from an unlock event triggered by the ’unlock’ button in the car. Setup: I lowered the windows, stepped outside the car, placed my hand through the open window on the ‘unlock’ button. I locked the car with the fob. Car locks. Mirrors fold. I unlock with the button on the inside. Car unlocks, mirrors do not unfold. unlocking with the fob is not the same as unlocking from inside the car.
How can I tell if it is the remote? I could set up some elaborate software-defined radio to listen for rogue events. That seems like all the tedium of a stakeout with none of the character development and suspense of a movie stakeout.
Instead, I used a Bluetooth to OBD2 dongle (OBDLink CX) and an app on my iPad. It turns out that there is a register in the ‘body control module’ that stores the last code received from a remote key fob. lock is 1, unlock is 2. A button whose icon I have never understood, but which I think unlocks the sliding van doors registers 7.
If I find the car unlocked, I can check the register. if the last code was 2, then the car was likely unlocked with the fob. I can’t tell yet if this is a stray press or a short within the fob. What happens if I press and hold the buttons? Lock becomes 17 and unlock becomes 18. Also, press and hold rolls all the windows down. This explains an earlier incident and points towards unintentional button presses and a surprisingly long range from the fob.
The van hereby acquitted. When I went out into the grassy lot among the RVs where I remember leaving the van, I couldn’t find it. Perhaps it followed another irresistible impulse. Oh, never mind. There it is.