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I realize I can buy the JHP kit to add a button to lock and unlock the doors, but what I'm really interested in is something that would lock them automatically at a certain speed. Since the M6 already has a solenoid that locks out reverse above 3mph, I was thinking it might be possible to tap into that circuit to add another solenoid to trigger the door lock. Anyone ever thought of this and tried it?

About a year ago one of my friends had a guy walk up to his truck at an intersection and try opening driver's door. It was locked (autolocks) and he just floored it through the red and the guy took off running. Since then the autolock feature that I had always called annoying on my Expedition started seeming much less so.
 

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The only thing is you would have to figure out how exactly the reverse lock-out solenoid works. You would need to know if it gets fed a constant trigger when above 3mph or if it is pulsed then gets pulsed again when below 3mph...like a latching relay. If it works like that with pulses, it should be pretty sraight forward. Find the trigger, a couple of relays and you'd be done. However if it gets fed a constant trigger when above 3mph, you would need to use some sort of pulse generator and latching relay to pulse the doors once then use the latching relay to unlock them when you turn off the ignition.

You could also just buy a cheap keyless entry, not use it except for the ignition lock feature and just keep using your stock fob.
 

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If you wired it for ignition it would be alot easier than trying to wire it for a speed trigger. Like Diamond Joe says, a cheap keyless entry module will let you do this fairly easy.
 

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The reverse lockout, along with any other speed-based functions, happens in the ECU and is not triggered externally. The only thing you could do if you were so inclined would be to add a circuit that would tap the vehicle speed signal and send a pulse to the door lock circuit.
 

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I hope this isn't too late but I just saw this post today and I have a resolution. It's been a couple of years since I did this so my memory may not be the best on how I did it exactly, but here goes:

I used a 5-pin (I believe it's called a double pole) relay in junction with my parking brake handle. The parking brake feeds a negavite (-) signal when the brake handle us pulled, so I tapped off the parking brake wire and ran it to the negative post on the relay. I then brought in a constant 12v (+) wire to the positive post on the relay. After the positive and negative was taken care of I then wired one of the two remaining posts on the relay to the door LOCK wire, and the other remaining post was wired to the UNLOCK wire. Both the LOCK and UNLOCK wires are found in the steering column by the way.

So the way it worked was everytime I engaged the parking brake the doors would automatically unlock for me so that I could exit the car without having to manually unlock the door. Then when I would disengage the parking brake the doors would automatically lock. This works very well on M6 cars because you constantly use your parking brake when entering and exiting the vehicle. If you owned an A4 and didn't already use your parking brake when parking then you would just have to get into that habit. Well I hope this helps you if you haven't already found a solution.. and if so then I hope this helps others!

-Andrew
 

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The problem I see with your solution is that in either position the door lock circuit is being constantly closed. It's only supposed to be pulsed momentarily to lock or unlock.
Also anything tapped into the parking brake switch should be diode-isolated to prevent interaction with other circuits such as the DRLs and brake warning light.
 
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