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I'm trying to figure out how to lock and unlock the doors of a 2016 Mercedes Citan using the CAN bus via the OBD-II port. Is there some kind of OBD-II interface where I can control this with plain negative pulses?

thanks

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  • you might be able to do it with one of the more expensive testers that allow you to modify and turn on/off systems. Mar 8, 2017 at 18:04
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    Which testers are you referring to? Could you provide a link or more info please? Mar 10, 2017 at 17:55
  • Any OBD-II tester that does this will be a professional unit made by people like Snap On, etc. Testers from them are $5k and up. $1k a year to keep the software up to date for new model years. But, it does everything, and especially the manufacturer specific PIDs. Big money. How badly do you want to control your door locks from the OBD-II port?
    – cdunn
    Mar 29, 2017 at 21:10

4 Answers 4

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The OBD-II port supports a number of electrical protocols, but all of them are serial in nature. There is nothing you can do to any of the OBD-II pins with "plain negative pulses" to do what you want to do.

One path to what you want is a cheap ELM327 device sold by just about everyone on Amazon.com. They are very inexpensive and easy to use. If you get one that has a USB cable on it, you can talk to it with Putty, or any serial data application you like. Putty is free, and easy to use.

Once you have connected your PC / laptop to the ELM327, you can send commands to the vehicles ECM (Engine Control Module) and anything else that's connected directly to that bus.

Here's the hard part, the command you want is likely going to be a mode 8 PID. (See this link for general information about modes and PIDs Wikipedia OBD link

The reason that's the hard part is that most manufactures don't like to publish what their mode 8 PIDs are, or what the data format is. Getting this information could be difficult, or expensive.

But at the end of the day, you cannot just send single ended pulses to a pin on the OBD port and control anything. Most of the signaling protocols are double ended (differential) btw.

References:
ELM327 Data Sheet

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    1st, thank you for your extensive detailed ,concise and clearly explained comment. When you stated you can send commands to the vehicles ECM (Engine Control Module) and anything else that's connected directly to that bus What else would be connected to the bus? What is so hard about mode 8? How is voyomotive able to do it? VoyoMotive Getting this information could be difficult, or expensive. Do you mean that they would have to license this from the auto manufacturers? What do you mean by Most of the signaling protocols are double ended (differential) btw. Oct 30, 2017 at 16:14
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I have been trying to find information on the same thing and the conclusions I arrived at are that it would be easier to repurpose an old/new key fob to do just that in combination with a BLE/WiFi microcontroller. Matt Frost did that here:

https://gitlab.com/milagrofrost/esp8266-car-key-fob-iot/-/tree/master

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If you read Voyo FAQ it states they use “wireless relays” to add Convenient functions that when an “authorized” device is in range can trigger relays.

So Voyo doesnt appear to be using the OBD port to trigger locks. OBD in the lock scenario is just a coordinator for the Voyo wireless addons relays.

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Voyomotive does somehow lock/unlock doors and pop trunk without anything additional than their obd2 unit. I have one so I can vouch for this. The relays (which they still haven't come out with yet) have some other functions like starting/stopping the engine.

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