Not sure where to post OBD questions, I don’t see an “OBD” forum. My questions are about OBD shields like the one linked below. I’m working on an Arduino OBD gadget. To prevent it from draining the car battery, I want the gadget to be powered by the cig lighter, so that it’s 100% off when the engine is off. But the gadget will still be linked to the car’s OBD plug via the OBD shield. In my car, the 12V in the OBD is live even with the engine off. Questions:
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Does the OBD shield draw a current (via DB9 from the 12V OBD pin), if the Arduino with his shield is powered down completely?
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If not, can I simply just cut off the 12V pin on the DB9?
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If the OBD 12V is needed for the shield’s operation, can I use a relay to kill the 12V on the OBD, if the Arduino is powered down?
https://www.sparkfun.com/products/13262
The [schematic on the product page you’ve linked has answers to all your questions.
From the schematic, 12 volts passes through the OBD cable, through the shield to the VIN pin where it’s passed to your Arduino. Cut the VIN pin between the shield and Arduino and you’ve isolated the 12 volt supply and no current should flow from your battery.](http://cdn.sparkfun.com/datasheets/Dev/Arduino/Shields/SparkFun_CAN-Bus_Shield_v13a.pdf)
OK, thanks. One more question: reading up on the CAN bus, it seems the the stubs (branches) off the bus should be <30 cm (1’) to prevent noise (1st link below). Does this apply to the OBD/DB9 cable between the car’s OBD and the shield? I see that Sparkfun’s OBD/DB9 cable is 5’. Seems to me, this cable also branches off the bus. Is it not an issue?
https://youtu.be/YvsGuK9Up0E?t=434
https://www.sparkfun.com/products/10087
You’re attaching to the OBD connector that’s on the end of the bus so you’re not creating a stub, just extending the bus by 5 feet. Shouldn’t have any trouble. 