D-GND vs Chassis GND

General Discussion
#1

I know on most car OBD ports, pin 4 and pin 5 are both ground.

On some car schematics, it says pin 4 is chassis ground, and pin 5 is sensor ground…on other car schematics, it just says theyre both “ground”.

But even on the cars that have the two pins labeled as “separate” grounds, if you put a multimeter across the two pins, you always still get 0 ohms.

What gives? Ive seen some scan tools that have pin 4 and pin 5 internally tied together…and others, like the Macchina, that only use pin 4, but still have pin 5 tied loosely to it.

Im working on an unrelated car/OBD project just using an 8-bit AVR, and I could never decide what to do about the pin4/pin5 thing. Tie them together? Tie them together through some loose isolation, but basically only use pin 5? Just leave pin 4 open, and only use pin 5?

Thanks
Ben

#2

Pin 5 is suppose to be an isolated sensor ground. Most manufacturers run them both to chassis ground but in different spots. I have built several bench top setups and I just tie them together. As long as the grounds are clean and electrically tight, there should not be any problems.