OBD2 Vehicle Adapter, chassis ground tied to signal ground via 1M and .100uF?

[NaDobraNich]

I found this OBD2 vehicle adapter. It's a breakout / Y-adapter.

In the vehicle's, a PIN4 is a chassis ground, and Pin5 is signal ground. Depending on the vehicle they may be tied together at a point with a wire splice or go through some module's clever circuit.

This adapter ties the two together with a 1M and series .100uF. I have a feeling the designer knew what they were doing, but I have issues with this.

https://github.com/macchina/OBD2-TEST-BOARD-hardware/blob/master/SCH-01028%20R0%20SCHEMATIC.pdf



My concerns with this.... Well, what if I was using this without a vehicle. So, I have a device that plugs in and extensively uses chassis ground, and another device that plugs in an extensively uses signal ground. These two aren't really the same UNLESS, you're plugged in to a vehicle that ALSO makes chassis ground and signal ground effectively the same somewhere.

If you were just using this on a bench, it seems like chassis ground IS NOT going to be a good ground, right?

Thoughts?

(in the photo, the 1 and 2 are chassis and signal grounds)

[Benta]

Decoupling with the 100 nF cap, plus a discharging resistor.

[NaDobraNich]

Right, but… like… this doesn’t make them equal grounds.


The 1M resistor is so large it’s doing little, and the .1uF is a DC block, no?
My concern is that I plug this in on bench, one side uses signal ground, and the other side uses chassis ground, and I am not connected to a vehicle, then I’m in for unpredictable trouble no?

[Andy Chee]

Will your bench setup include a starter motor, or alternator, or spark plugs?

If not, then it's ok to combine the GNDs together on the bench.

[NaDobraNich]

Will your bench setup include a starter motor, or alternator, or spark plugs?

If not, then it's ok to combine the GNDs together on the bench.

If I use it in a vehicle, yes. If I use it on the beanch, then no.

I’m seeing if I can avoid needing a jumper to sometimes connect chassis to ground.

[NaDobraNich]

I think I just need someone to tell me what would happen here.

1. If I have one device that attempts to use VBAT+ (12V) and Chassis Ground, as is, no vehicle. What is going to happen? That isn't going to function as "real ground" though that 1M and 100nF right?

2. Let's say I have two self-powered devices and one uses chassis ground as a reference, and the other uses signal ground, I try and read a 5V signal from one device on another, same thing right? I don't really have a shared reference. And rather this DC filter is of strange sizes so (1M 100uF) so what would I expect on a changing signal just transitions right?

[NaDobraNich]

Welp, no answers as to what would happen with that resistor and capacitor setup and no vehicle. So I just put a jumper on to tie Chassis to Signal grounds if there is no vehicle.