Am planning to install a carputer (Raspberry Pi) in a 2005 Ford Mondeo, replacing the car stereo. The Quadlock connector to the stereo apparently includes CAN bus, so I am thinking I will get a USB OBD-II adapter and just solder the relevant pins.

enter image description here

6, 14, 16 are CAN bus and battery plus.

On 4 it says "chassis ground" and on 5 "signal ground". I think I understand linguistically what that means, but am not sure why you need both. (Is that even the CAN bus signal?)

Since I only have one "ground" on the Quadlock, which I assume is battery minus, connected to the chassis, what should I do?

Both 4 & 5 to ground or ignore OBD-II pin 5?

Clarification: I am not going to use the car OBD-II port, only the Quadlock connector.

Edit: what I am really wondering is why the OBD-II standard has both chassis and signal ground. I find it hard to believe they are just the same.

More clarification: the car has an OBD-II port under the steering wheel. I am not talking about that one. The one I mean is the "in" connector to an adapter for interpreting the bus computationally. The adapter is connected to the computer via USB. I am going to remove that connector and connect the pins I need to the car via the Quadlock connector.

  • 1
    Both are usually grounded at the same point on the chassis.
    – Ben
    Commented Mar 3, 2018 at 13:17
  • Welcome to Motor Vehicle Maintenance & Repair! Commented Mar 3, 2018 at 13:29

2 Answers 2


You are over thinking things, just connect your devices (OBD & Pi) to a good chassis ground:

The difference between chassis and signal ground is:

  • Chassis Ground Pin 4: Is exactly that, connected directly to the chassis or battery negative terminal.
  • Signal Ground Pin 5: Is designed to provide a 'clean' ground isolated from any noise from devices such as radios, alternators and poorly designed switching regulators.

Usually signal ground comes from the Engine ECU, diagnostic gateway or nearest ecu/module, signal ground always ends up to ground through either a cleverly designed ground plain in the ECU/Module or through a number of 0Ω resistors or connected at a point far away from any noise.

So Signal ground and chassis ground always end up connected to the battery negative terminal but the signal ground is just isolated from any noise.

Only use signal ground as a reference when monitoring data, for sensors or diagnostics. This is because you cannot guarantee it is designed to carry significant current.

Now in the real world and especially on cars with little electronics such as your Mondeo you usually find that signal and chassis ground are just connected directly together somewhere in the loom.

CAN bus data is resilient enough to use chassis ground as a reference. You have no problem connecting everything to a good chassis ground.

While 99% of the time you will be ok connecting signal and chassis ground together, it's really not good practice as any noise on the chassis ground could now disturb any thing sensitive such as a diagnostic computer connected to signal ground.

Now when it comes to connecting your USB to OBD device connect pins 4&5 on the device to chassis ground. Don't connect pins 4&5 together on the actual vehicles OBD port for the above reasons. As for the lives you also don't have to worry about getting them from the OBD port, just connect to the required permanent and ignition lives on the quad lock.

  • Agreed on the "overthinking it" portion. Commented Mar 3, 2018 at 14:25
  • I don't think there is permanent & ignition on the OBD port, it is just "power" (the Quadlock has both). I guess I will use the ignition sense, which is apparently fused at 7.5A.
    – Tomas By
    Commented Mar 3, 2018 at 14:51
  • On your car that is probably right, on some vehicles there is an ignition live on the OBD port such as Nissan. My point was just theres no need to connect anything to the OBD port is everything is available on the quad lock. Commented Mar 3, 2018 at 14:55
  • 1
    @Aditya not really no. Ultimately all grounds end up at the same place, the battery’s negative terminal. Signal ground, analog ground, digital ground, shield ground etc etc are all at the same potential they’re just isolated in some way to stop the noise on one circuit affecting another. Like my answer states in some cars they’re just crimped together in the loom or sometimes they use a clever ground plane or circuitry in a ecu or module. Commented Nov 12, 2018 at 19:30
  • 1
    This will be by last comment if you want anymore detail then ask your own question and I'll be happy to answer it. There's really no need to use a shielded cable, but if you have to just connect one end of the cable shield to ground not both ends. Also remember that if the device is to be powered from a separate supply to the vehicles battery then you need to connect the devices ground to the cars ground. Commented Nov 14, 2018 at 19:29

As Ben stated in the comments, they are both usually grounded to the same point on the chassis. My suggestion to you is to just ground it to the chassis and ignore the #4 & #5 pins altogether. The more you mess with the wiring directly, the more issues you're going to have. Chassis ground is almost always a good bet, especially when all you are looking for is a point for a grounding source.

  • I think maybe I failed to explain properly. The OBD connector is between the Quadlock and the computer. I will throw it away and solder the four (or five) pins I need to the quadlock. I don't think I can ignore both 4 & 5.
    – Tomas By
    Commented Mar 3, 2018 at 13:45
  • @TomasBy - Why wouldn't you if the negative on the Quadlock is only a ground? In this case a ground is a ground. You are looking for a power path and nothing else. You're not looking for an exclusive signal path, just the ground. Remember, the DLC is just the end point for whatever you are connecting to it (think: scanner). It provides a connection so a device will work. It does nothing if there's nothing there. If you are installing a Quadlock into your system which has nothing really to do with the DLC, then there is no issue whatsoever. Find a grounding point and attach it. Commented Mar 3, 2018 at 13:52
  • Also, I want to be able to switch reasonably easily between carputer and stereo, which is another reason to use only the quadlock connector and not install my own wires to e.g. the chassis.
    – Tomas By
    Commented Mar 3, 2018 at 13:53
  • I am not installing the Quadlock. It is the connection to the car stereo, with the speaker outputs etc.
    – Tomas By
    Commented Mar 3, 2018 at 13:55
  • Your question why I wouldn't. Because they are inputs to the adapter. I feed it plus, and the CAN signal (L & R) and then I also need to provide ground, I firmly believe.
    – Tomas By
    Commented Mar 3, 2018 at 14:00

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .