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Ground constant power supply?

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  • Ground constant power supply?

    Hey I have a MINI Cooper S and am in the process of swapping out the HU.

    Euro cars don't come factory with a constant power wire just a ignition switched wire. I am going to run a wire to my battery for the constant power. Do I have to ground this wire? I assume I do since the fuse board is grounded and I am not using OEM harness etc. Also someone suggested I solder the constant,switching and switching wire thats provided together.
    This is incorrect from everything I have learned but I would like someone to clarify the problem with doing so. Just to recap: My car only has a ignition switched power wire for the headunit.

    I need a constant source and am wondering if I need to ground the constant source? Also just to skip both those steps would it be wise to use the ignition switched wire and solder the constant and switched wires from the aftermarket harness? Any confusion please let me know. I will most likely just run a separate wire to the battery independently and ground it. Also is there a preferred way to splice into a constant power wire and run a ground off it? I am assuming I would just expose the bare wire and solder bare to bare and ground? If I am incorrect let me know.



  • #2
    You do not ground a +12v wire from the battery, that is a direct short. The wire should be connected to the "+" terminal on the battery, go straight into a fuse holder as close to the battery as possible then continue into the passenger compartment. If you pass through the firewall, make sure it is not rubbing on the edge of the steel. If it is put a grommet in the hole to protect it from wearing through. If I were you, I would add a relay to this wire as a second output and control the relay from the factory ignition wire. Then you can hook what ever you want to it (within reason) without worrying about the factory circuit being over taxed. You could also add a heavier gage ground wire if you wish and connect it to a bolt passing through anything steel in the dash frame or firewall. Then you truely have a stand-alone power circuit. That way if you ever have electrical isssues with the vehicle, the dealer should not question any "modifications you have made.
    My 2007 Ford F350 Work Log located HERE


    • #3
      Well said PhilG.
      I too interpreted vTwin as saying "connect +12V to ground" as opposed to grounding the LOAD (ie, the HU).

      I always have a direct & dedicated +12V to the battery +12V (and ground to the 0V = battery negative) for my HU.
      That way I listen at any time. (I used to have a switch controlling a relay - ie, ACC (or IGN) and constant; but I no longer bother.)

      It can be a problem if you forget to turn the HU off.
      This has only been a problem for me since its flouro display blew (a case of over-voltage thru an external-regulator ground drift), but I run a spare battery so it's no big deal.

      Makes sure the Cooper S is negative earth/ground/chassis if following the above polarities. (Rule is always fuse "hot" cables near he battery; and (FYI) that the chassis shall be disconnected WHEN doing electrical work.)


      • #4
        ^+1 to what has already been said.

        the general rule in car electronics is to have a fuse within 12" of the battery on the positive lead, and there is also a pretty common saying to keep grounds shorter then 12"-- though i rarely use that-- this is primarily to keep the resistance of the ground cable low, though there are other methods around that(like using a larger cable)
        My OLD 2001 Mitsubishi Eclipse GT:
        "The Project That Never Ended, until it did"

        next project? subaru brz
        carpc undecided