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How does the switch work.

Nathan Buffery
Nathan Buffery
in Ford Focus Club

Featured Replies

In the old days, a switch had live in and live out, but the switches on my mk3 focus don't seem to work like that. For instance, the Heater Rear Screen switch has a neutral, the illumination, the switch for the HRS and the LED. So apart from the illumination there is no live feed going into the switch. 

So I'm wondering....when the switch is pushed, does it connect to the neutral? 

  • Quote


    Modern car switches are just signals to the control modules to say you have asked for that item to activate, then the control module turns it on or off

  • Quote
    • Like 1
    • Author
    5 hours ago, DaveT70 said:

    Modern car switches are just signals to the control modules to say you have asked for that item to activate, then the control module turns it on or off

    When you say "signal" @DaveT70 is there a voltage that I can check for with my multimeter? 

  • Quote
    • Author

    Thats a bit more sophisticated then my multimeter. So presumably, you need the switch to be wired in to be able to test it?

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    buy a 15 quid cable - install forscan on a laptop and go from there

     

    https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR2.TRC1.A0.H0.Xels27.TRS0&_nkw=els27&_sacat=0

     

    basically without modern diagnostic kit there is NO point trying to do anything on a car

  • Quote
    • Author

    I've got the OBD reader and forscan, but was trying to understand how the switch actually works.

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    Screenshot_20191218-202130.png

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    • Author

    That's the diagram I've been looking at @iantt and that is also why I was thinking the switch terminal/wire is picking up the neutral. I understand that the module is receiving the signal and doing the actual switching. So if I put my (cheap) multimeter across termials 1 and 12 when I depress the switch, will I see any change?

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    Yes you will

  • Quote

    Yes, with the switch closed it will read zero and with it open it will read some voltage but not necessarily 12V. The line from the BCM will be pulled up  by a resistor so It all depends on the voltage used in the BCM electronics. For many microprocessors this is commonly 5V but CMOS circuitry can operate at 12V or above.

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    • Author

    Right I've got it now. Just another piece of the car I've got a bit more understanding of. Thanks all for your input. 

  • Quote

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