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I am installing a dash cam in my 2011 Ford fusion and hardwiring it to my fuse box. in the instructions it says it must be connected to a constant fuse and a switched fuse.

I have a test light and when the car is off and the key is out of the ignition some of the fuses light up as they are constant. when I put the key into the ignition and turn it to the on position, none of the fuses light up at all. Even the constant fuses that I tested earlier do not light up.

Am I doing something wrong here?

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    Where do you connect the other end of the test light?
    – Solar Mike
    Commented Mar 2, 2020 at 19:05
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    Sounds unlikely. Perhaps the testing method is faulty.
    – Harper - Reinstate Monica
    Commented Mar 3, 2020 at 0:47
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    it is being connected to an exposed bolt underneath my dashboard. and from there i am testing the fuses. maybe that is my problem? not connecting it to the correct place? i'm new to this stuff but i thought you could connect the test light to bare metal as its for a ground. thank you for the replies by the way!
    – Marcus Gendron
    Commented Mar 3, 2020 at 23:30

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It sounds to me like you have connected one end of your test light to the positive side of the battery rather than the negative. This would give the results you are experiencing.

I believe that this is what @solarMike is also suggesting in the comments.

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  • I agree it sounds like user error but I don't see how you can come to the conclusion that its clipped to positive. Are you suggesting the user is moving the clip after testing with ignition off? I can not think of any application where a fuse is grounded during ignition - off and then hi when ignition is on. In the controlling modules the outputs for these fuses are always floating and when controlled by relays then they are simply open.
    – narkeleptk
    Commented Mar 3, 2020 at 17:13
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    @narkeleptk Use the engine ECU for an example. When the ignition is off, its fuse would be disconnected from the positive supply. If you then connect a bulb from the battery positive to the fuse, you are in effect providing power to the ecu through the bulb, therefore the bulb could glow. When you turn the ignition on, both sides of the bulb will be at the same potential so the bulb will extinguish.
    – HandyHowie
    Commented Mar 3, 2020 at 17:39
  • Interesting.Thank you for explaining more in detail. If you are completely oblivious to what you are probing and only watching the light while randomly poking around then I suppose that is possible on a few circuits. I'm going to experiment with that now.
    – narkeleptk
    Commented Mar 3, 2020 at 18:15
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    hey guys, so i had the test light connected to an exposed bolt on the inside of my car as i couldn't find a better place to ground it. and i was testing on the interior fuse box not the fuse box under my hood. i really appreciate all of the comments
    – Marcus Gendron
    Commented Mar 3, 2020 at 23:27
  • @MarcusGendron Looks like you had it connected correctly.
    – HandyHowie
    Commented Mar 4, 2020 at 8:02

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