I have a pair of LED lights wired on my truck to a 12V 4-pin relay all correctly with a switch. The system worked great every time until I decided I wanted to add a small 5mm LED, so when I turned them on I had an indication in the cab of the truck that my exterior lights were on.

I ran a ground for the LED and then wired the positive leg to the output side of the switch.

When I flip the switch "on", the small LED lights up like it should, but my exterior light no longer come on. That was the only thing I changed but it gets weirder.

If I take the small 5mm LED out of the equation and rewire it exactly how it was working before, it still doesnt work. I tried swapping in a new relay and still nothing. Any idea?

wiring diagram

  • You must have blown a fuse, right?
    – vicatcu
    Commented Nov 13, 2017 at 0:22
  • That was my first thought too but nope fuses are good
    – user168694
    Commented Nov 13, 2017 at 0:36
  • 1
    I suggest you start with by a multimeter to measure the voltages at each terminal of the relay (with respect to 0V / chassis) with the wiring still connected to it, when (a) the relevant switch is turned off, then (b) when the switch is turned on. A few types of fault (e.g. ground connection fault) are not obvious from that initial set of readings, but as the first stage of troubleshooting, it does allow some possible faults to be eliminated. Next actions depend on those voltage readings.
    – SamGibson
    Commented Nov 13, 2017 at 1:03
  • Thanks for the flag, I tried testing voltage and it's getting a solid 12v at the positive input to the relay and then when I flip the switch on again solid 12v going to that pin from the switch. The output pin that runs to the positive connection with the exterior lights is outputting between 2-3 volts regardless of the switches position. How can that be?
    – user168694
    Commented Nov 13, 2017 at 1:13
  • 2
    I wired the 5v indicator led in series with an appropriate resistor to safely drop the 12v to a level appropriate for it.
    – user168694
    Commented Nov 13, 2017 at 1:38

2 Answers 2


you wired your relay wrong. see pic.

relay diagram

  • I didnt draw my picture accurately just to show and idea of it. I wired it correctly and as I stated it functioned perfectly until I tried to add an indicator led to the output side of the switch, then it stopped working
    – user168694
    Commented Nov 13, 2017 at 8:32

Finally figured it out, turns out the ground wire at the exterior light, under my rear bumper and tucked way up in the frame (thanks past me) had broken off when I hit a rock off-roading. Easy fix but hard to find. Thanks for all the suggestions

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