Can't get relay to work, feel like idiot.



I've used relays before to wire electric fans but I can't figure out what I'm doing wrong this time. I've tapped into the reverse circuit to trigger the relay and power and ground wires are coming up to the bottom of the opening there, they go directly to the battery. I've tried 2 different style relays I have around I can't get it to work. Trying to use the factory reverse lights as the trigger the relay and send power through to the auxiliary backup light.
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Rendezvous Conspirator
Without any other context or description, I can only offer this:

Relays are typically 5-terminal devices.

Two terminals are for the coil (the low-current part that's usually driven by a switch, button, etc.) And the other 3 terminals are the (COM)mon, (N)ormally(C)losed, and (N)ormally(O)pen which are the high-current "working" part of the relays. Very occasionally one of the coil terminals may be shared with one of the main relay terminals. This is awkward, but can be made to work. If there are fewer than five "wires", you'll need to know which ones are shared. Likely its the common and one side of the coil, but you must be VERY sure before proceeding.

The coil is rated for a specific voltage and current. If it's functioning correctly, it must be driven by the appropriate voltage in order to generate enough force to close the relay. One side must eventually supply a voltage, the other side must eventually connect to ground. I say "eventually" because different systems use either a switched hot or switched ground. Eventually though, it will be +12v------CCCCCCC------>Ground (with a switch, button, etc. somewhere in there). In a normal magnetic relay there is usually no polarity to the coil, so either side can connect to +V. Some magnetic relays and most "solid state" relays may have a blocking diode on the coil, therefore there may be a positive and negative side of the coil. This must be respected, as reversing polarity may damage the relay.

The Common is always in play (is common), and it connects to the NC terminal when the relay is idle (not powered), and the NO terminal is left disconnected. When the coil is activated, COM is connected to the NO terminal instead (and disconnected from the NC). NO is never connected to NC (unless something is broken).

I'd say verify that you have the correct relay (12V for an auto system, typically), and that your pins assignments are correct. There is a chance the relay has failed, but it would be by either too much current/wrong voltage on the coil, or too much current through the COM terminal to one of the switched terminals. In a normal all +12v system, this is pretty rare unless the relay was undersized for the load or was the wrong relay.

Hope that helps.
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Verify your grounds.

And if you are trying to light up those LEDs, also make sure your polarity is correct.

It matters with LED
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I've used five wire and four wire relays i have around.

I dont know how i could be doing it wrong, theres a picture on the side of the dam thing.

I'm trying it right at the battery at this point
I connect the the led ground straight to the battery ground. I take a wire fron the positive and go to the switched side of the relay. I take another wire and run it from the other side of the switch and connect to the led light. I then energize the relay by putting power and ground on that half.

The relay clicks but power doesnt go through i checked continuity with the fluke, no continuity . I reversed polarity on the trigger side, no change. Reverse polatity on the triggered side, no change.


Many of the Leds I bought for my camper use black as the positive. This is an industrial standard, but most people expect the black to be negative/chassis ground.

Sent from my Passport


I've just got it powered off the reverse circut now, no relay. 14w light only pulls something like one amp right? Its a 15 amp circut and the factory reverse lights are led as well. Still frustrated.
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If you're using a standard automotive lighting relay, the terminals on it should be numbered: 30, 85, 86, & 87 (some relays occasionally have a 87a terminal also)

#30 is Common (as per Herbie's description)
#86 & #85 are the relay coil (12V across these terminals energizes the relay, polarity doesn't matter)
#87 is the Normally Open terminal,
#87a (if present) is the Normally Closed terminal (won't be used here)

Wire 12V power from your battery to terminal #30.
Terminal #87 then goes to your aux reverse lights.

Ground terminal #86
Connect terminal #85 to your reverse light circuit.

You should have aux backup lights now.

If you want to put a switch inline to disable them (or turn them on separately), the switch goes inline with the feed from your reverse circuit to terminal #85.

I also always suggest to connect a small diode (#1N4001 or similar) directly across the relay coil to clamp the voltage spike that occurs when the coil is de-energized. Cathode (striped end) goes to the side receiving +12V (terminal #85 in this case). This protects the contacts in your switch (if used) as well as prevents the spike from traveling up your reverse light circuit.

Here's a diagram also that may help (sub your reverse light circuit for the headlamp circuit on diagram):
Ok, seems Photobucket is effed up for some reason, Attaching it instead

Something I also just thought of, it may be possible your vehicle switches the ground side of the reverse light circuit rather than the +12V side (you can verify this by the presence of 12V on the reverse light circuit even when not in reverse).
If this is the case, remove the relay terminal from ground and connect it to +12V instead.



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