Wiring aux reverse lights.

[Kevin B.]

I've got a set of lights that I'd like to wire as auxiliary reverse lights. I'd rather not switch them, I'd like them to turn on and off with the existing backup lights. They draw enough that they blow my engine fuse as soon as I pop it into reverse, so I can't just tap into the existing reverse light circuit.

I know just enough about wiring 12v to get myself in trouble, so check me on this - what I need to do is wire a relay into the existing reverse light ckt, and then bring 12v off my auxiliary fuse block through that relay to actually power the lights, right? And in order to determine what size relay I need, I need to measure how many ... amps? ... the lights draw when they're on?

 

[TurboMinivan]

what I need to do is wire a relay into the existing reverse light ckt, and then bring 12v off my auxiliary fuse block through that relay to actually power the lights, right? And in order to determine what size relay I need, I need to measure how many ... amps? ... the lights draw when they're on?

You've got the general idea, yes. Using a relay is definitely The Right Thing To Do since it will (a) provide proper power for the lights and (b) won't overload the circuit that toggles the relay. You can get some more specific ideas here:

http://www.jpmagazine.com/techartic...arch_2011_randys_electrical_corner/index.html

As you'll see, his preferred method is to toggle the relay with the ground wire rather than the hot wire. This is also my preferred method, and it's how I have installed numerous relays (two-stage boost control, compressor bypass valve switch, extra fuel injectors, etc) on turbo minivans in my past. You can do a lot of neat things with relays and solenoids.

 

[JL Rockies]

I know a guy who is pretty good at 12V.

 

[Kevin B.]

I know a guy who is pretty good at 12V.

I haven't gotten back in there yet, but I picked up a relay and did some reading (thanks for the link, Dempsey!) and I think I've got it covered. If I run into any trouble I'll ping you, thanks!

 

[Manlaan]

I'm curious as to how one would use the ground to toggle the relay. Could you give more details on that?

The easiest way to wire it in my eyes would be to do the relay switch piece as follows: light power -> relay -> ground, so when the lights have power, it turns on the relay.
Of course the high powered side would be from the battery -> fuse -> relay -> lights -> ground.

 

[JL Rockies]

Relays are electromagnetic switches and it doesn't matter if the trigger is +12V or -12V. I don't agree that using the relay to supply power ground is the best idea since steel is already a crappy conductor without adding the inductance of the relay coil but yeah, it will work. Since copper is superior to steel as a conductor, the inductance would be less of an issue on the + 12V side.

I would ground the lights to chassis and use the relay to supply +12V for best performance.

 

[TurboMinivan]

I would ground the lights to chassis and use the relay to supply +12V for best performance.

Oh, totally--that's what I would do, also. When I said I'd toggle the relay with the ground wire, I meant I would trigger the relay (ie: the low-power side, the side controlled by your dash-mounted toggle switch) via the ground wire. This way there are no additional hot wires coming through the firewall into the cabin.

Sorry if I wasn't clear.

 

[Kevin B.]

Oh, totally--that's what I would do, also. When I said I'd toggle the relay with the ground wire, I meant I would trigger the relay (ie: the low-power side, the side controlled by your dash-mounted toggle switch) via the ground wire. This way there are no additional hot wires coming through the firewall into the cabin.

Sorry if I wasn't clear.

Meaning, you'd put the switch between the relay and ground, right? Not between the relay and battery?

 

[BCGPER]

You don't need any switch, use your exsisting reverse lights to energize the relay and the new lights will fire from there. All you'll need is one wire from the battery to the relay.

 

[Kevin B.]

You don't need any switch, use your exsisting reverse lights to energize the relay and the new lights will fire from there. All you'll need is one wire from the battery to the relay.

Dammit, right, sorry. I've got switch on the brain because I'm roughing in a set of rock lights too. That's been my plan, is to take power off the reverse light circuit to trigger the relay.

Now I'm confused all over again. Is Dempsey suggesting that I locate the relay between the reverse light and ground, instead of between the reverse light and battery? Right now I've got a wire tapped into the harness plug behind the tail light that will terminate on the relay, what's the better way to do that?

 

[JL Rockies]

Ground lights to chassis, reverse light trigger than supply to battery.

Reverse light to 86
Chassis ground to 85
Batt to 30
Lights +12V to 87

Make sure you fuse the batt connection at source.

 

[BCGPER]

Battery + to relay.

+ from exsisting lights to relay.

New lights - to chasis ground.

Clear as mud now? It's a piece of cake if you can visualize it. The hardest part is getting the + wire from the battery to the relay. On my JK I installed the relay inside the tail light opening, so the run to the auxillary lights was very short. If you want to take a peek at mine, you'll probably understand it as soon as you see it.

 

[JL Rockies]

If you had me help, you'd be done... just sayin'

 

[TJDukit]

If you had me help, you'd be done... just sayin'

Then you would be expo approved.

 

[JL Rockies]

Then you would be expo approved.

Let's not go there.... modz have a limited sense of humor these days.

 

[jentzschman]

I have been wanting to do this for a long time. Trouble is finding a place for the reverse aux lights in my case. I will be interested to see how you end up doing it kevin.

 

[Kevin B.]

I have been wanting to do this for a long time. Trouble is finding a place for the reverse aux lights in my case. I will be interested to see how you end up doing it kevin.

I haven't decided for sure where they're going to end up. Right now I've got them on the spare tire bracket underneath the truck - very out of the way, but I think I'm going to move them back to the rear bumper where they'll throw more light.

 

[TurboMinivan]

Now I'm confused all over again. Is Dempsey suggesting that I locate the relay between the reverse light and ground, instead of between the reverse light and battery? Right now I've got a wire tapped into the harness plug behind the tail light that will terminate on the relay, what's the better way to do that?

Sorry, I was confusing you because I forgot you wanted them to only come on with your existing lights. My comments were all centered around installing a separate toggle switch to control the extra lights. Follow what JL said and you'll be golden.