Dash light circuit?

Relays allow a low current switch to control a high current load. It only take about 1 tenth of an amp to activate a relay that handles 35 amps. Safer for you since you only touch the part that's low current and the wiring in the cab and through the firewall can be thinner while the wiring from the battery to the relay and on to the accessory can be as thick as it needs to be. Whether it's necessary or not depends on the wattage of the lights and the rating of the harness wires and switch. That pic I posted above is typical of a lot of aftermarket switches that max out at 3A and personally I'd only use them for 1A or less or to activate a relay. Some switches have much higher ratings and don't require a relay but still would require an adequate harness. so you need to know what you have and what you need.

 

Relay coils pull almost no power, so you can use an existing circuit to control the relay. Say you want to have some hella bangin' something or other come on when your DRLs do. Control the relay with a tap on the DRLs and let the added separate circuit carry the load of the hella bangin' something without risk of burning up your DRL wiring.

I'm running three bitch ass amber LED grill lights and I still used a relay to control them because I may want to add some hella bangin' something on that situation later and now I'll never have to worry about adding more infrastructure to pull it off.

 

Ok thanks for all the info guys... where along the wiring harness would a relay go? What type of relay do I buy and from where?

 

I mounted the relay behind the grill next to the radiator. There was a beep module there that made a beep when you unlock/lock the doors with the key fob that I disconnected on day one of owning the truck. I relocated that module to the passenger side fender and bolted the relay in its mounting hole.



I grabbed the power for the lights I added from the engine compartment fuse box. There was an empty fuse slot with constant power on it so I ran a wire through the main box entrance on the engine side, made a hole in the divider and installed a grommet to run the wire through to the fused lead.



The relay you need is a simple automotive relay with four terminals. The five terminal version gives you both normally open and normally closed contacts which you don't need for adding lights unless you want to get fancy with control options or something.

Essentially you power your ON/OFF switch (which controls the relay coil) in the cabin off of an add a fuse, run the power out of the switch to the relay's coil contact marked 86, and ground the other relay coil contact marked 85 in the engine compartment. This enables you to run a single wire through the firewall for controlling the relay (use automotive primary wire).

The power you steal in the engine compartment fuse box goes to terminal 30 on the relay, and your light positive wire connects to the NO (Normally Open) relay contact numbered 87. When the relay coil is engaged, it connects the two terminals and passes power on to the accessories that are connected to it. Your negative for your lights needs to be connected to a decent convenient ground location to complete the circuit.

Power from the switch and power feeding the lights are isolated from each other now, and you can use the appropriate sized fuses to protect each circuit.

 

So is that relay after the lights, before the fuse?

 

I was moving pictures from my phone to a PC where it's easier to assemble them for use. See my updated comment.

 

This is why I freakin love this forum. Thanks!