3 Way Switch Truck Cap Light - Electrical Tap

jeffluecke · February 24, 2009

Sorry for the long post I am a newbie! This past month I just sold my old 1992 F250 with 460 and purchased a GMC 2500HD CC LB with the 6.0. I have to say I was hoping to get better gas mileage with the 6.0 but it is still at huge jump up from the old 460. I am averaging 13 mpg on the highway with a ladder rack, and hoping with no ladder rack to jump up to 14-15 running 65-70 mph.

I've spent a couple hours searching on these boards and google and have not found the answer I'm looking for. I want to wire up a truck cap light like a 3 way switch in a typical house. I have found some clean looking on-on switches with 3 terminals that I think will work out nicely. My plan is to wire one switch in the cab in the location just under the cargo lamp and then wire the other inside the truck cap, it would be easiest to source constant (I think I want to turn the lights on with ignition off) 12 VDC inside the cab but I'm having difficulty figuring out the cleanest way to source constant 12 volt power inside the cab.

I have just purchased a Tekonsha P3 brake controller and I like the way it plugs into the interior electrical panel. I checked some of the other slots in the panel and found several hot posts, I assume I can purchase appropriate connectors at the dealer? Are these dealer only connectors, I assume they are all fused how do I trace where the fuse is located for these slots?

Am I overthinking this should I just get a fuse extender at Autozone and hook directly into one of the accessory fuses? Has anybody had luck wiring up a 3 way switch for a cap light? Does anybody have a link that covers what all the extra spaces could be used for in the interior panel - I'm thinking about adding air bags and not sure if the truck is already wired for them, and I'm curious what the cell phone slot is?

Thanks for your input!

pwall · February 24, 2009

Welcome to

Sorry, can't answer your question in regards what the other slots are in the panel. However, the one marked 'cell phone' is most likely for OnStar.

bill190 · February 24, 2009

The right way to do this would be to find an existing circuit, turn on everything which uses that circuit, then use an amp meter to measure how much amperage everything is using, then look at the fuse rating and then determine how much spare amperage is on that circuit.

Note: If you are measuring a 20 amp circuit, you would need an amp meter with a 20 amp or more capacity - many meters sold in auto stores can only read 10 amps!

So maybe you have a circuit with a 20 amp fuse (and this is the correct fuse for the size of wiring used) and with all the devices on that circuit turned on, the circuit uses 16 amps.

So you would have 4 spare amps available on that circuit. (Actually drawing 20 amps might blow the fuse, so let's say 2 or 3 spare amps.)

Then measure the amperage used by the light bulb. Let's say it draws 1 amp. So this would be safe to add to that circuit and not have the fuse blow. Especially if you would not have everything on that circuit turned on at the same time.

As a general electrical rule, it is a good idea to not draw more than 80% of the amperage continuously on a circuit. But if everything would never be turned on at the same time, then you can go higher than this.

So far as wiring a 3-way switch set-up, you would need to use two switches. Each switch would need to have a "common" connection. Then the switch flipped one way would contact one connection and the switch flipped the other way would contact the other connection. Then run two wires between the two switches and these would connect to the non common terminals on both switches.

Then use the two common terminals to connect between +12 Volts and the lamp.

Like this diagram from a home wiring set-up...

(Black shown should be red and +12 Volts. White would be ground (black) and does not need to run through the switches of course. And just one light.)

jeffluecke · February 25, 2009

Bill,

Thanks for the repley that is the exact circuit I was thinking of using, I've had a little difficulty finding the exact switch I want rated for 12 VDC but I have spoken to a couple manufacturers and they feel the switches rated for 120 VAC will work just fine.

I was under the impression there were several "spare" circuits available to tap for this type of application, I'm just not sure where to find them and the best way to "tap" into them, I think I read on one of these forums that some people buy a "fuse extender" or "fuse tap" from an auto parts store which allows you to tap 12 VDC directly at the fuse box. I was also hoping I could tap into one of the un used ports of the interior electrical panel where the trailer brake controller plugs in.

Additionally does anybody know if there is a location on the cab that makes it easy to run tires outside the truck?

bill190 · February 25, 2009

So far as the voltage and the switch, a higher voltage rating would be fine. The amperage rating is most important. The amperage rating should be higher than the amperage the connected devices will be using. So if it is a 2 amp 12 volt DC light and you have a 120 volt AC switch rated for 10 amps, that would be a higher rating for both and would be fine.

The only way I know of to know if there are spare circuits and where to connect to them would be to get a factory wiring diagram for the vehicle. (And be able to understand wiring diagrams of course.)

The factory wiring diagram would list what all is on each circuit.

Then did you mean run "wires" outside the truck instead of "tires"?

If you want to run a wire from inside the cab to the outside, I find it easiest to run wires through the firewall into the engine compartment. Then from there I can run them under the truck to wherever I want outside. Sometimes I need to drill new holes. Sometimes I can use an existing hole and fish the wire through. If drilling a new hole, be sure to use a rubber grommet to protect the wires from being cut by the sharp metal and shorting to ground.

jeffluecke · February 25, 2009

Bill,

I did mean run the wires not tires outside of the truck. I will spend some time trying to find suitable spot on the firewall to pull the wires through.

I have some factory service manuals on order, but from my owners manual it seems like there are several un-used circuits that I can tap into. It seems like there are several locations in the interior electrical panel that are fused either in the internal fuse panel or the fuse box under the hood that would make this a clean install. I was hoping this was a common practice and somebody would know which connector to use and where to get it. I'm going to go down to the dealer today to get a build sheet for my truck and discuss purchasing some connectors to tap into the spare circuits.

Thanks for the help.

bill190 · February 25, 2009

You might be able to see spare/unused circuits in the fuse panel, however finding the right wire would be next to impossible just by looking in my experience. I find it impossible to trace any wiring on a vehicle as a lot of it is in hard to get to places.

But with the handy dandy factory wiring diagrams (1 of 4 factory service books with my truck), it tells you which circuit, which connectors, which wire colors, and where the wires and various connectors are located.

And then what may be unused in one vehicle may be used in another.