Phase 1: Planning

I have two rectangular 55W halogen H3 lights mounted to the rear of my rack. I had considered just making a new run of wiring from the engine bay with appropriate relays and a spare switch panel and spare fog light switch i picked up from the junk yard. After doing a little reading I realized there was a much easier solution that didn't entail making a long run of wiring from the front to the rear of the truck. In the rear near the passenger side tail light there are three wiring connectors for the trailer wiring:

See below for pic

C400 is a 7 pin connector:

see below for pic

the pin out for C400 is as follows:

pin 1 - red/orange - right rear side lights
pin 2 - green/purple - brake lights
pin 3 - green/white - right turn signal
pin 4 - green/red - left turn signal
pin 5 - black - ground
pin 6 - red/yellow - rear fog light
pin 7 - red/black - left rear side lights

connectors C424 and C425 are single point connectors.

C425 is a switched 12v fused 20A lead from satellite fuse box 2
C424 is a feed from the reverse lights.

Now to make sure that the lights wont overload the stock wiring we have to figure out how much current they will draw. The lights are two 55w lights so they will draw a total of 110 watts @ 12V DC. We need to figure out how many amps this will draw, in order to do this we divide watts by volts to get amps. 110/12= 9.2 amps. So our stock power lead from the satellite fuse box 2 will handle the load from our lights just fine.

Now, how to get everything connected and working nice and happy:

What we are going to use here to get things up and running is a hella 25 amp automotive relay:

see below for pic

Now this relay is a little different as it has a built in fuse holder. Why add a fuse when the power feed for the C425 connector is already fused? Well, a few other things live on the circuit namely the raido, interior lights, and cluster so i'd rather have a fuse on the relay like a 15 amp that is rated lower than the main circuit fuse so that it would go first. A 10 amp would probably work in reality, but may pop on in the initial start up surge since the amp draw is really close to that.

- we are going to use the C425 connector to supply power to our lights
- we are going to use pin 6 of the C400 connector to supply the trigger voltage for our relay
- we are going to use pin 5 of the C400 connector to supply ground to trigger side of the relay
- we are going to going to find a nice chassis earth to use as our grounding point

Now for those that aren't very techy, a relay is just an electromagnetic switch. It's has a coil inside that when energized closes a set of contacts and completes a circuit. Makes switching electrical components on and off a lot more efficient in that you don't have to make long runs of heavy gauge wire to keep voltage drop to a minimum. You can keep your runs of heavy gauge wiring short and then run long runs of light gauge wire to switch the coil in the relay.

If you look back to the pin out of the C400 connector you'll see that the pin 6 is for the rear fog light. Fortunately we just so happen to have a rear fog light switch installed in our trucks already and since we don't live in a terribly foggy climate here in socal I don't feel bad re-purposing it to trigger the relay to turn on my rear work lights. In the event I'm driving somewhere and it's imperative I use my rear fogs I can pull over and remove that 15 amp fuse from the hella relay and my rear fogs will work as they were designed.

On the bottom of a relay there will be numbers near the blade terminals. We will make our connections to specific terminals. The terminals on the bottom of the relay are as follows:

30 - relay main power in ( connection from C425)
87 - relay main power out ( to lights )
86 - switching power ( C400 pin 6 )
85 - switching ground ( C400 pin 5 )

That's the groundwork and concept of what we're going to be doing, I'll update this with the actual installation, that will be phase 2.

 

good stuff.

take some pictures

im curious to see where/how you end up running the wiring out of the truck

 

I highly recommend running a better (larger) wire from the front to a fuse panel and running off that.
While the 14ga factory wire will support 110 watts ok, it leaves you no room for additional loads (or swapping in higher wattage bulbs) without getting too high a volt drop.
Overall volt drop is the more important figure to look at. If you keep that under 0.5v then you know your wire size is good for the load.

 

I agree, but I am kind of a push-over for good wire. Here is what I use for power wire sometimes: Knukonceptz product detail for KORD ULTRA FLEX 12 GAUGE SPEAKER WIRE

Maybe just use the simple setup and save this for when it fails?

Either way, this is turning out to be a great contribution, thanks.

 

This is true, running nice run of 10 gauge to the back would be preferable and if in the future if I find i need to run higher wattage lights or actually tow something then maybe i will. For right now though I'll have a workable solution using currently un-utilized factory wiring that is sufficient for the application intended at present. If I find the wiring starts running hot or i start popping fuses then maybe i'll just run a single 55w light for the time being. Just trying to spread some info around..

 

can you post a pic of how they look on the rack? considering doing this also.

 

I'm new to the Rover world, and this article was spot on for helping me put a work light on the back of my Disco 1. Thanks for the great details and diagrams! I wired everything just as indicated, and it works perfectly. Had my helper not blown a fuse "checking for power", I'd have been done in an easy afternoon.

As for the light mount, I followed the directions from here:

http://www.discoweb.org/worklamp.htm

I centered mine over the third brake lamp directly on the top of the door.

My one son fabricated a very nice bracket and my other son did the soldering and finish work. Since they're the drivers of this rig, they have some investment in it as well.

Again, great directions. Thanks for posting them!

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Greg
1995 Disco 1