Originally Posted by Manimal
TS888, I'm going to follow your advice and upgrade the stock wiring with 12AWG cable. Currently I have it set up so the two headlight circuits go to relays in the fusebox. The headlight switch on the ignition only powers the relays. My aux lights are on a separate relay also I the fusebox and they come on with the high beams. Stock wire is 16AWG, right? How difficult is it to replace the two headlight circuits? Is it necessary to disassemble the dash or can it be done without too much hassle?
Carl, I did all of mine under the "bonnet", never touched the dash. In your case, you need to run wires from the fuse box (and I am assuming this the one over the tunnel, I'm not familiar with NAS trucks so perhaps they have a different fuse box?). You may have to remove part of the dash to get to the harness so you can route the wires out to the lamps.
I used a small "project box" from Maplin (UK version of Radio Shack) to hold the relays and fuses, screwed to the edge of the fender left front fender about 12 inches behind the left headlamp. It's not completely water proof, but short of complete immersion (which would take driving through water deeper than 4 feet, not on my list of things to do), it will keep the bits dry.
The box holds 3 fuses and 3 relays, one each for low, high and aux (planning ahead, I don't have aux lights yet). I took supply from the back of the alternator via 10awg wire that splits to the three fuses and relays. Max load for my headlamps and planned aux lamps is 30 amps, the 10awg is good for at least twice that on a run under 5 feet. I could have taken supply from the engine compartment fuse box, but the alternator was 2 feet closer and it's the same electricity! I made a common ground point on the fender next to the relay box and ran a wire from there to the engine block (I think to the mount bolt for the oil dipstick).
The switch signal for the low and high beam relays is taken from the left headlamp wires from the factory harness -- headlamp plug cut off, factory wires run into project box. The replacement harness is all 14 awg, for the right headlamp it runs along the frame cross member at the lower front of the radiator, in split plastic wire sheath. The factory harness for the right headlamp is just zip-tied out of the way. I used new headlamp plugs so I did not have a wire gauge step down right at the plug.
The supply to each relay is fused. The supply from the relay to the lamps is not fused. Why? My logic -- the run to the lamp is short, well protected, and the lamp bulbs are effectively fuses. In the unlikely event of a short between the relay and the lamp, the worst case is I have to replace a relay, a bulb and a few feet of wire. There's nothing else to burn or melt.