From F1.3 (30A fuse) in the BJB which is hot at all times, to the main light switch and the MFS. Out of the MFS to the (a DRL relay if so equipped) the CJB. There power is distributed among four fuses, F2.16 (High Beams), F2.26 (Right Low Beam), F2.27 (Fog Lamps), F2.28 (Left Low Beam). No headlight relay suppyling battery power directly to the headlamps.

If and only if the truck is equipped with Autolamps, then there is a headlamp relay that is in the Auxillary Relay box 3. Then this relay is between F1.3 and the MFS. Again, no headlight relay supplying battery power directly to the headlamps.

As we know, headlamp switches and MFS do fail on these trucks. If a relay was in the circuit after these switches supplying battery power via the relay contacts then these switches the would enjoy extended life in our trucks since they then would be switching very low current to turn on the relay.

There is a lot of added color in the article referenced that has to be taken for what it is worth. The basic idea however is good and that is what caught my attention.

 

OK, I'm convinced. Thanks. The FORD Owner Guide & Haynes show the full capacity of the fuze panel/wiring harness & the normal full compliment of components does not include the headlight relay shown.

Guilty of only looking it up on paper, my '98 does not in fact have the headlight relay indicated in the Owner Guide. The OEM fog lamps do have a relay. I'm also unfamiliar w/the mysteries of what's inside the MFS. Still hard to believe a late model FORD still routes, not just the full amps of headlights, but all lights, through the dash switch. SAABs have had headlight relays under the hood since the 60's & tapped into alternator output for max V. Looks like a crappy system in the '98 F150 by comparison.

I'd also thought you needed to remove the headlight to disconnect the plug. Noticed the P side is easily reached so I checked the V & found it the same as the battery w/o any indication of V drop, despite the "torturous path".

Another thing I have to agree w/you on is the cheap wire FORD used. I'd honestly thought it was thicker & it plainly appears to be pretty thin for the application.

Just another disappointment to add to others like: the difficulty of adding Remote Entry to central lock or upgrading Power to Signal Mirrors. Think the same villian planned the lighting system & HVAC system too.

 

The issue of voltage drop through too small a wire is a common issue with motorcycles. I am not surprised it's also an issue with trucks. It is significant enough that vendors supply aftermarket wiring relays to make sure the bulb is getting full power. My KLR 650 has this very problem and putting a big bulb in does not fix the problem. Installing an aftermarket relay that supplied power directly from the battery, did.

 

IMO it would be foolish to expect a "big bulb" to "fix the problem".

If "big bulb" means a higher wattage, quite the opposite is indicated.

The last motorcyle I had was an old HD w/no battery & a headlight that worked right of the magneto (?) & got brighter w/rpm & just glowed orange at idle.

 

A higher wattage bulb is the angle taken by most people not realizing what the problem is. Your right, it doesn't fix the problem and only puts a lousy band-aid on it. Tuning the system to make it more efficent provides better results.

 

In my experience replacing 65W bulbs with 100W in this misguided pursuit, is responsible for many of the melted down wires & burned up connectors in headlight wiring.

Maybe expected to be a "band-aid" more often the Stick the Breaks the Camel's Back.

 

As I stated in my post, tuning it for efficiency is the way to go.