Actually,I've never found a good reason for the fender relay.

The low current small gauge wire from the ignition switch to the fender relay could go directly to the solenoid mounted on the starter.

 

And that's the same relay, in typically the same location, Ford has used in most of its vehicles since, oh, the invention of fire. I remember some of the old Mavericks, et al, that rusted so badly, there wasn't enough fender left to ground the relay.

 

pull your steering wheel. where the key tumbler goes in and contacts the teeth you are talking about there is a little pin about 1/8 or smaller by about 1/4 inch long. it has teeth on on end. that pin sits inside the end of the teeth rail. the other end is supposed to be in an aluminum horseshoe looking thing on the right side. your ingition switch rod hooks into it on the top side. if that pin is not in there you will still be able to turn the key to the start position and all your lights will come on. but you will have to jump the solenoid to get it to run. in my case also had to dump the clutch in fourth to shut it off. the pull the batt cable to kill power....... real pain in #*$... good luck.



oh if the pin did fall look by the tilt spring in the column( if you have tilt) that is where mine always landed..... finally got it to stay though brake clean and jb weld. havent had problem yet...

 

Not really,...............

The older trucks had a high current solenoid mounted where the relay is now, and had a fat cable the same gauge as the battery cable running from it down to the starter motor rather than the lighter gauge wire that runs from the relay down to the solenoid which is mounted on the starter on these trucks.

 

As I recall reading somewhere it is an isolation relay to prevent the large voltage spike from entering the electrical system that is generated when the magnetic flux collapses and cuts through the windings of the starter solenoid.
This preventing destruction of the four computers and the radio that are in the truck.

 

BINGO !!

Yep, makes sense,...Separate big cable directly from battery to starter solenoid. All the rest of the truck is run off the other cable.