Headlight switch failure mode

[fiddler]

I was installing LED park and turn signal 1157 bulbs in my 1974 c10 stepside and I did something I had not done since I owned the truck.  I pulled the headlight switch to the first detent.  The headlights came on instead of the parking lights.  I pulled the switch the rest of the way and the headlights and running lights are on..  I am heading to the parts house hoping it's a switch issue as the only other alternative is somebody miswired something somewhere in this truck's 43 year lifespanl and its under the dash and I don't bend as good as I used to.   If you have seen this I would appreciate your input. 

thanks,

[bd]

That is not a common problem.  It could be in the switch if the internal contacts have been dislodged or if there is a conductive "floater" inside the switch, but it is unlikely.  More likely is that wires have been swapped in the headlamp switch connector.  Use the attached image to compare wire colors of the harness connector in your truck.  The respective terminals are circled in their corresponding color.

• red = fusible link protected headlamp power into switch (B+)
  
  
• orange = 20-amp fused running lamp power into switch (B+)  [Typically, there are two orange wires connected to the same terminal.]
  
  
• brown = running lamps out (tail, marker, park, license plate)  [There maybe two brown wires separately connected to the switch.]
  
  
• yellow = headlamp power feed out to dimmer switch
  
  
• green = dimmer rheostat feed to 5-amp instrument lamp fuse and dash lamps
  
  
• white = dome lamp override

[fiddler]

So the new switch changed things remarkably.  Now all lights come on in all positions...(except off).  I checked continuity between the yellow, (blue on my truck), and brown lines.  They are dead short to each other.  No way one can get powered without the other.  So my guess is somewhere along the line the headlight switch failed to light the headlights, and someone has wired the headlights and running lights to the same line to save 20 bucks.....  Now to find where in the heck the splice is.  Except...I don't know how the original condition of headlights only on at first detent happens.  Maybe I need to disconnect all of the bulbs out to test this. Now I am thinking with my fingers here....I'll let you know what I find.  I actually love wiring problems...  Avionics engineer by "trade" (retired). 

[fiddler]

Got it!  So the old switch had failed, and I found a blue splice, (like the kind you use to splice your trailer wires into your running, stop and turn signals with) about just too far back to get to or see, above the headlight switch plug, splicing the headlights and the running lights wires together.  So to save 20 bucks someone spliced the wire.  Some peoples kids!!  Was miserable getting tools in there to get the splice cut out, but once I did, the new switch works perfectly.  I coated the broken insulation with sealant.  Hope the wire isn't thinned down too much causing voltage drop or heat....  Maybe get LED headlights and cut down the current draw there as well!  The diagram and your description of what is supposed to be was most helpful.  Thank you much.

ricky