Courtesy light fuse keeps blowing

[Seabeckredneck]

I have a 1979 Chevy 3/4 ton 2 wheel drive Camper Special with a 350 and I'm having trouble locating the short which is causing the 20amp fuse to blow every time I plug the courtesy light orange wire into the fuse block. I don't electrical troubleshooting very well, but I must have let the smoke out!  Any help or suggestions would be greatly appreciated!

[bake74]

     I am not quite sure what "have let the smoke out" means in relation to the courtesy lamp fuse blowing.  But I will see if I can walk you through the steps and show you the links to the wiring diagram so you can see what I am talking about.
     First off the complete wiring diagrams can be found in the tech section.  (if you did not know, the bold green words are a link to go to the wiring diagrams, if you do then ignore this message, I never know if the other end of this conversation knows that or not).
     OK, in the 77-80_cab_inter.jpg diagram to the right side you see the courtesy lamp switch.  It has 2 wires attached to it.  .8 WHT-156(which is the door jamb circuit) & .8 ORN-40B.  This .8 ORN 40B wire is the one I assume when plugged into fuse block it blows correct ?   
     The first 3 things we need to check are as follows. 
     #1:  let's make sure your courtesy lamp is not shorted out.  You will need to take the lamp out of the courtesy light and check it with a continuity meter.  When you take the lamp out it should be a open circuit,  Take your continuity meter and check and make sure there is no continuity with the lamp removed, this will tell us that your courtesy light is not shorted.
    #2: We need to make sure you door jambs switch/switches are not shorted the same way we did the courtesy lamp.  This is a little harder to explain.  In the 77-80_cab_inter.jpg diagram, you see the dotted line that goes to the block labeled 156.  Now if you go south west with your eyes you will see a group of blocks, the top left corner is labeled 156.  So I do not write a book here, I am skipping how to trace this wire.  But on page 77-80_instrument_pg2.jpg on the right side of page, top and bottom corners are your door jamb switches.  If you want, you can trace the circuits between pages.  These door jambs are grounded at the door, that is why they only have 1 wire going to them.
     It is important to make sure the lamp or door jambs switches are not shorted, This is where I believe you have the problem you are describing.
    #3:  Let's make sure you are plugging the orange wire into the correct slot in the fuse block.  If you were to trace the .8 ORN-40B wire (it leaves the courtesy lamp and goes down to a block which is labeled 40 and connects to a wire that becomes .8 ORN 40A) from there on page 77-80_cab_inter.jpg to the next page which is 77-80_instrument_pg2.jpg (same page you have opened for the door jamb switches), going to the bottom left corner (after you blow it up by clicking on it), counting upwards the wires, (6 solid wires, 1 dash wire, 7 more solid wires) you get to the .8 ORN-40A wire that is continued from the previous page.
      Follow that wire and make sure you are connecting it to the fuse block in the correct spot.
      Try these steps and report back, this would be the first step in troubleshooting and we can help you further is one of those items are not the problem.

[bd]

The factory 1979 Wiring Manual is year specific to your truck.  Download the manual to your hard drive for faster access, then refer to page 8.

For clarification, the "courtesy light" (the lamp under the dash) is powered through an orange wire running off the headlamp switch from the tail light/courtesy fuse.  The tail/courtesy circuit is hardwired into the back of the fuse block.  On the other hand, the "dome lamp" plugs directly into the face of the fuse block, is powered through the horn/dome fuse, and I believe is the 'courtesy light orange wire' to which you are referring.  Though the same color wire is used, the courtesy and dome lamps are separate circuits.

Regarding the dome lamp, orange circuit 140 runs from the fuse block horn/dome fuse directly to the dome lamp (and cargo lamp if your truck has it) underneath the driver's side carpet and sill plate, then up the left rear pillar to the dome lamp (and cargo lamp).  The most common 'shorts' occur where the wire runs under the sill plate, but can occur anywhere between the fuse block and the dome (cargo) lamp, inclusive.  A grounded "white" wire between the dome lamp and the door switches would simply cause the dome lamp to illuminate constantly.