[Edited title - not the issue I originally thought]
Hi guys.
I have searched this great forum and found many threads on issues with power door locks, but I think mine is unusual.
I have a 1990 F-250 Lariat XLT 4x4, 2 door extended cab, with a 460 V8. I bought this truck earlier this year from the original owner. It has just 90,000 miles and runs strong.
All electronics seem to work except the power door locks. Neither door lock works and there is no noise from the door area when pressing the lock button, so I figured it was just a fuse. I swapped the door lock fuse with the power window fuse (same amps, same part # on fuses). Locks still don't work.
Upon further inspection I see there are no wires to the backside of the fuse box behind the PDL fuse! And I see no obvious loose wires hanging around.
What color wires am I looking for? Do the wires from the lock actuators in both doors go to a central point behind the dash then to the single spot in the fuse box?
Thanks.

I inserted a twist tie next to the fuse to make it easy to find from the rear.
Back side of fuse box. Just to the left of the white twist tie is where the PDL wires should be.

 

The wiring:



/

 

Thanks for the diagram sunford. I'll check it out tomorrow.
If I am reading the diagram correctly, only 1 wire (BK/W) runs to the fuse box for the PDL?

 

Yes you are reading it correctly, a black wire with a white stripe.
An odd color for a power wire though.

 

The only black wire with a white stripe I can find is this. The Blk/wht wire and a thicker all black wire both run through this oblong shaped plastic (no idea what it's called). Then both wires run to the grounding spot on the dash sheet metal where the fuse box is mounted.


Both wires connected to ground with same lead. Looks like factory? I assume both are grounds? Arrow points to Blk/Wht wire.The white wire with the yellow lead runs to the trailer brake controller but is cut off at the other end so does nothing. There is another all black wire grounded at this spot as well.


I don't suppose anyone with a similar year/model can check behind their fuse box to see if you have the same all black and blk/wht wires grounded like this?

 

This is the black/white wire your looking for and the pin is right beside the zap strap


the door lock motors do like to seize up on the older trucks or the wires in the door jam break, I would start there and put the breakers back in for the door locks and Windows, they should look like tiny metal cans

 

ifry, I believe you are correct. What threw me off is that wire is above the fuse contact points, not next to it where I had the twist tie. And I did not notice it was blk/wht from the pic. You have to be a contortionist to get under the dash to see with your own eyes - I'm not that flexible anymore.
I had to go buy a new multimeter, I think my brother "borrowed" my last one. I do see the fuse contact has 12 volt power so the fuse box is not the issue. I will take the door panels off and see what I can find.
I guess this is not so unusual after all.
Thanks all.