OK, I finally got a chance to return to my door lock problem. Here's a detailed solution.
Summary: Door locks act weak and barely work if at all.
My findings: Carbon powder from the motor brushes accumulates inside motor providing an alternate path for current.
This, combined with warm days when the motor's thermistor is at a slightly higher resistance will cause weak or nonfunctioning door lock actuators.
Solution: The best solution is to replace the actuators. My dealer quoted $250 for both doors. Actuators can be found for about $30-$50 each on the web. For those on a budget and willing to do a little labor, an alternative is presented here. This involves removing the actuators, removing the motors inside the actuators and opening the motors to clean out the carbon dust.
Disclaimer: I did this repair to both doors on my F-150. The passenger side works great. The driver's side works most of the time but not like new. When it stops working I must wait about 10 seconds for the thermistor to cool down then try again and it usually works. I have not yet tested in hot weather.
I took many pictures throughout this process but I do not have a web site to post them. I can e-mail to somebody who does, however. (I will be gone for a couple of weeks so it will take a while to reply.)
Materials and tools needed:
1. #9 torx
2. long flat blade screwdriver
3. philips screwdriver
4. small short needle nose to open motor case
5. drill and a set of bits
6. 11mm socket and wrench
7. some #4 1/2" sheet metal screws to put actuator back together
8. Electrical cleaning spray (critical)
9. one or two bandages
10. a good facial grimace or two
11. about 2 hours of time
Instructions (2000 F-150 extended cab)
1. Make sure window is up then pop off vertical plastic trim above door panel
2. Pop off trim under inside door handle.
3. Remove two screws found under the trim pieces.
4. Pop up the panel with lock and window buttons. Lift the edge closest to door hinge first.
5. Disconnect connectors
6. Lift up on door panel to remove. Careful, lightbulb will still be attached!
7. Remove lightbulb. (Turn on interior lights and use this light as an aid later.)
8. Pull down the plastic stick-on barrier.
9. Remove 11mm bolt near door edge under latch to free up window guide.
10. Remove two more 11mm nuts to the horizontal metal bracket to make some elbow room.
11. Pop off the plastic lock button on top of rod.
12. Remove inside lock stem. There are two plastic holders. Reach behind the lower holder and squeeze to release. Pull rod down through upper holder. Leave hanging.
13. Use long flat blade screwdriver to snap open the holder for the outside door handle rod. This holder will be at the latch side -- not the door handle end. My snap holder was yellow. Once opened just pull the rod out.
14. The key lock rod is the hardest. Reach a couple of fingers behind the rod then use a flat screwdriver to pry open the plastic holder while pushing the rod out. This holder is hard to see and is hidden behind the window guide. Move the guide a bit to see the retainer.
15. Use a #9 torx to remove three latch screws
16. Now, holding your mouth just right, wiggle and turn the latch assembly to get it out. The rod that went to the door lock may fall off during this step. Remember that the small loop in the rod goes to the lock and the other end fits rather loose into a hole lined with plastic dip coating.
17. Disconnect wires to the actuator portion. Other wires can remain connected.
18. Remove the actuator. Use a flat screwdriver to pry up on a locking tab located between two metal guide rails. While prying up slide the actuator off the rails.
19. Drill out 6 plastic rivets. (Do not remove metal rivets at the end.) Use a drill bit slightly smaller than plastic head. Don't drill too deep. You will use a smaller bit to drill deeper to clear holes for the #4 screws used to reassemble the case.
20. Pry open the case and remove the motor.
21. Bend out metal tabs holding plastic motor back on.
22. You will likely find the inside cap and motor windings covered with a black dust. This comes from the wear of the brushes. This needs to be removed. I used an electrical cleaner called Electro-Clean. Spray your cleaner on and this powder should come off rather nicely.
23. I removed the small thermister element along one side and cleaned it off as well. Remove using a small needle nose. Do not loose this piece! Also, do not replace this piece with a short circuit or you will burn up your motor! The motor is not designed for a 12v stall current of about 4 amps!
24. Assemble motor and replace. Make sure two slots are up for the electrical connection. (Look inside housing and you should see two terminals sticking down that go into motor when the case is closed.)
25. Use 1/2" #4 sheet metal screws to close the case. It may be necessary to first use a small drill bit to clear a hole for the screws at each rivet location.
26. Slide the actuator back in place and connect the wiring.
27. For the fun of it, you can test at this point by reconnecting the door switch temporarily.
28. If the rod for the door lock came off earlier slip it on now.
29. Hold your mouth in whatever fashion worked for step 16 and wiggle the actuator back in place.
30. Before replacing the three torx screws, I reattached the rod for the door lock because it wants to float around a lot.
31. Now, replace three latch screws.
32. Pop the door latch rod into place and snap the keeper over it.
33. Slip the inside lock rod up through the upper holder and replace the lower holder. Put the plastic lock button on the rod now before you forget.
34. Replace 11mm bolt and two nuts.
35. Test again.
36. Replace plastic barrier, door panel and other trim.
37. It's possible that the plastic slot for the manual lock will cause additional friction during unlock. You might need to lubricate slightly or remove door panel and bend the rod a bit.
Hope this works for you! Again, I have not tested in hot weather yet.
Roger
n9sxz@arrl.net