Very strange door problem, all doors will unlock via the remote key pendents,
they will unlock from drivers door master unit, they will unlock from passenger door, but the will not lock. Went to junk yard and got used master controller for drivers door and same problem. I must go from door to door to lock manually, rear tailgate must be locked via the key. Is there a some mystery to this problem. Please Help.

 

Sounds like a wiring problem but without knowing the year and whether this system is stock I can only guess to help you further. Check for voltage at the 16 gauge orange wire at the switch.

 

You know, I kind of had this problem and somehow it got solved by itself, and I didn't know what caused it, probably due to a false contact because I installed a Viper alarm and hooked the alarm's built in lock/unlock relay wires. When I had this issue, I took both door swithces off and checked them on both sides, then with a light tester, found out that the passenger's side door lock switch was not sending the signal to lock the doors, then I gave up, turned on and then off the truck and they worked back again and they have never failed again.

If the truck is between '88 and '92 (I'm not quite sure on the wiring of older models), it should have the reversing-polarity switches, which eliminates (I guess) the use of relays, so I think the switches of both doors depend upon each other somehow, so if any switch goes bad the circuit is not completed, thus you don't get the signal.

In order to test the door lock switches, take the switch off but do not disconnect it. On the back of the switch you will see 5 holes with some sort of wire terminals arranged as a 5 on a dice. You should also have 5 wires: red, orange, pink and other 2 wires, (which I don't remember the colors) where orange and pink are the lock/unlock signals and the red wire is a constant 12V. Clasp the light tester to a ground place, then you should have a light on the center hole, which means the red wire is hot. Then, press the switch in order to lock the doors and test the 2 terminal holes on the top of the switch (or using a little common sense, you will determine which hole belongs to the orange wire). You should have a light on the tester if the switch is OK, if not, either the wiring must be checked (which is going to be a painstaking task) or the switch must be replaced.

I hope this info, complements other replies or posts you may get about this problem.

Good luck.

Good luck!