There is a common problem with the door lock problems on 1st gen w-bodies where the door lock actuator operates but does not lock or unlock the door (and often times the door won't work really well from the interior lock knob as well). This problem comes up in threads every once in awhile and I just had a member PM me in my part-out thread to sell him the door lock actuators, so I'm making a thread. New door lock actuators will not fix this problem, or if they do it is usually only temporary and they will go back to doing the same thing. If the actuators are moving in and out electrically, they are most likely OK. There is a common problem with these older style actuator systems GM used from the 70s through the 90s in that something in the linkages or the latch that gets too much slack. I'm not exactly sure what or why they get like this.
Here's the easy fix though:
On a w-body remove the door panel, and find this metal pivot piece the door lock actuator attaches to. First take a pliers and squeeze the tangs that are covered in rubber so that they are closer together. They can only be squeezed so much, so then take 2 zipties and tie 1 around each of the tangs. All we are doing is taking up the large gap in this pivot so that the lock rod doesn't have to move so damn far before it is actually moving anything on the latch. It is also a good idea to lube up everything while you have the door panel off. Lube the pivot piece in the picture, put some grease on the channels the lock rods run in and also lube up the latch mechanism really well inside the door. You could also lube up your window regulator and guides and get your window to move faster while you're in the door. To reinstall your panel, it is always a good idea to get new clips. These can be purchased cheaply at any auto parts store usually out in the aisles somewhere sold in small packs, just take an old one and match it up.
I have no idea why these systems were designed with that large gap in the pivot piece, but I have fixed several doors with this problem. Strangely my 96 GTP with 200,000 miles on it still works ok, but neither door of either the 90 GP SE in the picture or my 87 Monte Carlo (which has that same actuator & system design GM used for 20 years) had working power door locks, but all the actuators operated. Performing this repair fixed the lock problems, no purchasing of actuators required. I hope this helps some of you as well.
