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Automatic Door Locks

Replacing old OE part with a new actuator

We made a list of all the subsystems on our old Mustang. It's pretty long. What does this mean? More subthings to break. It's Murphy's Law of Coincidental Failure. Parts will degrade and at the most inconvenient moment they'll break. The trick is to stay ahead of what we call the "inclined degradation and breakage curve."

How They Work

Consider your automatic door locks. They're driven by an electric motor hidden within the rearward lower corner of the car door (note: access is a royal pain) and linked to the door lock mechanism by a length of forged helical rod steel. That motor is isolated by two rubber grommets, suspended in a plastic bracket. When the motor turns one way (hit the unlock button), it winds in the length of steel and pulls the locking lever down, disengaging the lock and raising the manual lock button. The opposite selection on the automatic lock button reverses the motor, extends the rod, engages the lock and lowers the thingy.

When this motor fails, it's usually because age causes the threads on the shaft to wear off; thus, the motor can't raise and lower said shaft—it grinds and buzzes instead. You'd best replace both locks even if just one breaks because logic suggests the other one is close behind (get ahead of the breakage curve). Figure the extra amperage available to the second motor when the first isn't working gives #2 the extra oomph to make chunks of its threaded shaft.

The Joy

Frankly, working inside the doors of any car built in the U.S. from '60-something to '90-something is about as pleasurable as setting a compound fracture in your kitchen. Difficult access, loads of rough metal panels rarely factory-de-burred and about as sensibly arranged as VCR instructions in a foreign language. Problem is, the mechanisms within have been installed once and that's it—meant never to be removed or repaired. This is in line with the prevailing, "A car should last 100,000 miles" design theory that pervaded much of the products turned out by the Big Three during the high-V, low-Q Halcyon days beginning roughly with Vietnam, lasting through the end of the Cold War. Currently, a much more level automotive playing universe has brought to bear revised standards for functionality and longevity, but that doesn't make replacing the door-lock motors in our Fox-body Mustang any less of a pain.

Shade-tree Shady

This is one of those little things you think you can ignore, the automatic door locks, so at first when they fail you just manually unlock the door and have a nice day. Cars age, right? Funny thing about even the simplest technology, it'll sneak up on you�and suddenly the driver's side door-lock actuator freezes up, shorts out the circuit and you're left to climb through the hatch, so you can let your date into the car. Golden moment there. So swallow your wallet just a little and pick up some new actuators and a box of Band-Aids.

To the OE manufacturer's chagrin, there's a host of aftermarket and small-scale parts suppliers ready and willing to ante up better hardware in place of the oft overpriced and less functional stuff the factory puts out. Door lock actuators for the Mustang are one of these parts (the factory drivers are pricey and slow). Companies like AutoLoc (800.873.4038, autoloc.com) build a solenoid that does the job more quietly, more quickly and for less cash. We ordered the whole kit for our Mustang, and it cost less than a cheap night at the pub.

The Installation

The tricky parts of this gig included figuring out just how to remove the factory lock motor from its bracket, and deciding how to locate the new door-lock solenoid. Turns out a bit of leverage and a twist popped it out of the two-sided plastic receiver, and of the two locations available for the solenoid one proved easier to make the lock operate correctly.

Getting into the door cavity is not as difficult with the Fox-body Mustangs ('79-'93) as with many other Ford or GM products, but you're still worming your way past a lot of metal. A suggestion for those of you using a Dynamat-style product to keep road noise out—installing it in smaller sections makes removal for this sort of work less complicated. Stock up on those little plastic door-panel fasteners if you plan to be doing stuff inside your car doors to parts like stereo speakers.

When you remove the old lock motor, the bracket it lived in is obvious and you can choose to mount the solenoid in that spot. A long bolt can be used to park the solenoid in between the bracket forks, but this will place more stress on the actuator rod, as it will then also function to keep the solenoid from shifting excessively. Our suggestion is to attach the AutoLoc unit to the inside of the inner door panel in line with the OEM manual lock shaft. This shaft runs through the door lock mechanism, and arranging the solenoid to operate this portion of the door lock system works just as well. Two Phillips screws through two well placed holes and the new actuator is fixed in place, allowing the Allen-head tie-block to do just that, tie the new actuator to the manual lock mechanism.

A few thoughts: make sure you've tested the wires (just two of them) so the "Unlock" on the lock switch corresponds to a similar door-lock movement. Consider that the new solenoid has a longer stroke than the OEM actuator, and take this into account when locating the unit. Be patient while aligning the piano-style wire rods so you don't wear them out (and yourself) bending and re-bending to match the factory hardware. Don't be afraid to order new window motors too, if you plan to be in the door for a while, because by the time those OEM door-lock motors wear out (100,000-ish miles), the window motors will be getting tired. And wearing a long sleeve shirt will probably help save money on bandages.