Troubleshoot and Fix Power Door Locks

Diagnose and fix power door locks that don’t work

Power door lock problems most often caused by a bad actuator or relay. In rarer case, the problem can be a bad switch or broken wiring in the door hinge area. I’ll give you some tips on how to fix power door locks on your vehicle.

Start by testing the actuator

This is a typical door lock actuator. The linkage connects to the door latch lock.

Keep in mind that all power door lock actuators have the same number of cycles on them; they all open the same number of times and close the same number of times. So, if you find one failed actuator and replace it, don’t be surprised if the others also start failing.

A lock actuator is a simple motor or solenoid device. The door lock switch toggles power and ground to the actuator to make it lock or unlock.

Remove the door trim panel on the problem door. Disconnect the electrical connector to the actuator. Connect a multimeter to the pins in the connector. Then operate the door lock switch. You should see +12 volts or -12 volts and the reading should toggle as you move the switch from lock to unlock. If you see the toggling, the switch and all the wiring are good and the problem is a bad power door lock actuator.

If you don’t see voltage or it doesn’t toggle, you’ll have to refer to a wiring diagram to locate the fuses and see if it uses relays.

Refer to a wiring diagram

In this setup you see two relays (door unlock and door lock) and a mess of wiring that also connects to the remote control receiver that unlocks and locks doors from your keyfob.

Here’s how this setup works

Power flows the center of each door lock switch. When you move the switch to unlock the doors, power switches to the unlock contact. From there, power flows to the input side of the control coil of the unlock relay. The ground side of the control coil has constant ground, so by applying power to the input side of the unlock control coil, the relay pulls the contacts closed which sends power to the door lock actuator circuit breaker. Power flows through the circuit breaker and into the power door lock actuator motor. To get ground, the ground path goes through the non-energized power door LOCK relay (which is in the ground position when not energized) and then to ground. The process is similar when you move the switch to the lock position.

When you use the key fob to lock or unlock the doors, the receiver provides power to the unlock and lock control coils and the system operates just like if you had used the door switches.

What to check

Start by checking the fuse. Then check for power at each door switch. If you seen battery voltage, operate the switch and check for power coming into the control coil on either the lock or unlock relay. If the relay is getting power when you operate the power door switch but the doors don’t lock or unlock, check the control coil ground. If the ground is good, remove the door trim panel and check for power and ground coming into the power door lock actuator. If the relay and switch are working properly but you’re not getting power and ground at the door lock actuator, check the wiring harness in the hinge area of the door. That’s where most wire breaks occur.

Here’s how a body control module controlled door lock system works

Here’s a power door lock wiring diagram for a 2009 Chevrolet Malibu.

The body control module (BCM) supplies a voltage to each door lock switch. When you move the switch to the lock position, the switch grounds the supply voltage and the BCM sees a full voltage drop. It interprets a full voltage drop as your request to lock the doors.

When you move the switch to the unlock position, the switch sends power to a 1.5KΩ resistor and then to ground. So the BCM sees a partial voltage drop. It interprets that partial voltage drop as your request to lock the doors.

To diagnose this system simply disconnect the door lock switches one at a time. If the door locks work after removing a switch, replace the switch you just removed–it is shorted inside. Done.

If a single door actuator doesn’t work, check the wiring harness for that actuator.

©, 2019 Rick Muscoplat