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Power door locks don’t work

Power door locks don’t work condition

Trying to diagnose and fix power door locks that don’t work can seem like a daunting job. And it can be on older vehicles where the wiring was far more complicated. This post can make it a little easier to diagnose and fix power door locks that don’t work or work intermittently. Here’s the power door lock wiring diagram for a 2000 Chevrolet Malibu

power door locks

In this setup you’ve got 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 those power door locks work

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.

Diagnose those early power door locks

Start by checking the fuse. Then check for power at each door switch. If you seen battery voltage, operated 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 power door lock system works

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

power door locks

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

Posted on by Rick Muscoplat

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