Power Door Locks

Security Button – Power Door Locks Then and Now

According to at least one internet source, the first power door locks appeared in 1914 on the now-obscure and mostly forgotten Scripps-Booth motorcar, but power locks didn’t catch on as a common option until cars until 1956 when Packard included them on one particular luxury model. It’s interesting that you can still buy a 1956 Packard power door lock actuator on eBay ($9.95).

Today, power door locks are as ubiquitous as power windows, and just about anybody who has wrenched on cars has dealt with these amenities, particularly when a  customer is accustomed to using the button on a fob to lock and unlock his ride. Volvo put a heartbeat monitor on some key fobs to inform an approaching owner of someone who might be lurking in their vehicle.  GM has a fob for its high end SUVs that will have a liquid crystal screen that will enable the customer to remotely check tire pressures, fuel level and odometer readings, radio station settings, and more. Further, these GM fobs will have four to six times the range of a conventional fob.  Power locks make us safer, and while their failure isn’t as much an irritant to most customers as an inoperative power window, they typically enjoy a fairly prominent place on repair orders when a car comes in with inoperative locks.

To troubleshoot any power door lock system, understanding that particular system is key (pun intended).  In the beginning, power locks were basically reversible motors with a threaded armature that would spin around a rod with matching threads. Spinning the armature moves the rod.  The other end of that rod is connected to the part of the door latch mechanism that operates the lock.  Lock versus unlock is handled by simple polarity reversal, and there isn’t anything mysterious about that element of power locks unless you’re electrically challenged.


Troubleshooting Older Systems

On the older systems, power lock switches were a lot like power window switches – positive and negative feeds to the switch are delivered to the actuators with first one polarity and then the other when the switch position is changed from lock to unlock. positions.

Here’s a schematic of door lock systems as they used to be. B+ power is fed through a circuit breaker to the switches and works in tandem with a ground fed to those same switches. Changing the switch position changes polarity at the lock actuators, which have built in circuit breakers to prevent actuator damage if the kids get to happy with the lock buttons. It’s important to note that the switches are wired in series, so if the normally closed contacts on the driver side switch fail, the passenger side switch can be rendered inoperative. The logical best place to test for power and ground is between the second switch and the actuators.

To troubleshoot those, you check for power and ground at the master switch first, and if you have good voltage there you connect a test light or a meter in parallel with the lock actuators at an easy access point (check the schematic and test at a point downstream of both switches, since they’re wired in series), operate the lock switch, then look for battery voltage. If that’s good all the way to the actuator, look for problems with the actuators themselves. They fail fairly regularly. If the light or meter reads substantially less than battery voltage during your test, use your electrical know-how to determine whether the missing element is power or ground.  Both sides of the actuator should show ground with the switches at rest. If the passenger switch is the only one that doesn’t work, realize that a bad driver side switch can have corroded normally closed contacts and prevent a passenger side lock switch from operating, so disconnect the actuator to break the circuit loop and, with all switches at their rest positions, check for a ground at both connector terminals on the actuator side of the switch.  If one side is missing a ground, check both feeds coming from the driver side switch.  If that ground is still missing, check that circuit feed at the driver side switch.  No ground?  That’ll finger a bad switch.  If the ground is there but isn’t at the passenger door switch you have an open circuit between the two switches.


The BCM Factor

Today’s power locks are computer-controlled on most higher end cars, with the Body Control Module (BCM) receiving inputs from the door switches, external key pad, or the fob.  With the BCM a part of the picture, snazzy things can be programmed in. Example: The door locks can be locked either when the vehicle is shifted into drive or after a certain road speed is reached (15-20 mph). Another cool deal is that if the BCM knows the airbag has been deployed, many of the modern platforms will unlock the doors to allow egress for first responders.  Furthermore, if the BCM sees the key in the ignition and an unwary driver locks the doors and closes the driver’s door, the BCM will unlock that door to prevent a lockout.  Ford Taurus and Sable have had that function built into their GEM modules since 1996.  Ford’s GEM module did the job for awhile, and the latest Ford vehicles use some of the circuits in a Smart Junction Box (a fuse panel with a built in computer) for door lock operation. In a similar fashion, late model Asian vehicles typically have the door lock relays built into the fuse panel so the fuse panel needs replacing if one of those dandy little relays goes south.

Early Remote Keyless Entry systems led engineers to add a pair of relays to the door lock system – in these systems, the door lock switches aren’t hard wired to the actuators –they’re simpler and lighter, and they operate a pair of relays with each side of the door lock motors connected to the relay common terminals. With this arrangement, when either relay is operated with the other relay at rest, the triggered relay sends a 12 volt burst to one side of the actuators while the other side remains grounded through the normally closed contacts in the non-triggered relay. Rocking the switch toward “Lock” toggles the Lock Relay, and rocking the switch the other way triggers the Unlock Relay. Most cars have a dedicated relay for the driver’s door that is driven by itself when the RKE is used (for security purposes), but this schematic doesn’t show that.

Some 2004-05 model Chrysler Vans have a programming error whereby the door locks can simply stop responding. When I scan tool one of those vehicles, I notice that the BCM is receiving commands from the switches and the fob but it simply won’t operate the doors, not even when commanded to do so by the scan tool Active Command function.  Removing the battery terminals and shorting them together to reboot the door locks typically straightens that mess out, but the Chrysler folks recommend a reflash.


The 2006 Sonata

This Korean made Hyundai Sonata was built about 80 miles north of the place where I work, and when the door lock function went away, we dove into the factory shop manual looking for answers.  What we found was a BCM that takes input from the switches (0.6 volt is fed by the BCM to each side of each switch, which shorts the voltage to ground with switch operation), then the BCM triggers micro relays in the interior junction box/fuse panel to drive the locks. The BCM also knows if the actuators are in the lock or unlock position, even if they were operated by hand (most modern systems are designed this way).  Well, the scan tool showed us everything was copasetic regarding the inputs and BCM outputs. The BCM was fully aware of lock and unlock switch inputs and was actually sending commands to the appropriate relays in the junction box whenever the lock or unlock switches were activated.

There were times when the door locks would actually try to work, so in obedience to the infinite wisdom of the Hyundai service engineers, we removed the junction box. The shop manual gives no instructions on how this is to be done, by the way, and it’s something of a pain to get if off. The knee bolster has to be removed along with several screws securing the panel. There are a half-dozen or so wire connectors that have to be disconnected (some of them behind an annoying plastic cover that can’t be removed until the box is unbolted).  Well, we finally got it off and bench tested the box using the wiring schematic and common sense.  Our investigation seemed to indicate that the relays were all functional, so we were done with our bench test and we reinstalled the panel.

With these EZ Hook ® probes connected to the door lock actuator we were able to connect a meter and eventually an oscilloscope to measure what looked like acceptable voltage coming to the actuators from the junction box, but the locks still wouldn’t work. This voltage was slightly lower than system voltage, which indicated a voltage drop problem.
Following Hyundai shop manual procedures and disconnecting the driver door harness connector to isolate this circuit, we applied battery voltage directly to the actuator wires and saw crisp action. With the measured voltage the same at the junction box as at the actuators (11.4) and system voltage at 12.4, we determined that there was a voltage drop concern inside the junction box.

The first time we operated the locks after reinstalling the panel and reconnecting the battery, some of the locks worked on the initial attempt, but they wouldn’t work the second time.  This led Amanda to remove the passenger side door panel for access to the wires that lead to the actuator in that door. Using some EZ Hook ® probes, she accessed the wires and we connected a meter, which showed 11.4 volts being delivered to that door on the lock AND unlock commands, yet the actuator was largely unresponsive.  System voltage was 12.4 volts – we were losing a volt somewhere. Connecting an OTC Solarity scope to the two EZ Hook leads, I selected a couple of channels and operated the locks – the trace showed that the polarity was switching nicely, but lock operation was nothing more than a mild bump – sometimes one door or another one would lock or unlock but that was the exception rather than the rule.  Since both legs on the actuators are grounded with the junction box relays at rest, Amanda disconnected the door harness connector and we delivered voltage straight to the actuator – it operated very crisply.

Final analysis revealed voltage drop somewhere inside the junction box. We didn’t catch this on the bench test because we were using a low impedance test light to test the secondary relay circuits, which wasn’t adequately loading the relays.

Calling the local Hyundai parts department, I was smacked with a $747 price tag for the junction box.


The practice of building relays into the junction box is becoming more common than ever before, thus the cost of junction boxes (particularly ‘smart’ junction boxes) is rising exponentially.   What each customer has to decide is how much their door locks are worth.

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