Auto Repair Help
Reiner’s Corner – Auto Repair Help
DIAGNOSE – POWER DOOR LOCKS DON’T WORK PROPERLY
PART 2

By Reiner
Continued from Part 1
POWER DOOR LOCK REPAIR TIPS & TRICKS

Apart from what has been stated in Part 1, diagnosing problems with power door locks is not always a straightforward affair, but one way to avoid confusion and misdiagnoses is to approach the problem logically. Thus, ask yourself the following questions in the order they are presented here, address one issue at a time, and base your actions on the repair options provided in this guide for each question. Also, be sure to check if the problem is resolved after each step both to ensure a successful repair, and to save time.

Are all the door locks affected?
  • If the key fob does not unlock any of the doors, check that the battery in the fob is serviceable, and replace it if there is any doubt about its condition.
  • If the fob battery is new but the door locks remain inoperable, consult the manual to locate the relevant fuse(s), and replace as required with fuses of the CORRECT rating. Note however that blown fuses usually indicate a short circuit, so do NOT replace fuses until the short circuit had been found and repaired.
Do the door locks work now?
  • If the fob battery is new and there are no blown fuses, it is possible that the key fob had lost some or all of its programming. This happens more often than you might think, so have the fob’s programming checked by the dealer or other competent repair shop. In some cases, reprogramming the fob resolves this kind of issue even if there are no obvious programming issues present.
Are the door locks still not working?

If none of the above steps resolves the issue, suspect either a defective door lock switch, or wiring issues. Proceed as follows-

  • Consult the manual on how to remove the switch that operates all the door locks from where it is mounted, and determine the function and color-coding of all the wires that are connected to the switch. Identify the wire that supplies power to the switch, and use the multimeter to verify that the correct voltage is present. If there is no voltage on the input wire, refer to the wiring diagram to find and repair the open circuit.
  • If the input voltage is significantly below the specified value, find and repair the abnormal resistance. This kind of problem usually (but not always) occurs as the result of poor connections across connectors and/or slip joints in wiring.
  • If the correct input voltage is present, it is likely that the switch is defective. However, it is also possible that there is a communication failure between the key fob, the module that controls the central locking/anti/theft system, and the switch. Do not replace the switch (which is expensive) until you are certain that there are no communication issues between control modules. Verifying this is best left to the dealer or other competent repair shop that has access to diagnostic equipment that can access control modules and the CAN (Controller Area Network) bus system.
NOTE: The steps outlined above will usually resolve the problem if all the door locks are affected. Proceed as follows if only some door locks are affected-

Do the affected door locks fail to work consistently?

  • If the failure is consistent, suspect either a wiring issue, or defective door lock actuator, but note that both possible issues require removal of the door panel. Consult the manual on the correct procedure to follow to remove the panel to prevent damage to the panel. Remember to roll up the power window before removing the door panel.
  • Once the door panel is removed and placed safely out of the way, inspect all visible wiring for signs of damage such as chafing, rubbing, burning, or bad connections. Make repairs or replace wiring as required.
  • If no damage to wiring is apparent, consult the manual or wiring diagram to determine the color-coding and function of all wiring that is associated with the door lock actuator. Disconnect the wiring from both the actuator and the main wiring harness and perform resistance and continuity checks to verify the condition of the wiring. Compare all obtained readings with the values stated in the manual, and repair or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

NOTE#1: Pay particular attention to the wiring in the boot between the doorframe and the door itself. Continual flexing of the wiring as the door is opened and closed can sometimes cause conductors within the plastic insulation to fail, with no sign of the failure being apparent during a casual visual inspection of the wiring.

NOTE#2: Be sure to include the affected actuator(s) itself/themselves in this step. Measure the resistance across the two terminals on the actuator(s), and replace it/them if there is no continuity, or if the measured resistance deviates from the value(s) stated in the manual by more than a few percent.

Does the manual switch now activate the affected actuator(s)?

  • Reconnect all wiring, and operate the door lock manually with the switches in the dashboard/console/door panel. All switches should now activate the actuator(s), provided the actuator(s) itself/themselves are in perfect working order. However, since door lock actuators work in both directions because the polarity of their input circuits are reversed by the switches that operate them, it is possible that a switch many be defective if it works in only one direction. For instance, an actuator either locks or unlocks a door, but does not both lock and unlock the door if the switch is manipulated.
  • One way to check if a switch is defective is to connect current from a battery charger directly to the lock actuator. If the actuator works in both directions when the polarity is reversed, the switch is defective.
Do some door locks fail intermittently?

If one or more power door locks fail intermittently, the cause is almost always bad electrical connections. Note though that some types of intermittent failures can be extremely challenging even for professional technicians to find and repair, so if an intermittent fault is suspected, repeat all resistance and continuity tests on all wiring (while vigorously wiggling) the wiring until the fault is found and repaired. Alternatively, refer the vehicle to a competent repair facility for professional diagnosis and repair.

DIAGNOSING MECHANICAL PROBLEMS

Note that many problems with power door locks involve mechanical failure(s) of one or more components, with sticking and/or binding moving parts following close behind. One particular problem, that of locks cycling rapidly (especially on some Honda models) is described below-

Locks that cycle rapidly

On some applications that are fitted with door locks that use DC motors, the control module depends on a voltage spike that occurs when the actuating lever reaches a built-in stop. In these cases, the actuating lever comes to rest against a rubber stop in the switch casing at the end of its travel, which causes a voltage spike that the control module interprets as a signal that the door lock had either locked or unlocked the door.

In many cases, the built-in stop either breaks off or wears away after long use, thus failing to produce the expected voltage spike. When the control module does not receive the expected voltage spike, it either interprets the lack of the signal as a failure of the system, or more commonly, to lock and unlock the door in rapid succession for as long as power is applied to the affected actuator.

Note that the only reliable, long-term remedy for this condition is to replace the actuator with an OEM replacement part to ensure proper operation of the door lock.

Actuators that move slowly

This happens more often on actuators that use DC motors and gears than on actuators that use solenoids. On geared actuators, either moving/rotating parts wear out after long use causing them to bind or stick, or the lubrication inside them dries out, causing the entire unit to bind. Replacement of the door lock actuator is the only reliable remedy.

An example of a typical geared power door lock actuator is shown below. Note the presence of lubricant on the gears that can cause the unit to fail when it dries out, or to fail due to excessive mechanical wear on one or more parts.


Other mechanical issues include mechanical linkages that bind or stick, or door latches that stick or bind due to the build-up of dust and dirt inside the mechanism. Ideally, door latches should not be lubricated, but many mechanics spray all manner of lubricants onto (and into) door latches in efforts to “cure” some power door lock issues. Since few car doors are dustproof, dust and dirt eventually combine with excess lubricant to form a sort of “glue” that inhibits the free movement of parts in the door latch mechanism.

To check if the door latch mechanism is the problem, disconnect the mechanical linkages that connect the power door lock actuator to the door latch mechanism, and activate the door lock. If the actuator now moves freely, the door latch mechanism is sticky or binding. Replacement of the door latch mechanism is the only reliable long-term solution.

NOTE: In some cases, the power door lock actuator is incorporated into the door latch mechanism. On these applications, the entire assembly must be replaced with an OEM replacement part to ensure proper operation of the locking system.

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