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Thread: Tailgate lock motors

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    Default Tailgate lock motors

    TAILGATE (OR OTHER) CENTRAL LOCK MOTORS
    Preventing Burnout

    I have had 3 tailgate lock motors fail on a Ford Escape / Mazda Tribute SUV - the factory fitted one after the car was around 10 years old, and two replacements subsequently, about 3 years apart.

    This is what I have done to address a problem that is outrageously expensive to have fixed by a garage, and likely to recur. I was quoted AUD$800.00 to replace the entire tailgate lock mechanism, whereas the easily accessible and replaceable failed element costs under AUD$5.00, and the difference in time to fit an entire assembly compared to the time to pull the assembly apart and replace just the failed element was around 30 minutes. In fact the total repair time turned out to be only around one hour. To me that's a return rate of around $795.00 per hour and I'm happy to work at that pay rate.

    I am not blaming the garage for their quote. It is likely they are being similarly gouged by their automotive parts suppler. It all comes down to the modern unwillingness to analyse a problem when a no-thought replace-the-whole-damn-thing quick fix is available. In electronics it's the choice between board (or assembly) level replacement, vs component replacement. There's no definitive answer to that quandary, but I tend to poke and pry before throwing away, which is why I offer this story.

    I found replacement locks that looked identical to, and claimed to be from the well respected Mabuchi company, which is the same brand as the OEM motor. Mabuchi seem to be the lock motor supplier of choice for many brands and models of car.

    There is no limit switch to cut off power once the lock has actuated: 12 volts is applied for around 2-3 seconds whenever the central locking is activated. It only takes around 500mS to fully lock or unlock the mechanism, so the remaining time the motor spends in stall conditions, which means maximum current, heating and, so it seems, eventual burn out.

    After the first failure (and baulking at the quoted repair of $800.00) I pulled the tailgate lock assembly apart, and found the only failed component was the motor. These were readily available for between $3.00 and $4.00 from Ebay and other markets. I purchased two - one for spare - just in case.

    After the failure of the original I simply replaced the motor. After the failure of the second motor, again some 3 years after the first, I looked into the failure mode more closely and this time fitted a poly-fuse (PTC resistor) rated at a little below the stall current. Poly-fuses act like a wire link when cold, but when heated, as by a current higher than their fuse rating, they switch quickly to a much higher resistance state so limiting current. When they cool, they return to their low resistance state. Poly-fuses have a limited number of cycles before their on state resistance goes higher than when new, but it is still in the thousands of cycles depending on the degree of over-current. And a poly-fuse is 11 cents. Suffice to say the second motor also failed after about 3 years, and the poly-fuse had suffered too, increasing it's on state resistance.

    I have used a poly-fuse very successfully as the lock motor protection for a self fitted centrally locked motorhome, where I was concerned that the radio unlock signal might be sustained if, for example, I inadvertently sat on the transmitter and continuously activated its button. The radio control kit I was using did have a timed option, but that time was much longer than that needed to run the lock motor, so it afforded little protection.

    The idea of applying current limiting after a short delay still seems the best solution to protect such a stalled DC motor, but for the third replacement I wanted to avoid the change in poly-fuse on state resistance that I had measured, so I elected to use an 18Watt 12volt tail lamp in series instead. This functions much the same way as a poly-fuse (the resistance is low when cold, but goes higher when heated). Apart from filament failure a lamp has no comparably limited life. By the way, the choice of poly-fuse over lamp the first time round was because I had both on hand but the poly-fuse was a smaller, neater fit...and I had used one before)

    Because a lamp's resistance-current-time characteristic is non linear, choice of a suitable lamp is best left as a matter of trial and observation. The lamp shows no brightening for the milliseconds that the motor is driving the mechanism, but brightens to almost full for the remaining second or so while the motor is stalled. After that the car's central locking system removes power anyway.

    A more elegant solution may have been to reduce the time the lock is energised, but not only is access to the firmware of the car computer fraught, but I think it reasonable to assume the manufacturers have chosen the lock actuation time to be that which works best for most combinations of battery voltage, locks, and motors.

    The simplest solution available in a back yard situation was to address the problems of the one lock that has been failing, and leave the rest alone.

    The details of my fix are as follow. Your vehicle may differ, but almost certainly only in details.

    • Remove the actuator housing from the tailgate (3 screws, and a wiring connector)
    • Remove the motor housing cover (more screws)
    • Take some time to see how the whole thing works, and where the loose components go.
    • Remove the motor - carefully - as there are small components and gears that can fall out of the rest of the assembly
    • Measure the motor dimensions - particularly the shaft length and diameter, so you can order the right replacement.
    • Go searching (Ebay, Ali, wherever you like)
    • Lay in some beer and drink it over the weeks you will wait for your motors* to show up.
    • Optional: refit the whole thing while waiting because Time and Beer are not Memory's friend
    • When your motors arrive back to work....
    • There is a keyway press-fitted onto the motor shaft. It comes off, but needs considerable persuasion. Be careful not to shatter or deform it. You will need it later to fit onto the shaft of the new motor. Because your motor is already screwed don't worry about destroying it utterly in getting the keyway off.
    • Press the keyway onto the shaft of the new motor. Best way is applying a vice between the shaft's ends. DON'T press between one shaft end and the motor: That only puts stress on the rotor and bearings. Avoid hammers and other shaft bending implements.
    • Reassemble into the actuator housing and test. (Careful the whole thing doesn't sproing apart if the cover is still off)
    • Refit into the tailgate.
    • Check again that it all works



    Adding Protection

    • Solder some insulated wires onto the terminals of a single filament tail light lamp** (NOT an LED type)
    • Fit the lamp inside a short length of electrical conduit, or similar tubing. A snug fit is good but not imperative
    • You might care to insulate it thermally from direct contact with the inside of the plastic conduit with a bit of aluminium foil or thin sheet. This is purely precautionary as the lamp (should) never be on long enough to ever get hotter than touchable. I just don't like any glass lamp envelope in contact with any kind of plastic.
    • Run the lamp in series with the lock motor. Polarity is irrelevant.
    • Test it and confirm the lock operates the same way with or without the series protection. Listening to the sounds it makes before and after is as good a test as any. There should be no difference. If the motor seems sluggish then the lamp is too low wattage. Try a higher power lamp.
    • Confirm that the lamp comes on brightly if lock power is sustained, and that in that condition the lock current is significantly less than when measured stalled without the lamp. Mine measured less than 25% of stall current. If you don't have a multimeter or know how to use it you can omit that step.
    • If the lamp is not a snug fit in the conduit, then use some kind of non flammable / non melting insulation to prevent it rattling inside its housing.
    • Bog the ends up*** so the lamp cannot fall out.
    • Cable tie or otherwise fix the lamp housing to the panel work of the tailgate. I used cable ties, and put it inside the double skin.
    • Cable tie the second motor somewhere close - just in case, and include an explanatory note for yourself (recall the time/beer/memory dependency) or for the next owner. Hopefully you will never see or need it again, or you will sell the car before ever it is needed.
    • Replace the trim and test again.
    • Feel satisfied, then see if there's any beer left.


    Q.E.D.



    * Buy two - they're cheap enough and you never know.....

    ** I found a 12V 18W globe just right, but YMMV. And why a lamp and not a resistor? Because a resistor is a constant resistance and a lamp has the right non-linear voltage/current characteristics for this application.

    *** I used hot melt glue - just to show the whole thing never gets hot. Two end caps, some non flammable wadding, even heating and squeezing the ends a bit will do to stop the lamp falling out.

  2. The Following 2 Users Say Thank You to PAFMelb For This Useful Post:

    fandtm666 (15-07-20),VroomVroom (20-07-20)



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