Home UpS Type Lock Actuators

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The central locking on my 2000 V8 S Type didnít work when I bought the car. When operated by the remote or drive-away locking the system tried to lock but immediately unlocked again. From information obtained from the various forums it appeared that the lock actuators were likely to be the cause of this problem. It seems they can fail either mechanically or electrically. A mechanical failure, i.e. something broken in the lock, was indicated if you could hear the lock motor running but nothing happened.

In my case I could see that the front passenger door lock didnít move when I operated the remote but there was no sound so I thought it more likely to be an electrical problem. Most probably the actuator itself was at fault but before diving into the door I made some other checks.

Briefly, the system works like this. The Driverís Door Control Module (located in the driverís door) receives signals from the remote and sends out the appropriate Lock, Double Lock or Unlock command directly to the actuator in that door. It also sends data (Ī 5 volts pp) onto the SCP bus. One of the modules that answers is the Rear Electronic Control Module (located behind the trim in the right hand side of the boot). On receipt of the data signal the RECM decodes it and sends Lock/Unlock commands to the two rear doors and the front passenger door. All the Lock/Unlock/Double Lock commands are in the form of a 12 volt pulse of between Ĺ and 1 second duration.

I checked that data was being received by the RECM and that in response to that data appropriate 12 volt pulses were being sent to the door actuators. All the actuators have a switch in them that closes when the door is shut. Also, the front door actuators indicate their lock status (the rear doors donít at least for the age of my car). So when a lock signal is sent to the front doors the control modules look for a change in the lock status from unlocked to locked. If the modules do not see the correct status the lock command is cancelled which is why with a faulty actuator the central locking immediately unlocks again. If any of the door shut signals are missing the locking system is inhibited.

So having checked that the RECM was functioning correctly I bought a second hand passenger door actuator from JoJags, took off the door card and fitted it. Doing this for the first time it took me about three hours mainly because I wasnít initially prepared to be brutal enough with the interior handle escutcheon. With the replacement actuator installed the door now locked properly and the central locking worked indicating that the original actuator was indeed faulty.

Unfortunately my joy at having a car that could be locked was short lived. Almost exactly two weeks later it stopped working again and this was caused by the second hand actuator having failed like the original one. At this point I didnít ask the supplier to replace it because the wisdom of using salvaged parts for this application was now very much in question. I thought it was time to see what goes wrong with these locks and to see if they could be repaired.

Taking the actuator apart is fairly straightforward if you are determined enough. Start by removing the two ĎCí clips carefully as they are inclined to spring off and disappear. Then work around the various plastic catches, again being very careful as some are easily broken off, until the cover can be unclipped from the rest of the actuator.

This will expose two levers one of which connects to the interior door handle and lock. There is a small spring that pops out from underneath the other lever. Note the position of this spring for refitting (see later). Once the two levers are removed the two Torx screws can be unscrewed and with a bit a wriggling and prising of the metal tang, the plastic housing that contains the motor and all the electrical parts can be removed which will leave the mechanical part of the lock exposed. The main gear (the big blue one) may stay in the housing or remain in the lock. Itís important to note its position for reassembly. I did this by marking it with a Sharpie pen.


Examining the housing you will not find any internal wiring visible because it is all moulded into the plastic. There is a white plastic gear on the end of the motor shaft but again, the motor appears moulded-in. Four very small micro-switches are visible and it seems to be one of these that causes the trouble.


Two of them, C and D (made by a German company called Marquardt) are respectively the Lock/Unlock status and door closed switch. The other two switches are made by Cherry. Switch A is for the Lock/ Double Lock function and switch B switches off the motor at the right point at the end of the Lock function when it finds the shallow section on the side of the main gear.

Normally, this switch is closed until the main gear turns to the shallow section whereupon the switch is allowed to open and so removes the motor supply. This leaves the mechanism in the Locked position whereby the locking pin has been moved out of engagement to disconnect the outer handle but the interior handle can still unlock the door because the arrangement of levers. One pushes the pin back into engagement as the other opens the latch.

However if Double lock is now invoked the main gear rotates further until the peg engages in the Double Lock hook. Once this has happened operating the interior handle can no longer move the locking pin and lever just moves the latch plate against the spring tension but the door cannot be opened from inside or outside the car.

The micro-switch that had failed in both my faulty actuators was switch B and it may be that the premature failure (the manufacturerís specification says it should be good for half a million operations) is caused by the side loading effect of the gear turning rather than simply pressing the switch button in to operate. Alternatively, I might be that any stiffness in the lock or the motor itself would cause more current to be drawn which ultimately exceeds the 2 amp maximum rating of the switch.

I identified the faulty switch as being a Cherry DJ or SJ series and since these are still listed in their catalogue I didnít think it would be too much trouble to obtain a replacement. That turned out to be a false hope. Cherry told me they are manufactured to order and that the smallest quantity would be 4,000. I tried their distributors in the UK, US and Australia but no one had any. A very helpful person at Cherry UK kindly sent me a few samples but other than that they are simply unavailable.

So if these actuators are to be repaired I needed to find an available alternative and given the very small size of this device that is quite an ask. After much research I found that a sub-miniature micro-switch made by Omron could be fitted and that most of the crucial dimensions were at most only 0.35mm different to the Cherry switch and the switch is also rated at 2 amps. The Omron part number is D2HW-BR201DR. You need this exact part number to get the mounting pins and the contacts bent over on the correct side. They are readily available from Farnell (part number 2069681) and other distributors for less than £2 each at the time of writing.

All the micro-switches are soldered in and then back filled with silicone to keep out moisture. It is easy to dig out the silicone which reveals the contacts which can be unsoldered and the faulty switch removed.

The switches mount with two plastic pins which are slightly closer together on the Omron switch than the Cherry one so one of the mounting holes needs to elongated slightly and the pins very slightly shaved down to get them to fit. The pin nearest the switch button is in exactly the same place relative to the button as the original so there is very little work to do to fit the replacement.



I then re-siliconed the connectors.  An electrical grade of silicone is needed here, not the bathroom type as this cures with acetic acid.

Re-assembly is just a matter of making sure that everything goes back as it was. I found it better to put the main gear in the plastic housing to be certain that the switch button was correctly placed against the edge. Also, adjacent to switch C, there is a plastic cam with a spigot. This cam rotates to operate the switch and the spigot goes inside the square end of the lever. I missed this first time and so the lock status switch was absent.

The trickiest part of re-assembly is fitting the little spring that provides an over-centre action on the for the lock lever. After various experiments I found it easiest to put the cover on with the ĎCí clips and then manoeuvre spring back with a spring hook.

After repair the actuator can be tested if you have a 12 volt power supply that can provide approximately 2 amps. I have included a schematic and a plug pinout to assist in this.

All the foregoing relates to the passenger door actuator. The driverís door one will be similar but it will have an extra switch to arm the alarm. The rear doors do not include a Lock/Unlock status switch on vehicles of the age of my car. I donít know if this changed on later MYs. It seems a bit of a strange omission because it means that the RECM cannot detect the result of Lock/Unlock commands sent to the rear doors.

Electrical Description

Switches A and B operate as a pair and they are connected as I have shown in the schematic diagram. The connections to the RECM from pins 2, 4, & 10 are normally electrically grounded by the RECM but each one of these connections are raised briefly (≈1 sec.) to 12 volts as required.

Starting with the passenger door closed and unlocked, on receipt of a lock request via Standard Corporate Protocol (SCP) data, the RECM checks to see if it is in receipt of the appropriate door closed signal from switch D and if the door status switch C indicates unlocked. If these conditions are met a 12 volt pulse is sent to pin 2 of the actuator. Since pin 10 of the actuator is already grounded the motor will run until the main gear cam changes over switch B thus disconnecting the supply. At this point the mechanism is locked from the exterior handle but can still be unlocked by the interior handle.

The RECM will check to ensure that it has received a locked status indication, in other words a ground on pin 3 via pin 6. If the actuator does not indicate Locked status in this way the lock function will be reversed immediately.

As the main gear turns its peg moves the locking yoke, opening switch A enabling Double Lock and the new path to ground will be through switch B and then switch A out through pin 4. An unlock command causes a 12 volt pulse from the RECM on pin 10 so the motor runs backwards. Shortly after it starts to run the main gear cam will cause switch B to change over to the Lock side and so the ground will now be via pin 2. At the end of the rotation switch A changes back to enable Lock.

I make the assumption that if the 12 volt pulse is still present at the end of the rotation it is terminated as soon as the RECM receives an unlock signal via switch C by means of a ground on pin 1 but I havenít been able to verify this.

Double Lock from Lock causes the motor to rotate forward via a 12 volt pulse on pin 4 allowing the peg on the main gear to engage with the Double Lock hook and comes to an end when the main gear cam closes switch B back to the Lock position.

Unlocking from Double Locked follows the same pattern as unlocking from Locked. However, it seems to me that there is a serious weakness in the design in that a failure of switch B whilst the door is Double Locked renders it impossible to be opened from inside or out. I have no idea how one would open the door if this happens and a vehicle presented for an MoT in this condition would result in a failure.