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I know there has been much on here about this topic but here goes anyway. I understand that there are 3 microswitches in the driver's door latch. One of them switches the earth/ground for the rest of the door latches, so if it plays up the other locks lock/unlock randomly as mine have begun to do, but the driver's door remains unlocked (whilst the car is being driven). I've looked at a Youtube video which shows the black plastic piece in the door latch moving across the microswitch with the red/black connections, depressing the pip. I understand that this is when the driver's door is locked and that it interrupts the earth connection, thus de-energising the other lock motors. It isn't what I expected and I may have misunderstood the narration. What I want to know is how I can temporarily control the other locks with a manual switch until I am ready to remove the door latch and sort it properly. Is the problem with that switch due to the pip wearing a groove in the plastic so that the pip is not depressed properly and the earth is intermittently connected, because that would suggest that the pip is depressed when the driver's door is unlocked, which is not what the chap on the video seemed to say? Are the necessary components to effect a permanent repair available? Presumably the black wire on the back of the double microswitch is a common earth for the blue wire (door ajar) and the red wire (C/L), is it? In which case if I disconnect the red wire and provide a temporary 'permanent' on/off switch to the black wire, I can control the locking of the other doors with that switch? I know there is a fellow who can supply reconditioned latches, but I'm looking for a temporary remedy now and my own repair to the latch/switch later. Thanks..

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JeremyAldermartenQC wrote:

There must be an easier way!

There is, but you won't like the price, https://www.island-4x4.co.uk/front-door-latch-assy-9499-fqj103220-p-1772.html for up to 99 or https://www.island-4x4.co.uk/front-door-latch-assy-9902-genuine-fqj103260-p-1774.html for 99 onwards. It is Marty who does refurbished door latches (www.p38webshop.co.uk) but he is currently in NZ so not able to supply at the moment. The main problem with changing the switches is that while they are the same size as standard microswitches, the mounting pins do not line up with the mounting holes on a standard microswitch.

You've got it right, when unlocked the CDL switch is closed so the pip is pushed in but with wear it doesn't get pushed in far enough for the contacts to make (ignore the self proclaimed experts on Youtube as most of them seem to have got it completely wrong). It's a failsafe, if the connection is lost, the doors will lock. Assuming you've got it right and it is the Red wire (it's the one that connects to the Green/Red in the loom to the outstation), grounding that will cause all the doors to unlock, except the drivers door as that is the one supplying the ground therefore it must be unlocked already. However, putting a manually operated switch in there is almost certainly going to cause you problems. If the switch is left closed (so the doors are unlocked) and you try to lock the car with the fob, the outstation will pass a message to the BeCM that the drivers door isn't locking (as the ground won't be removed), so you will at best, get a mislock, at worst, it will lock then immediately unlock all the doors. If the switch is left open, and the microswitch has failed completely (or has decided at that particular moment to be permanently open), it won't try to lock the drivers door as it is already locked. If you try to lock with the key, it will probably set off the alarm and/or trigger the immobiliser as they will be a pulse from the keyswitch but not a change of state from the CDL switch.

You could, as a temporary measure, fit an external switch that is operated by the movement of the rod or linkage from the sill locking button, so when the button is up (i.e. door unlocked) the switch is closed and when the button is down (locked) the switch is open.

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Excellent information. Thanks. It appears that I will have to fix it properly! If I unplug and remove the driver's door latch from the car with the battery still connected, will that cause any problems with the immobiliser or the BECM? You are right in thinking that I wouldn't like the price of a new latch!

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All that will happen is when you unplug the latch, all the other doors will lock as the ground will be removed from the Green/Red wire. To the outstation it will appear that you've pushed the sill locking button down. Just ground the Green/Red to cause them all to unlock again.

Door latch for an MG TF is the same as the pre-99 latch and is a direct swap although there are two versions, one with key locking only and one with key and central locking, it is the latter you need. I've bought a couple of secondhand MG latches in the past but you've no way of knowing if the switches are good or on their last legs and about to fail. One I bought failed within weeks of fitting it.

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Unfortunately my car is a 2000 model, so I can't use the MGF lock. Might the earlier lock have a compatible microswitch unit? I presume that it is permissible to use the sill lock when the car is being driven. The driver's handbook states that the sill locks on both front doors will lock the car from inside, but doesn't make it clear whether that can be done in motion, or how the car should subsequently be unlocked. I presume it is by opening a front door. My thinking is that if I sill-lock before moving off, the locks won't keep jumping, because it will be stable with the earth applied. You previously stated that, when the mechanism is worn, the pip doesn't get pushed in far enough to make the circuit, but it would actually be to break the circuit, wouldn't it? The earth is applied when the switch pip is released and it is not pushed in far enough to break the circuit reliably as it wears. I'm not wishing to be pedantic, but i'm anxious to get a sound comprehension of how it works. I'm thinking that I shall remove the bit of black plastic mechanism that operates the switch and build it up so that it fully depresses the pip. It should be easy enough to test it electrically before and after reassembly. Thanks for your patient and informative replies! I've just looked on Ebay and note that driver's door latches are over £100 secondhand for my car!

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The difference between the pre and post 99 latches is the connecting plug. Early ones have two plugs, a 6 way and a two way, where the P38 only uses one wire but the MG uses both, whereas the post 99 has a single 8 way plug. The latches themselves are the same even down to the wire colours so you can chop the two plugs off an early one and fit the plug from a later one. The internals are also identical so the switches and motors can be swapped over too. Pushing the sill locks down while driving locks all the doors, in fact, many modern cars do that automatically once you reach 10 mph. To unlock them, you just pull the button up again when you find that your passenger has accidentally pushed the button down with their elbow and the door won't open. The switch makes the circuit when the doors are unlocked (the same ground through the CDL switch is passed to the tailgate release, ground there and the tailgate can be opened, no ground means the tailgate can't be opened), again a failsafe so a broken latch for whatever reason keeps the doors locked.

Without a latch in bits in front of me, I'm not 100% on whether the pip is pushed in or open to close the circuit but I'm fairly certain the CDL switch needs to be pushed in to close the contacts. If you look at the way the switches are arranged, you'll see that they can be either, and two of them work one way round while the third works the other way.

Rather than try to build up the plastic, one option that I did on one latch (where I broke the pip off completely) was to add a metal wiper strip as used on some other microswitches and bent it so the switch was operated with only a very small amount of movement.

Like I said, secondhand is a gamble. Buy it, test it and find it is fine but how close is it to being worn out and putting you back at square one?

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Thanks for that extra help. I do recall that the tailgate won't open without the earth via the driver's door microswitch. That is understandable as the tailgate latch is powered and therefore needs an earth. What is surprising is that the door latches need an earth to remain unlocked, but I suppose they must, although that earth perhaps operates in the BECM rather than in the latches themselves and merely switches the earth to the actual latches. It must be the case that the earth is removed when the pip is out, otherwise the wear wouldn't lock the doors, so you were right to say that the pip wouldn't go in far enough to MAKE the circuit when wear has occurred. I have loads of microswitches from Xerox photocopiers of the 1970s, some with wiper strips. They may prove useful for the first time since I acquired them over 40 years ago! Thanks again.

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No, the earth from the microswitches goes to the door outstation to report the status, That then informs, over a data line, the status of the switches to the BeCM. When locking or unlocking is required, the BeCM tells the door outstation to change the state of the locks which applies power to the motors. Or in the case of the rear doors that don't have an outstation, the BeCM powers the motors directly. The ground going to the tailgate switch is the only time it is actually used to pass current, the rest of the time it is merely to signal a low state. Think pulling inputs high or low rather than applying a ground.

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Great stuff! I'd forgotten about the 'outstation'! What a mad design! I must'nt complain as I knew these were tricky cars! I'll let you know how I get on!

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Far simpler than the 2007 Merc my other half owns which is all Canbus. That has door outstations too (although Merc call them Door Control Modules) but they have to be programmed, like nearly all of the other 20 odd modules, to tell it what engine is fitted, what gearbox is fitted, whether LHD or RHD, etc. Had to change the passenger door outstation only to find that the passenger electric window button on the drivers door wouldn't work but the one on the passenger door worked perfectly. Couldn't find a problem with the switch or wiring only to discover that the outstation comes programmed for a LHD car so was ignoring the input from the drivers door. £60 to Mr Mercedes to plug his computer in and tell it that it's fitted to a RHD car so can expect inputs from two switches.

It's all done to keep down cost and weight. Copper wire is both expensive and heavy so a pair of wires can supply power to the outstation and it can, under control of the BeCM over a data line, supply that power to the door latches, electric windows and mirrors (2 supplies required, one for the side to side motor and one for the up and down motor) rather than using 4 pairs of power wires. That reduces the amount of heavy current wire required, the number of wires needed and the overall weight. On the E32 7 series BMW, which used conventional wiring rather than an outstation controlled system, the wiring loom was the heaviest single component on the whole car, heavier than the bodyshell or engine even.