Central Locking problems

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Central Locking is a wonderful thing... when it works. If it doesn't, check this standard checklist to get all your doors locking at the single turn of a key.


The central locking system uses a series of microswitches and actuators to power all of the locks open or closed when any one lock is turned. Power is sent to the actuators by the central locking relay, and power goes through the driver's door loom and back to make it all happen. Therefore, the two biggest problem areas are the central locking relay, and this power wire known as the Red/Black wire.

The E30 Zone now considers a modification to the Red/Black wire to not only be a standard cure, but also an essential preventative for future central locking problems.

Red/Black Wire

BMW use a logical wiring system where all red wires are constantly live. One of these wires, coded with a black stripe, makes a loop into and out of the driver's door via the A pillar socket. This is known to cause corrosion and general mayhem both within the socket and to other wiring in the area.

The fix is to eliminate the live loop through the door. The modification to the red/black wires will cure 90% of locking faults, and it is not necessary, or desirable, to dismantle the door hinge pillar plugs and sockets to do so. All the work can be done through the speaker hole.

Note that Cabriolets have a slightly different wiring arrangement, for some reason known only to the E30's original designers, and you will have three red/black wires to connect together.


Remove battery earth lead.

Remove the driver's side kick panel/speaker cover.(one screw)

Remove the speaker(three more screws)

Reach into the hole and pull the hank of harness going to the A pillar socket into view(try not to pull the socket out if possible)

Locate and cut the red/black wires,saloons have two,cabbies have three.

Solder and heat-shrink the harness ends together,insulate the ends going to the socket.

Replace speaker and cover.

Re-connect battery.

Central Locking Relay

The other common cause of central locking failure is the relay. Because of its location, this relay often gets wet due to localised rust, until it eventually burns out. The only solution is to replace it.


The central locking relay is well hidden, but you’ll find it following these instructions.

- Remove the plastic trim that covers the driver’s side speaker in the foot well. This requires removal of the single screw at the bottom left of the speaker grill. This panel can then be pulled free of the door-seal that holds it in at the bottom.
- Remove the speaker – should be only three screws left as the fourth also held the plastic trim that you’ve already removed. Store it well out of the way to avoid any damage to the cone.
- Undo the two screws just below the speaker void – see photo below:

Cl 001.jpg

- Reach down inside the speaker void and you’ll feel the central locking relay, mounted on a triangular metal plate. Withdraw this carefully – don’t want to damage any wires! You should be left with this:

Cl 002.jpg

- At this stage, try operating the central locking from the driver’s side lock and listen for the relays clicking. If you hear nothing, then there is probably either a fault with the relay itself (circa £70 from your local dealer) or somewhere in the cabling. See point 15 below onwards.
- You’ll find a single screw on the back of the metal plate which fixes the relay to it – remove this for ease of handling.
- You then need to remove the white connector from the relay. This is done by easing a small flat-bladed screwdriver to release the catch, as shown below:

Cl 003.jpg

- You can then prize apart the two parts of the black housing, thus removing the end section covering the connector pins, as shown below:

Cl 004.jpg

- Pull out the circuit board from the black casing.
- The photo below identifies the main components that I’m aware of:

Cl 005.jpg

- The component that was the cause of my faulty central locking was the vertically mounted resistor in the bottom right of the board, just below the connector pins. This is a thermal cut-out device which gets hot when there is a fault and hence melts the low-melting point solder at the top, which causes the strip of metal attached at this top joint to spring away, resulting in an open-circuit. Think of it as a thermal fuse.
- If this has sprung open, try soldering it back as per the photo above – preferably with low-melting point solder to retain the intended purpose of the device. Reconnect the relay to the wiring loom and re-check the operation of the central locking system. Mine worked after this and I jumped for joy! I have heard of one other instance on the forum where this has cured the problem too, so it’s not uncommon.
- If this has cured the fault, put everything back where it came from by following the above procedure in reverse. Please remember to mount the central locking relay back as it was using the same fixings. Failure to do so may not allow the sensor that unlocks the doors in the event of a crash to operate as it relies on sensing the full impact. Leaving it hanging below the speaker will probably not allow it to work. Could be a life saver one day!
- If the above procedure hasn’t cured the problem, have a good look around the relay for any evidence of burnt out components or dry-joints on the solder side.
- Another typical cause of central locking failure is corrosion of the pins in the C404 connector where the flexible trunking from the driver’s door plugs into the A-frame. This is caused by rain-water running down the inside of the door, some of which collates in this flexible trunking and eventually seeps up to the connector.
Pull back the rubber cover from this connector as per the photo below:

Cl 006.jpg

Remove the connector by pushing the two side lugs towards each other and pulling the cable at the same time. You shouldn’t need to pull too hard, otherwise you might damage the cables.
- Inspect inside both halves of this white connector, i.e. in the A-pillar (photo below) and at the end of the trunking. If there is any sign of water ingress and/or corrosion, repair/clean as necessary.

Cl 007.jpg

- I found it easier to replace the black rubber boot on the connector before reconnecting it into the A-pillar.