The 90-model 124 central locking system only operates from the rear hatch. When you turn the key in the front door locks, you hear a click somewhere in the engine compartment, but nothing else happens. At some point, the car's alarm system has been disabled. Could this be related to the issue with the central locking?

There is a cable duct under the carpet in the right rear footwell of the car. There is a rather thick cable in the duct. However, there are several cables inside, and a bundle of red/white and red/yellow cables connected. It is very common for cables to break from that bundle when they oxidize. Reconnect the cables and the door locks will almost certainly start working again.
Other reason might be cutted wires in the driver’s footwell. There is blue/white wire, which opearates the locking, and it’s often cutted when alarm/remote is installed. I recommend to remove all the non original components and wiring from the car and solder all original wires back togrther.
Third reason might be just oxidized connector in the vacuum pump below the rear seat.

In my C124 the fault was precisely in the wire loom in drivers footwell/sill. Extra wires connected to god knows where... I took all extra wires away, and reconnected the original wires, and all good.

johnniebell kirjoitti: ↑Ti 26.11.2024 09:48 The 90-model 124 central locking system only operates from the rear hatch. When you turn the key in the front door locks, you hear a click somewhere in the engine compartment, but nothing else happens. At some point, the car's alarm system has been disabled. Could this be related to the issue with the central locking?

Many 124 models have built-in alarm systems that, if improperly disabled, can affect the central locking function. A clicking sound could mean that the relay or actuator is trying to operate but may be wired incorrectly or disconnected.

A car this age can have numerous issues because of work altering the original factory setup. As far as I know a -90 model 124 central locking should be similar to that of the W126 S-class. The central locking motor is also the same. So, the clicking sound from the engine compartment sounds strange to me, as there shouldn't be any separate relays in this system until later on in the early 90's. This could indicate an aftermarket immobilizer/alarm? Certainly, a source of trouble sooner or later.

If the fault on both front doors appeared at the same time, you most likely have an issue with the harness(going to, not from the doors). To rule that out, the motor and the harness can be checked as follows: There are two connectors at the motor, one is the power supply the other one with three wires blue, yellow and green, each one coming from their corresponding switch. Green and blue would be the front doors, the yellow from the trunk.

Each door-/trunk switch is supplied with ground and continuous power. As the locks are turned, they will send either ground or positive current to the motor. With all locks open, remove the round three-pin connector from the motor and check for battery voltage at all three pins. Then close all the locks manually, and all three pins should now show ground. If everything checks out at this point, your pcb in the motor unit is most likely toast.