Now then, I may have misunderstood the concept, but ...

In olden days, a switch was supplied by the battery and when it closed, a circuit was completed.

Nowadays, everything is connected to the battery by a form of relay. When a control is moved by the driver, a code is sent around the bus and only the intended relay recognises it, and then closes.

The advantage (?!) is that the code can be inhibited. So, with an old-fashioned circuit, the tailgate could be opened whenever the circuit is closed. However, with the bus, once the vehicle is in motion, the code can be inhibited, so you cannot open the tailgate when under way.

At the very least, you need a diagnostic system.

FWIW, I do not think that I could fix my current model car, manufactured in 2019.

The motor will still need 12V to reach it to make it go - but the switch that controls that is a relay that is under computer control rather than your finger directly

All the usual failure modes still affect it though, and I'd be looking for wires trapped in hinges, water ingress connectors pulled out blown or missing fuses etc.

There will be a bus receiver somewhere, where the un-switched battery power meets that relay,  and a small PCB monitoring the pair of skinny  wires that come from the computer somewhere sending commands in the form of cunningly crafted pulses - a number of similar bus receivers share the same pair of data wires, but recognize only the pulse sequences 'addressed' to them, and ignore the signals intended for other destinations. (Imagine a bit like idea behind the old telephone exchange  but solid state, not clonking switches)

To debug the data bus needs a scope or a debugging receiver, but more than half the problems, as they always used to be actually , are on the power side of things, the fatter wires that do the actual applying 12V to motor windings, lights or whatever, so check all that, and any earth studs, first.

And once completely unhooked from the bus receiver the motor can still be tested by applying some volts in the traditional way.

But don't get the wrong wires and apply un-fused battery to the 'door closed?' sensor switch, as welding the contacts does not help. A small lamp in series a battery or the meter on the ohms range is the way to check that sort of thing.

And the control computer does not appreciate its data lines being shorted or stuffed by the battery either, so stay off those.

Mike.

Whilst it's never a good idea to short wires or connect them to the wrong voltages, the CAN bus in a vehicle is protected to survive these sort of things. The electronics modules also have reverse battery protection for mistakes when the battery is replaced (though that does rely on the rest of the wiring being correct).

Most modules will use low-side switches (generally MOSFETs) to switch loads such as solenoids and single-direction motors, and H-bridges for bi-directional motors. If an H-bridge is being used, there will be no measurable voltage if the motor is not being actively driven.

Whilst it's never a good idea to short wires or connect them to the wrong voltages, the CAN bus in a vehicle is protected to survive these sort of things. The electronics modules also have reverse battery protection for mistakes when the battery is replaced (though that does rely on the rest of the wiring being correct).

Most modules will use low-side switches (generally MOSFETs) to switch loads such as solenoids and single-direction motors, and H-bridges for bi-directional motors. If an H-bridge is being used, there will be no measurable voltage if the motor is not being actively driven.

Thanks for all the advise,may start with a new lock unit from vauxall,the scrap yard lock

I bought did,nt look too good inside.

                                                               Regards,

                                                                             Hz