First thing to check: Is the slider coming in contact with one of the wheels?

Second thing to check: Disconnect and separate the two lighting strips.
Connect the ground and slider wires to each strip one at a time and see whether there is still a short
This will let you know whether there is a short on one strip.

Third thing to check is to check with a multimeter (Ohms) to see whether there is contact between the leads for the contact plates and the return.
Touch one lead to the wire and the other to the axles.

Let me know the results of these tests. In the meantime, let me get one of my coaches with lights and check it out and I will get back here in a couple of hours

Regards

Mike C

Did you modify the solder on the wires? The lead from the slider seems to have excessive solder on it.

A few more things that you should check...

1) Check the contact which goes underneath the baseplate and goes to the coupler to make sure that it is not coming into contact with the axle/wheels

2) make sure that the coupler is properly inserted and not bending or otherwise not properly contacting the contact strip in the coupling shaft.

3) DO NOT couple this coach to any older coach or car or locomotive with metal couplers. The old metal couplers were connected to the ground (return) via the chassis, so this would result in an instant short.

4) If you are using this with additional coaches, make sure that all equipped with lighting and 72021 are connected in the same way so you are not crossing live with return in different coaches.
If one coach is wired AB and the other coach is wired BA, this would cause a short.

Make sure that only one coach is equipped with a slider. the others should be connected with one return lead and baseplate leads on the opposite side (outer) to pass the live to the next coach via the couplings. The coupler connected to the locomotive should be equipped with a 7203 (non current conducting) or you can remove the baseplate lead on the corresponding end of the first coach.

Regards

Mike C

I guess the first thing to check is to make sure that you have the right slider. Depending on the type of coach, there are sliders with the pin in the middle and ones that have the pin offset.
This is because some coaches have the pivot pin in the centre of the bogie and others have it offset. If you use a slider intended for an offset bogie on a centre pivoting bogie, it will be too close to one of the axles and the same if you use the centred slider on a bogie that is offset.
The pivot pin of the bogie is moved on some models to reduce the overhang of the longer coaches on Maerklin's tight radii.

If you think that you have the correct slider and it is still causing a short, you can add a little piece of heat-shrink tape to the slider arm. On some models, you may be able to put the tape on the axle between the wheels. The other option would be to see if Maerklin has a completely isolated wheel set that can replace the problematic one. (Must be insulated on both sides to avoid short)

If you can provide the coach model Number, we can check to see if you have the correct slider
73404 Offset short https://www.maerklin.de/...te/details/article/73404
73405 Centered (Assymetric) https://www.maerklin.de/...te/details/article/73405
73406 Offset long https://www.maerklin.de/...te/details/article/73406

Regards

Mike C