yes, try to clean the "ground" pickups carefully. You can also check that the wires from ground pickups are intact, eg that none has come loose etc. If I remember correctly mine has a total of 4 wires going to the decoder.

Hi
I may be wrong, of course but seems to me Märklin ground return, on most (huge majority of them) is made by wheel axles.
Regarding the 39212, the ground return goes through a number of parts (see the exploded diagram below):
- the wheel pair
- its axle rings
- the bogie Zamak frame
- the loco Zamak chassis
Ground contact must go through all these parts and if any is unclean (old oil) / sludge then there is no contact
.
Ideally removing the bogies and cleaning using RS24 (Noch) and then drying / lubricating makes wonders.
Sometimes, only a little "practice" (let the loco run on a loop for a little while) is enough to restore a good contact.


Cheers
Jean

Hi
This model (at least the later ones like mine) is a bit different as I believe it's basically a "DC" model. It, therefore, has two "track" pickups on each bogie. And on the AC model the left and right track pickups are just soldered together, making it 4 wires going together as ground. (2 wires from each bogie)

The track pickup is a small copper slide on the inside of each wheel. In the picture below I have coloured it yellow.

so it is these contact areas that need to be cleaned, if they are dirty.
and of course, the cause of the problem might also be something else....

Good point!
But if loco stop and the light are out then in case it´s very bad contact with the Märklin locos and this do happens to me often.
This happens when oxid are the problem.
Always clean locomotives and the tracks!

The lights do not remain on when it stops. I saw no excess oil, dust, or grease on anything. I used canned air, and plastic safe contact cleaner. I let it dry out 2 hours before running it. No difference. It is in curves that it quits the most. I thought perhaps that the decoder is being interfered by a lighting wire that passes under the tracks, or by the point where my ski lift power feed is connected to the track. Since others have the same issue though, it's unlikely.
I realize that these locos are mostly decommissioned from road switching and mainly used for dockside marshalling or in small yards, so it's not a major thing. I do however like running it around the layout with a moderate length freight consist.
Thank you Jean for the exploded diagram!

No bph, but I will. Thanks.

I had this problem my track 3rd rail was low so pulled up with pliers and i was not cleaning inside slider


hess

Yes this is one of the problem that is often miss.