The insider 36159 Crocodile used a "beefed up" 6080 decoder, so it used the old style AC field coil as used in the 3015 Crocodile. The anniversary set 31859 lists all three Crocodiles as having MFX decoders. Did Marklin make special permanent magnet field coils for the green and white Crocodiles in that set?

ESU is releasing a new item # 51965 - a permanent manget für 3015 type crocos, and the other crocos based on this model.

Old three-pole motors can not be upgraded to mfx with the current Märklin decoders. The mld and msd decoders are designed for modern five-pole oder Faulhaber motors. .For the 3015 crocos however, there isn't any five-pole conversion kit. A three-pole motor, digitalzied with a mld decoder, will not run properly - I already tested this; finally, I replaced the decoder a an ESU Lokpilot 4 (the loco is now running with DCC).

To digitalize an three-pole motor with mfx, you will have to take ESU's Lokpilot mfx (also called "Lokpilot M4" - but M4 and mfx are the same system: ESU had to use another name for it, as "mfx" was a registered trademark by Märklin).

However, for a price of CHF 185, Morep offers a Faulhaber conversion kit for 3015 crocos; this motor is fitting exactly into the original motor holder; drilling, cutting or grinding are not necessary. With this motor, you can use Märklin's mfx decoders.

Regards,
Florian

Hello
Reading your topic and having a 3015 crocodile coming from a flea market (I'll explain later) I felt the need to digitalize it at minimum cost.
I re-used a C90 decoder (removed from another locomotive for sound additions).
I wanted also to remove / modify as little as possible from the 3015.
The change is based on keeping the field coils and driving the entire motor (with its coils) as a single DC current motor (like a Märklin HLA -HochleistungAntrieb). For this to work, I had to add two powerSchottky diodes each one in series with each field coil but in opposite direction (one has the anode to the Field coil#1 while the second one has the cathode to the field coil#2.
The result is surprizingly good and cost me nothing and here is a modest video of the result:
Digitalisation of a 3015 Crocodile locomotive

The reason I mention the fleamarket and my restoration process is that I made no offense to the good Märklin workmanship because when I got the loco it was a wreck, a piece of rust without any paint remaining so I got myself the authorisation to do this strange modification.

Now the problem I have is I need to avoid collision with my other locs because, this is a real monster smashing everything on its way.
Have a nice Train (hem I mean a nice day!!)

Jean