Updated 27.08.2009

These two conversion sets are identical when it comes to the five-star high power motor itself.  The big difference is the digital decoder.

The decoder in 60903:

In the 60903 it was (not produced anymore) a so called c91 decoder, with a row of eight switches to set the decoder address and two potentiometers to adjust the maximum locomotive speed and the acceleration and deceleration time.  The decoder only receives data from the digital train controller.  With a Märklin control device you may select between 80 locomotive addresses.  The decoder dimensions are 36 x 21x 9 mm.  If required, it fits into a holder 258820.

The decoder in 60923, picture stolen from the 2006 Märklin catalog:

In the 60923 we find a mfx decoder.  It means, among others, that the decoder is adjusted from outside, from the train controller.  You can set a lot of things, like the address (if you use a Delta controller or the 6021 Control Unit), the maximum speed, which can be set rather accurate, the acceleration and deceleration time can be set individually and you have some control of the different functions of the decoder.
If using The Märklin Systems, either with the Mobile Station 60652 or the Central Station 60212, the decoder is reporting itself to the control system.  It is not necessary to set an address. The decoder dimensions are 22,3 x 15,5 x 5,5 mm.  If required, it fits into a holder 456200.  This decoder is wrapped into a sort of rubber tube in the conversion sets.

Märklin 60903 and 60923 consists of a number of parts:

Part name	Pcs.		Part number
	60903	60923
Decoder, fx c91 with DIP-switches	1		603617
Decoder, mfx		1	611298
Rotor	1	1	210888
Field magnet	1	1	224615
Motor cover	1	1	224613
Brushes	2	2	601460
Noise suppressor	2	2	516520
Screw M2x12	2	2	785140
Bulb socket	2	2	276770
Bulb 22 Volt	2	2	610080
Soldering lug	1	1	231470
Capacitor	2		600760
Rubber tube	2	2	787570
Tool to adjust the decoder	1		610080	Parts from a 60923 set with a 60760 decoder. The 60923 mfx decoder looks identical, but has more wires. A 276770 bulb socket is already installed in the 3000's original bulb holder.
Manual	1		608899
Manual		1	611488

The two screws 785140 and the soldering lug 231470 is missing from the above photo.  The pick-up shoe and the small contact plate next to it is not a part of the conversion set, but belongs to the locomotive.

The decoder functions:

Function	Direction	60903	60923	Remarks for 60923
f0	forward	on, changing with direction	on, changing with direction	On the mfx decoder in 60923 these selections can be done with the Central Station 60212. In addition to the acceleration and braking delay, this decoder also have a shunting speed setting, normally also controlled with f4.  This may be set to e.g. f3 with the Central Station.  Also the output type and intensity can be selected, like light, flashing light, smoke unit etc.
	reverse	on, changing with direction	on, changing with direction
f1	forward	on	on, may change with direction
	reverse		on, may change with direction
f2	forward	on	on, may change with direction
	reverse		on, may change with direction
f4	forward	acceleration and braking delay cancelled	on, may change with direction
	reverse		on, may change with direction

A Central Station screenshot:

The left half shows an mfx decoder, identical to the one in the 60923 set, the right half shows a c91 decoder, like the one in 60903.  The different functions are indicated far left and far right in the picture.  From the top we have f0, front head lamps, f1, for instance interior light front end, f2, for instance interior light rear end, f3, shunting speed (not on the c91) and f4, cancelling the acceleration and the deceleration delay.

Installation in a locomotive

A number of locomotives may be converted without any problems, but some require additional work.  If you have some mechanical and soldering skills, it is normally no problem to install the conversion set into a locomotive.  The locomotive must of course be in a good mechanical condition prior to the conversion.  If you don't have the skills yourself it is recommended to leave the work to professionals, like your Märklin dealer or the Märklin Repair Service in Göppingen, Germany.

This list shows some of the locomotives that are reported to be difficult or impossible to convert, with information regarding some of the conversions I have done:

Märklin model	Problem with conversion	My solution
3000 Br 89	This locomotive may have different problems, according to the production series. In my case, with a locomotive produced between 1974 and 1995, was that the lower screw for the motor cover was too long and blocked the lower gear from behind.	The simplest solution was to use a shorter screw. M2x11 would be perfect but as I did not have any I used a M2x10. It is a little bit short.  I used the motor from a 60903 set and a 6090 (c90) decoder.
30031 Br 24	On Märklin item 3003, Br 24, and other models based on this locomotive the head of the lower screw for the motor cover interferes with the rear  driving wheel flange.	I grinded off the screw head a little bit, to make sufficient clearance between the screw head and the wheel flange.  I used the motor and small parts from 60903 and the decoder from Märklin conversion set  60760.  The 60903 decoder also fits perfectly.
3016 Br 795	Some reports says that the motor cover needs some additional work.  My 3016 was made in 1988.	I did not need to do any additional work.  A plain conversion.  I used the conversion set 60903.
3099 Br 38	This locomotive, Br 38 or P8, and other models based on this locomotive, is reported to be impossible to convert.  The explanation is that the original motor cover is a special one and that the one from 60903 or 60923 does not fit.	The locomotive body hits the motor cover, so it will not fit properly.  The solution is to grind or cut off approx. 1 mm from the cover, horizontally above the rotor shaft.  Maybe the decoder from 60903 is a little bit too big.  The mfx decoder from 60923 fits perfectly, as it is smaller.
3103 T 12	On Märklin item 3095, Br 74, and other models based on this locomotive the head of the lower screw for the motor cover interferes with the rear  driving wheel flange.	I grinded off the screw head a little bit, to make sufficient clearance between the screw head and the wheel flange.  I used the motor and small parts from 60903 and a smaller c91 decoder, Märklin no. 603462. The decoder from 60903 also fits without any problems.
3148 Di 5	Märklin item 3064, Br 260, and other models based on this locomotive, is reported to need some modifications to the motor cover.  My 3148 was made in 1989.	I did not need to do any additional work.  A plain conversion.  I used the conversion set 60903 and installed Telex.
3171 D 109	On Märklin model 3171, D 109, and other models based on the same locomotive chassis, the motor cover is reported to be too thick, interfering with the inside of rear left driving wheel.	I did not need to do any additional work.  A plain conversion.  I used the conversion set 60903.  If the problem occurs, grind off 0,6 mm from the motor cover, in the area inside the wheel.

Some photos from the installations:

3000 motor:

The yellow arrow indicates the shorter screw, M2x10.  The long one, M2x12, sticks to the magnet for illustration.

30031 and 3103 motor:

Notice the modified screw head just above the driving wheel flange.  The decoder is in this case a smaller version of the c91 decoder, without the f1 and f2 functions.

3099 motor cover:

Notice the flat area above the rotor shaft.  This is cut away to give sufficient space for the locomotive body.  The photo is from the Märklin locomotive set 00380, Br 38 1967.

There are many internet sites regarding this kind of conversions and how to solve the difficulties.  Just search for Märklin 60903 or 60923 conversion.