Rebuilding Märklin 3103, T 12 7896 from KPEV, to digital with the Märklin 60903 conversion set

This conversion procedure works also for other Märklin models based on the 3095, DB Br74, chassis.

Updated 27.01.2014

Märkllin 3103, KPEV T 12 7896

Märklin 3103 after the conversion

Preparations for the conversion:
First of all, remove the reversing unit and the motor from the locomotive.  Also remove the pick-up shoe and it's contact plate.  Make sure that the locomotive works perfect mechanically, like the gears, wheels and connecting rods.  Clean it well before installing the new parts.  Carefully oil the rotating parts - if required.

The Märklin 3103 inside after the conversion

Conversion sets for Märklin 3103:
As this model has a Small Flat Collector Motor (SFCM) Märklin conversion sets 60903, 60623 and motor set 60943 may be used.
The only difference between these sets is the decoder.  60943 has no decoder, a decoder must be ordered separately, for instance the 60962 decoder set.

Installed at the same place as the original reversing unit. Initially I used the mini c91 decoder (Märklin part number 603462), not the bigger 60902 decoder from the 60903 set.

Later the decoder was replaced by one from the SBB Ae 3/6 II 10.421, Märklin set 37512 (part number 102155 or 611573, I think).  That is a remotely adjustable decoder with the same functions and the same size  as the mini 60902.  The reason for replacing was the maximum speed, which can be adjusted to a much lower speed than the potentiometers on the mini 60902 allows.  With the mini 60902 the maximum speed at the lowest possible adjustment was approx. 160 km/h.  Now, by adjusting the speed (CV5) to level 15 with the Central Station, the maximum speed is approx. 80 km/h.  Perfect!

Installation of the motor itself is quite simple. The diameter of the head on the lower screw for the motor cover must be reduced.  This is to avoid conflict with the rear left wheel flange.  I used a small grinding machine to do this.

The photo shows the initially installed mini c91 decoder

The locomotive body is insulated with self adhesive rubber foam to reduce the motor noise.


Front lamps:
The front lamp is installed in an old bulb socket (part no. 475030) glued to the front of the locomotive chassis, with a bulb socket insert from the 60903 conversion set.
For the front lamp I use 28 Volt T1 bulbs with wire ends, cut to suitable length. I think the 610080 (22 Volts) gives a too bright light.

Rear lamps:
For the rear lamps I drilled holes in the lower lamps and installed a light guide from the BR80 (3304, Märklin part no. 200690).

I made a bulb holder of some old plastic parts and a two-pole SIL socket.  The rear lamp is a 610080, as the light guide reduces the light intensity.

A future improvement will be to rebuild the rear light light to LEDs, including the top lamp.

The couplers:
As seen on the locomotive photos on top of this page, my Märklin 3103 does not have the original couplers.

Front coupler:
The front coupler is cut under the bumper beam , just in front of the air tank.

Rear Coupler:
The rear coupler is rebuilt with a close coupler from the Märklin coupler set 7205 (40 couplers for cars and 10 for locomotives), by cutting the original coupler and gluing in a car coupler.

First I remove the moving parts from the coupler, then it's cut at the red or yellow line, depending on the required length.

After shortening the new coupler, and cutting off the centering springs, glue it to the old coupler.  Secure it by bending in the lugs.

Be sure to aligne the coupler properly.

The locomotive is meant to run in a consist with the four coach set 4035.

A black wire runs from the luggage car to the green front car, partly visible in the photos.
Behind the locomotive: 1235, next 1418, then 1604, finally luggage car 3103.

Function decoder:
A Märklin 60960 function decoder and a relay is installed in the luggage car 3103 Pi.

The decoder functions, including car set 4035:
function Locomotive3103 Car set 4035
f0 f 3 front lights -
r 2 rear lights -
f1 - interior light in the connected three coaches, via a relay
f2 - interior light in conductor's cabin and one rear red light
f3 - interior light in the luggage area
f4 acc/brake delay off red LED inside the luggage area, just for fun

Screenshot from my Central Station 1:

Information about the T 12 prototype, from Wikipedia:
T 12 (Preußen, Elsaß-Lothringen)
DR 74.4 – 74.13
ÖBB 674, PKP OKi2
SNCB 69, SNCF 130 TC
Numbering DR 74 401–543, 545–1321
Number built: 1.014
Manufactorer: Union, Borsig
Building time: 1902 – 1921
End of service: 1968
Type: 1'C h2t
Track: 1.435 mm
Length: 11.800 / 12.100 mm
Service weight: 65,90 t
Axel weight: 17,36 t
Max speed: 80 km/h
Power 640 / 669 kW
Driving wheel diameter 1.500 mm
Front wheel diameter 1.000 mm
Cylinders: 2
Cylinder diameter: 540 mm
Piston stroke: 630 mm
Steam pressure: 12 bar