Converting Märklin 3171, SJ D 109, to digital with Märklin conversion set 60903 or 60923, and adding close couplers

Updated 16.05.2012

The 3171 is weathered with snow, otherwise it is identical to the Märklin 3170, the same locomotive without the snow weathering.  In other words, this conversion may also be used for 3170 and 3670.  3670 was the digital version of the 3170.  One version of the 3170 and 3670, from the very nice wooden train sets 2670 (digital) and 2870 (analogue), had close couplers originally fitted.

I think that the chassis in Märklin model 3170, 3171 and 3670 (SJ type D, wood body) basically is the same as in Märklin 3018, 3019 and 3030 (SJ type Da, steel body)

My 3171 after the conversion

Preparations for conversion, photo from the identical locomotive in Märklin train set 2870

The motor and the reversing unit was removed, so were all the wires, except for the black wire from the pick-up shoe contact plate to the power source selector switch (to switch between current from the pick-up shoe or the pantographs).  The contact tongues for the front and rear bulbs were carefully removed from their brown plates.
All the mechanical parts remained in the locomotive, as this was practically not used before the conversion.  It is very important that all the gears and axels rotate freely before the new motor is installed.  Oil the rotating parts very carefully, if required.

Conversion sets
Both the older Märklin conversion set 60903, with a c91 decoder, and the newer 60923, with an mfx decoder, can be used for the conversion.  The only difference between the two sets are the decoder itself.  The motors and the small parts in both the sets are the same.
Edit June 2011: The 60903 and 60923 sets are no longer produced by Märklin.  They are replaced by motor kits (in this case Märklin 60943) and separate decoders.  The closest to the 60903 decoder is the 60962 mLD decoder.  See this Märklin web site for detailed information.

Motor
The 3171 has a Small Flat Commutator Motor (SFCM), so the motor from the conversion set, Märklin no. 60903 (or 60923 or 60943), was installed.  I cannot recall any difficulties with the motor installation.  Make sure that everything rotates freely before installing the brushes in the motor cover.

I have seen reports about a small problem on these locomotives.  They say that the rear left driving wheel hits the motor cover, and that 0,6 mm has to be grinded off the cover, under the lower guide pin in the photo.  I did not experience that during my conversion.

Decoder
The decoder was installed at the same place as the old reversing unit.  I left it in the original cover and glued it to the locomotive chassis with some detachable green stuff from Faber-Castell.

The decoder functions (would be the same with the 60962 decoder):
Function Wire colour Connected to
f0 forward Gray Front headlight
revers Yellow Rear headlight
f1 Brown/red Centre interior light
f2 Brown/green Rear interior light

All the functions are connected to the orange return wire (which is actually +, the function outputs are -)

The locomotive inside after the conversion, with the original couplers.

Front lights
First, as a test, I pressed a new bulb socket, Märklin part no. 624510, into the hole where the old bulb used to be.  Then the 22 Volt bulbs 610080 fits, but it is too long, so it interferes with the locomotive body.  Even the shorter 28 Volt T1 bulb, which I often use, interferes with the body.

The result was to use another bulb holder, Märklin spare part no. 259920, glued to the locomotive chassis behind the bulb holes.  This way the 610080 bulbs fits perfect, in both locomotive ends.

The photo shows a 28 Volt T1 bulb.

The glued bulb socket from behind, including some dust:

Interior light
I think that it is a waste not to use all the function outputs of a decoder, so f1 and f2 was used for interior light in the locomotive.  One small bulb holder was glued to the decoder cover, another on the motor.  28 Volt T1 bulbs are used, to get a dimmer light than the 22 Volt 610080 would give.

Close couplers
I prefer close couplers, as they give less free play between the units. If a close coupler holder isn't available as a spare part, I modify the original holders.  In this case I found two couplers, identical to the original ones, in my spare part box.  I did not want to destroy the original weathered once.  One version of the 3170 and 3670 locomotive have close couplers originally fitted, but I have not been able to find their spare part number.

The original coupler. It is a little bit high, but very easy to adjust.

The original coupler parts, disassembled:

After disassembling, I cut the coupler at the red line, or, if I want the coupler to be a little bit shorter, at the yellow line, at the centre of the hole.

The modified coupler (sorry about the poor sharpness):

I cut off the "flat part" of a Märklin 363950 coupler pocket.  A pocket with a shorter "flat part" is also available, no difference, as the flat part is not used.  The pocket is glued into the modified original coupler.  The "ears" that held the old coupler parts are bent in to squeeze the pocket firmly to the coupler.

An original and a modified coupler:

The final result, after some fine adjustment:

The rear end of the locomotive, the small coupler hook on the bumper beam is missing:

From below:

Part list for the conversion:
Part number Pcs. Description
60903 1 Digital high propulsion conversion set, motor and small parts only,
would today be replaced by 60943 motor set and 60962 mLD decoder set
259920 2 Bulb holders
610080 2 Light bulbs
363950 2 Coupler pockets
70163 2 Close couplers, from a 7203 coupler set with 50 couplers
259920 2 Bulb holders
610080 2 Light bulbs
- 2 2-pole bulb holder, cut from a SIL connector
- 2 28 Volt T1 light bulbs

Some facts about the prototype:
Power type Electric
Builder ASEA
Build date 1925-43
Total production 333
UIC classification 1'C1'
Gauge 1,435 mm
Wheel diameter 1,530 mm
Length 13,000 mm
Locomotive
weight
80.4 tonnes (steel)
79.5 tonnes (wood)
Electric system 15 kV 16⅔ Hz AC
Top speed 70, later 75 km/h (Dg)
90, later 100 km/h (Ds)
100 km/h (Dk, Du, Du2)
110 km/h (Dr2)
120 km/h (Dr)
Power output 1,220 kW (Ds, Dg)
1,660 kW (Dg, later)
1,470 kW (Dk)
1,840 kW (Du, Du2)
Tractive effort 141 kN (Ds)
187 kN (Dg, original)
216 kN (Dg, later)
151 kN (Df, original)
161 kN (Df, later)

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