Get regulated speed control in Märklin MaK Diesels.

By Per Zander

This article was made by a request in a discussion about possible conversions of this type of loco in the forum.
Märklin have manufactured a number of MaK diesel models:
Model Number: Description:
33641, 37641 Dutch NS Cargo, red, 165 mm
33642, 37642 Swiss SBB, red, 145 mm
33644 Dutch NS, green-yellow, 165 mm
33645, 37645 German Tegernsee Bahn, blue, 145 mm
33646 Dutch NS Cargo/Railion, red, 165 mm
37646 German Dortmunder Eisenbahnen, green-yellow, 145 mm
37647 Austrian ÖBB, red, 165 mm





Figure 1. The Tegernsee Bahn MaK diesel, Märklin 37645.

I bought a 37645 in the spring of 2001. I immediately observed that, despite its 37xxx number, the loco didn't have regulated speed control. The same applies to the 37642 and at least all the 33xxx models. I don't know if it is the same for the 37xxx versions of the longer models (37641 and 37647). Furthermore, I noticed that the loco only understood the old version of the Motorola format, and that it only had 14 speed steps, not the 27 steps that the 60901 decoder has. The start of the loco was quite unpredictable. It usually started at speed step 2, but sometimes it stood still at speed step 5 on flat ground. Uphill and downhill grades had big impact on the speed, and made the starting speed step even more random. In other respects, the loco ran very nicely however, and it is extremely quiet.

The lack of speed regulation, and the lack of absolute direction control with the old Motorola format, is something you can easily live with if you operate the loco manually, digital or analog doesn't matter. The otherwise superb speed characteristics makes it very nice to drive anyway. But I'm planning for a more automated digital control in the future, and then you cannot have a loco that doesn't start safely in a known direction. Therefore, I decided to replace the original decoder and put in a regulating decoder instead.

Selection of decoder
I decided to use the ESU LokPilot decoder for the conversion, after a tip on the discussion forum. The LokPilot was selected for several reasons:

  • It is small enough to fit in the model. There is not much margin, as we will see later on, but it fits.
  • It accepts all different operation modes (AC analog, DC analog, Märklin/Motorola and DCC). I run the M/M format and I also want analog AC operation to work, without having to open the loco and change switch settings. This excludes a number of decoders that don't support both these modes, or needs manual switching of the mode. To get the DC and DCC modes too is an extra bonus.
  • It has got good reviews (contrary to some other decoders I have checked out).
  • It is on-track programmable, even with Märklin equipment.
  • It is low cost, 27 Euro.
The LokPilot is sold through Noch. This is good to know if you want to buy one.

Figure 2. The LokPilot decoder.

Preparation of the loco
Before the LokPilot can be installed, the original decoder must be removed. The decoder is unscrewed and folded over, and the two small LED boards at the ends are pulled up and folded out. The five wires connecting the loco to the decoder package are soldered off at the decoder. Then, all the wires are disconnected from the small LED boards, and the excessive solder is removed. I have documented the original wiring, and a drawing can be found here.

Figure 3. The opened loco, with the original decoder folded out.

Initial investigations
I made a lot of initial tests and investigations before I continued with the final conversion. If you trust my findings, the LokPilot quality and the stability of your operation environment, you can skip this section. If you are interested, it is described here.

Required components
You need the following material to complete the conversion:

  • The LokPilot decoder, of cause.
  • Four 8.2 Ohm 1 Watt resistors.
  • Four 1 kOhm 0.6 Watt resistors.
  • Two 1.2 kOhm 0.6 Watt resistors.
  • Two soldering point brackets.
  • Double-sided tape.
  • Shrinking tubing with different diameters.
  • Wires in different colors.
Mounting the decoder
The decoder is mounted with double-sided tape on the black plastic cover over the motor gear, in the long end of the loco (the end where the wire from the pick-up shoe comes up). The connection side of the decoder should be up, and the wires should go out towards the center of the loco.

Figure 4. Mounting of the decoder.

Chassis and pick-up shoe connection
The two brown wires from the bogies are connected to the chassis with two solder point brackets that are mounted with the screws that held the original decoder. The contact through the chassis is probably sufficient, but I mounted an extra wire between the two brackets, just to be safe. The black wire from the LokPilot is soldered to the nearest solder point bracket. The red wire from the pick-up shoe is soldered to the red wire of the LokPilot, and the soldering point is protected with a piece of shrinking tubing. The wire lengths are kept to a minimum, since there is no room for excessive wiring inside the model.

Figure 5. Soldering bracket for chassis connection.

Front and rear light connection
The Märklin MaK models have LED (Light Emitting Diode) front and rear lights. The LEDs are mounted on small circuit boards at each end of the loco, but the series resistors are mounted on the main board of the original decoder. Each LED board has five LEDs, three white (yellow, actually) and two red.

Figure 6. LED board schematics, seen from the green side.

The LEDs need series resistors. On the original decoder, 1 kOhm is used when there are two LEDs in series, while 1.2 kOhm is used for the single LED. I have used the same values. I have soldered the resistors directly to the LED board. The leftmost connection on the LED boards shall be connected to the blue function return wire of the LokPilot.

I want to have the long end of the loco forwards as the default driving direction. This means that the white wire shall drive the front lights in this end of the loco, and the yellow wire shall drive the red rear lights. In the short end of the loco, the white and yellow wires have the opposite connections. The picture below shows the LED board at the short end of the loco. As you can see, I have protected the solder points with pieces of shrinking tubing to avoid short circuits when I assemble the loco.

Figure 7. LED board with mounted resistors.

This light connection scheme is valid for the German, Dutch and Austrian models. The Swiss 33642 and 37642 models have a different light scheme, with three white (yellow) lights forwards and one white (yellow) light backwards. A schematic can be found here.
A more detailed description of the Swiss light scheme wiring in the 37642 can be found at the author's site.

Connecting the motor
There are two wires coming from the motor. One is red, the other is red and black. These two wires shall be connected to the orange and grey motor outputs of the LokPilot through a 33 Ohm resistor. Since it is difficult to fit a 33 Ohm resistor with sufficient power rating (4 Watt) inside the loco, I will use four 8.2 Ohm 1 Watt resistors in series instead. The resistors serve two purposes:

  • They make the motor run smoother. Without them it is almost impossible to get a clean run at the two lowest speed steps.
  • They improve the speed characteristics (read: reduce the speed) in AC analog operation. Unlike the Märklin decoder, the LokPilot does not apply the maximum speed and acceleration/brake delay in AC analog mode, it just supplies the full power to the motor.
With the default direction I have chosen, i.e. the long end forwards, the red motor wire shall be connected to the orange wire of the LokPilot, and the red/black motor wire shall be connected to the grey wire of the LokPilot. I didn't have the 8.2 Ohm resistors, so I have temporarily used three 22 Ohm resistors instead. This works well too,
but I may burn them if i run the loco with heavy load for a long time. I will replace the temporary resistors as soon as I get the right ones.

Figure 8. Temporary motor connection. The 22 Ohm resistors used have insufficient power rating and may eventually burn if not replaced.

Programming the decoder
Now, the loco is converted, and it should be possible to operate it, but it needs to be adjusted to get the best possible driving characteristics. You may also want to change the loco address. The LokPilot is on-track programmable, and there are several different parameters that can be changed from the control unit. If you use a Märklin 6021, the programming procedure is described in the instructions that are delivered with the LokPilot. If you use e.g. the Intellibox, the programming procedure should be even simpler, they say. I use home made 6021-like equipment to control my layout, and I had no problem to follow the instructions. The tricky thing is to find out the correct values of the regulation parameters. I have spent quite a lot of time on this (see Initial investigations above), and I think the following parameters are close to optimal:
Parameter number: Parameter value: Description: Comments:
2 6 Minimum speed. The value here is partly a matter of choice, but I recommend this value to start with. The default value (3) gives a very low minimum speed.
51 58 Regulation reference. This value is close to the default value (56), but I could observe significant differences in behaviour between the values 56, 57 and 58. There was not very much change with higher values than 58, so I concluded that 58 would be the optimal.
52 25 Proportional regulation strength. A too high value gives jerky behaviour at low speed. The default value (32) doesn't give a clean run at the two lowest speed steps. Without the series resistors to the motor, this value has to be set much lower  (below 12) to get acceptable performance. 
A too low value causes a tendency  to over-react on sudden load changes and to run away in downhill grades. A very low value (0-5) also causes general jerkiness.
53 50 Integrating regulation strength. This parameter doesn't have as much influence on the performance as the two previous parameters. In the LokPilot instructions, it is said that this value should be lower with a higher momentum in the motor. I have come to a rather high value, despite the fact that the model has flywheels and thus should have a high momentum. A too low value gives the same negative effects as a too low value on parameter 52, but not at all to the same extent. I saw very little difference with values between 40 and 79, but it seemed like the value 50 was marginally better than both the higher and lower ones.

There are other parameters too that can be programmed, like middle and maximum speed, acceleration and brake delay, and selection/de-selection of different operation modes. These other parameters are mainly a matter of choice, and I don't want to give any recommendations. A good idea is to set acceleration and brake delays to low values, and maximum speed to 63, until you are satisfied with the regulation parameters.

More information about the LokPilot, with parameter values for a lot of different locos, can be found at

Observations and summary of the results
The resulting speed characteristics in digital mode are very much improved over the original decoder. The loco now runs smoothly (comparable to 60901) with and without load on flat ground and uphill grades. On downhill grades there is still a bit jerkiness at the two lowest speed steps, and with heavy load there is also a tendency to "run away". The downhill behavior can be improved with other regulation parameters, but not without negative effects on the flat ground performance.

The speed characteristics in analog AC operation were better before the conversion, but with the addition of the resistors in series with the motor, the difference is only minor. Again, it is the downhill grades that cause problems, in this case a too high minimum speed. Unlike the original decoder, the LokPilot doesn't apply the maximum speed and acceleration/brake delay settings in AC analog mode, but I think this is also a minor problem.

The LokPilot offers 28 speed steps in Märklin/Motorola mode, but the way to reach the extra speed steps is different from the original Märklin method, and they can therefore not be reached with a Märklin 6021 control unit. The Märklin method is to go through odd speed steps upwards and even speed steps downwards. A command sequence with speed steps 0-1-2-3-4-3-2-1-0 to a Märklin 60901 decoder will result in the loco speed step sequence 0-1-3-5-7-6-4-2-0. The LokPilot codes the extra speed steps in the unused bit of the function trit. (For an explanation of the Märklin/Motorola format, look here.) With my home made equipment, I could add the support for the extra speed steps quite easily, since it is just a matter of updating the control program.

Under normal load conditions, the model consumes very little power, at least after the conversion. (I didn't measure it before the conversion.) On flat ground it requires around 100 mA / 2 VA , which is about one third of a standard loco. The power consumption increases significantly with heavy load, but it is still low compared to other locos. The highest current I have measured is 270 mA, but theoretically the maximum should be around 375 mA / 7.5 VA.


The original article can be viewed at Per's site.