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Independent headlight for MM Baby GMs

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Dhu Varren

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I recently took the bull by the horns and decided to look into the possibility of modifying my Baby GMs to have independent headlights, thus being able to switch the headlights on and off independently from the marker lights.

Investigations revealed that it would be a relatively simple task to do.

The first task is to remove the body and cabs. The cab remote from the radiator can be removed quite simply as it is a clip fit and can be pulled straight up with a slight twisting motion. The cab at the radiator end is glued to the body, so the cab and body need to be removed together.

The cab interior needs to be removed by removing the two screws. Once removed, the lighting board can be lifted, but beware, the marker light prisms underneath are very delicate, particularly those for the early type marker lights, and the lenses are easily broken off. The two that I managed to break were repaired by using short lengths of optical fibre.

Once the lighting board has been lifted, a cut needs to be made in the copper track as shown in the picture circled in red. It is difficult to see even with a good light, but it is there. This track is the –ve for the headlight LED.


Turning the board over, there are four terminations along the back of the board. Three of them have wires attached which are connected to the marker light LEDs, the fourth one has nothing attached. A wire needs to be soldered to this termination, see picture, circled in red.


The other end of the wire is soldered to pin 15 of the socket on the main circuit board, a very fine soldering iron is needed for this task. Pin 14 is connected to the lighting board at the other end of the loco. Pin 15 is the AUX1 output from the decoder, and pin 14 is the AUX2 output. Neither of those outputs is used for any purpose on the Baby GMs, and is not connected to any circuits on the circuit board. The heavy blue wire along the top has nothing to do with this modification, it is a repair to damage on a copper track on the circuit board which was there when the loco was purchased secondhand.


Once all wiring is complete on the first end, it can be reassembled, taking care not to damage the light prisms, and ensuring that the prisms are aligned in front of the LEDs. I found that the black insulation tape used for masking the LEDs was best fitted after both ends were reassembled, and using fresh tape cut to size. Once the cab interior is refitted, everything is now secure, and work can start on the other end. Before going any further, it is a good idea to ensure that the wires attached to the rear of the lighting board are not fouling the front of the drive bogie, otherwise the bogie may not pivot fully.

Once the second end has been done, black insulating tape needs to be applied to stop any light leaking where it is not wanted. It is very much a case of trial and error with this, but essential if you do any running in low light conditions.

All that remains to be done now is to ‘map’ the decoder with which function button you wish to use for the headlights. In addition to an independent headlight button, I have mapped the decoder so that the headlights only work when the marker lights are turned on. Turning F0 off turns off all lights.

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On 26/1/2021 at 10:37 PM, murphaph said:

Thanks for this. I intended doing this mod as well and actually had assumed that a single led would be found inside. It's a pleasant surprise to read that the required led is actually present but just needs isolating and rewiring. Great stuff. 

Yeah have to agree! I was considering using a couple of SMD's (0603 or 0805) and wiring them to the PCB. 

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  • 1 year later...

Seeing as this thread was resurrected elsewhere I can contribute something useful to it.

The above pictures show a design flaw with the babies leading to light bleed. There's a hint of white light being emitted from the red marker light. Paddy seems to have corrected this on the big GMs as I found out a couple of days ago when dismantling one. The light pipe design was modified thus:


The light pipes received a silver paint (NOT where the light enters or exits obviously!) to help refract light internally presumably but also that black strip of plastic was inserted between those adjacent light pipes to ensure no cross contamination as a belt and braces approach. This improvement could fairly easily be retrofitted to the babies if you are going to the trouble of fixing the independent marker lights problem.

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