My Märklin 37881 Era 2 steam loco a class 44 2-10-0 freight unit has always been a good and smooth runner.
Came from the factory fitted with the digital high efficiency kit with 5 pole motor and the first generation 6090 decoder.
The two main differences between the first 6090 and later 60902 decoders are the extra functions and that the later one has a common + wire for the functions.
On Märklin decoders this wire is Orange, others like ESU this is Blue.
With the older decoder the function only get a half wave pulse so the lights flicker, not so nice.
So with the help of the schematic for these decoders I could work out where to solder on the Orange wire and then lead these to the front and rear lights for flicker free lights.
With two sided tape I stuck a piece of circuit board onto an open spot on the decoder so that I could easily connect a few Orange wires.
Then I went one step further and also found the decoder minus which then allowed me to fit a few capacitors to allow the engine to run over dirty track.
Here a diagram how to connect these.
Pluspol being Orange or decoder Positive, Masse being decoder minus.
Here's a pic of the 5 caps that I fitted into the top of the tender, 5x 470µf 35V.
To test I stuck a piece of tape onto my test track just longer than the pick up shoes and had the loco run at a very slow speed and with out stopping she went right over the tape.
Although this is a simple mod to do, if you make a mistake or are not very good at soldering you can very easily fry the decoder, so do this at your own risk.
