You don't connect the blue wire from the decoder to the ESU tester - the LEDs pick up the common positive (blue) from the tester itself.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
I just bought an ESU 51900 decoder tester the package had every little about it in english. I did find a pdf file with a little bit more. I was wondering on it's useage. so far i have only seen the first two leds light up. Also on the decoder leads for the lights on the loco it has a a place for the front light (white wire) and the rear light( yellow) but nothing for the blue decoder wire. I have n gauge and wonder if I can check a drop in decoder. Any place to find more information?