My Dapol Terrier runs fine on analogue straight out of the box.
But with a Zimo MX644D (21 pin) installed I have a mysterious problem. In reverse only it runs and then stops for maybe 2 to 4 seconds then starts off again only to stop again after a short while and so on. When the loco stops the background sounds continue and the firebox still glows so it is not a pickup problem.
In forward direction it works perfectly. I have tested the decoder separately on my ESU test rig and it performs faultlessly. I have tested the loco on 12 volt analogue with and without the chip fitted and again it is perfect.
This defies any logic - I do not perceive any mechanical problems - the decoder works on the ESU test rig and the loco runs fine with the decoder fitted on DC. I am missing something but do not know what else I can check.
I thought there could be contact between the underside of the decoder and the loco wiring board but I inserted a slip of paper between and eliminated that as a possibility. I have double checked the wiring between the board and the motor.
I have tried it on 14, 27,28 and 128 speeds steps just to eliminate a possibility.
I have fitted the default capacitor that comes with the decoder but have not tested without this.
Any sensible suggestions will be appreciated?