Totally bewildered just found this post.
I have a Man Tgl 7.15 which I purchased 2 years ago for private use for chassis swop for old daf 45.
Never turned on tachometer because only private use.
Yesterday getting tacho malfunction and on board computer malfunction signs on screen as well as permanent engine management light and hazard light on screen.
Been quoted £1500 for new tacho plus calibration.
Is anyway it can just be disconnected Been told has to be on vehicle but I notice you can fill out exception form for testing.
I