On the Roland TD-17, the Ambience effect can be subtle in some factory kits, making changes hard to notice when turning the AMBIENCE knob. To check whether the knob is working, select kit #33 "Jazz Funk," #25 "ClassicMetal 80-90s," or #26 "60s Rock," and compare the sound at the minimum and maximum knob positions.
Monitoring with headphones is recommended. Turning the AMBIENCE knob does not show a value on the display; this is normal.