Subject: One treadmill, secured for free in non-working condition from Freecycle about a week and a half ago.
Problem 1: Treadmill belt doesn't run, though it can be turned by muscle power alone.
Solution: Found one 0.25" spade connector to the emergency-stop switch in the control box was disconnected. Reconnected it. Presto, treadmill works. (This was several days ago.)
Problem 2: Treadmill computer powers up, but never displays anything, all readouts stay at zero.
Preliminary diagnosis: "I'll bet the computer's not receiving any tach signal."
Solution: Trace tach signal line, find mis-aligned Hall-effect sensor, re-align sensor. Presto, treadmill computer works.
Currently it appears I can manage about half a mile at 3.7mph before my knees beg for mercy.