Discussion
Does it return to zero when the ignition is off?
no RTZ with ignition off usually means a stuck mechanism. Wedge tachos (if its the same Stewart Warner tach as a 390) are driven off a frequency to voltage converter IC which rotates a moving coil (i think... maybe moving magnet, but who cares anyway) against a clock spring. Ergo, it should RTZ from clock spring load when the juice is switched off.
They are take-apart-able, carefully. Very much NLA so dont break it!!
no RTZ with ignition off usually means a stuck mechanism. Wedge tachos (if its the same Stewart Warner tach as a 390) are driven off a frequency to voltage converter IC which rotates a moving coil (i think... maybe moving magnet, but who cares anyway) against a clock spring. Ergo, it should RTZ from clock spring load when the juice is switched off.
They are take-apart-able, carefully. Very much NLA so dont break it!!
Sounds mechanical then, like rust flakes jamming the armature. I repair radiation monitors with moving-coil meters and they often come in with tiny specks of crap wedging the pointer at some arbitrary position. As an aside, I've often thought it ludicrous that some manufacturers persist in selling hand-held instrumnts with taut-band movements in them, when they're so easily knackered if you drop them... 
- just what you need in a safety-critical instrument.
- just what you need in a safety-critical instrument.Gassing Station | Wedges | Top of Page | What's New | My Stuff