Replaced old non-working tach with new, tach zeroed out with power on. Started engine, tach indicated RPM, engine stalled, re-started, tach stopped responding and is "stuck" at 800 RMP. Had replaced igition switch but do not know if old tach died before or after replacement of switch.
Engine starts and runs fine with new switch. What could kill tach?
If the electrical system is fine (no serious overvoltage situation) then chalk it up to what's called in the electronics world as "Infant Mortality". If new electronics fails it generally does so very early in its life (45 -90 days).
Problem turned out to be a intermittent connection in 12 volt supply to tach. Trouble traced to 3 pin connector feeding voltage to igintion terminal on tach. Pulled pins from connector and scraped oxidation from pins, replaced and problem solved.