I know people who've swapped alternators and report about 10% variation.While we're on the subject, if the alternator is used (tach lead), then there must be a ratio that has to be maintained to get a reading that corresponds to crank rpm? how is that calculated and how accurate are they? Think "mixing and matching" alternators when doing these swaps. I'd prefer a magnetic pick up and sensor mounted to the crank pulley or flywheel...any sources for, and experience with?
I have a tiny tach under the hood on my carryall. It does infact have a transducer that clamps on an injector line. Besides digital display of Engine RPM, it also shows Max Rpm until reset, also engine on hours etc. Nice little unit, BUT.......its a throw a way. It has a internal battery that is part of the PC Board that can not be replaced. When the internal battery finally quits, buy a new one. I have had mine running since 2004. The Tiny Tach also requires external 12 volts along wiht the battery.Another option which gets some mileage on the iH8mud forum is a tiny tach. I've never used one but it sounds like it picks up injector pulses.