Originally posted by ghosthunter
Question: how does Autoguage tach gets its reading on the engine's RPM? Does it tap to the stock tach or where?

I'm just curious if such tachometers can be installed into the new Toyota VIOS since its stock tach is digital LED.
A tachometer is basically a frequency meter with its gauge face showing rpm. In the old contact-point ignition system, it connects to the contact-point (or at the (-) terminal of ignition coil) where a nominal 12-volt pulse occurs for each spark. The deflection of the tach needle is directly proportional to pulses per second, i.e., frequency.

In the modern electronic ignition system, the ECU sends the pulses to the ignition assembly. The pulses could be at 5 volt level only because 5 volts is the typical operating voltage for most digital circuits.

Surely, it is possible to connect analog tachometer on VIOS. You only have to find the proper signal, the shop manual at casa will show the signal as well as wire color.

The current analog tachometer (like an Autogauge unit) will probably work on 5 or 12 volt pulses, because it is the frequency that matters and not pulse amplitude. If a tach (probably an old tach) accepts only 12-volts pulses and the ECU outputs 5-volt pulses, it is not difficult to assemble an adapter to convert the 5-volt pulses into 12-volt pulses. The adapter may only require a transistor and two resistors and can be hardwired directly along the connecting wires.