Conventional Tach on LS2

[TR65]

I took a look at the tach signal that the ECM is putting out on the tach wire. This is for the E40 ECM so I don't know about other ECMs. The tach wire is used to drive the OE tach in the vehicle.

The ECM puts out a low voltage square wave that is two cycles or "tops" per engine revolution. That is the same as the number of times a 4 cylinder fires per revolution. So that is why we have to use the 4 cylinder setting to get the correct RPM. (V8 fires 4 times per rev.)

The first picture is the signal from the ECM. So the (-1.5 V) voltage is probably too low to fire the traditional tachs that are looking for around 12 volts.

The second picture is the signal with the pullup resistor installed. So now the voltage is up to 10V at the high. So the tach wroks fine.

The engine was idling at about 680 RPM or 11.3 rev per second. The frequency of the signal is twice that, around 22 cycles per second.

TR