tacho questions.... DFI vs. distributor

[RomanV]

This is regarding wiring my redtop engine (Which has coil pack ignition) into my '90 MR2 (Which ordinarily, had a distributor)

What I want to know, is how/where the tacho signal comes from, in each of these cases, and what it consists of.

We put the oscilliscope on the 'tacho' output of the loom on the DFI setup when the car is idling, and we get a 12v square wave output, at about 35hz.
Which would indicate an idle speed of 1050 rpm (35hz x 60, /2 = 1050)
I would have thought it would be a 5v signal, since it comes from the ECU? (AFAIK?)
Now what I dont know, is where/how/why the tacho signal is generated for the 3S engine that runs a distributor.
Why are they different, and how?
I've heard that toyota tachos changed from +97 onwards, but I dont know how or why.

Any ideas anyone?

[sergei]

As far as I'm aware, the tacho signal (IG- output on the diagnostics tab) on most toyota (if not all) is 12v Square.

[fivebob]

Earlier engines took the tacho signal from the connector to the ignitor. It was a high voltage signal (>20v) and probably comes from the coil trigger, but I don't know for sure.

Consequently aftermarket ECUs have trouble runninng the stock tacho if you're not using the stock ignition system, as most don't supply enough voltage. This will also apply to the late model ECU which, even at 12v, will not be enough to run the tacho.

[escortman]

hey jus bought a ae92 fxgt rolla i took my 3 1/4 tacho out of my ae82 and went to install it into this all went fine until i went to connect the line to the coilpack, how do i connect to this or do i take it from the dash, its got a small port redtop