RPM/Tach signal issue

[the tech]

After searching all of the 240sx/SR20 forums out there, I could not find the answer to this problem that I have. I recently picked up an 89 fastback with SR20det. Everything works well so far except for the tach. I have two DOHC clusters here and still can get no tach to work. After checking the yel/red wire, I come to find that it has been cut in several places. More research finds that it is not connected to anything in the engine compartment. I need to figure out where the wire is supposed to go and/or where the SR20 gets its tach signal from. I have checked the FSM and I can find nowhere where it tells where the ECU gets the RPM signal from.

All input would be helpful.

[DuffMan]

The ecu gets the rpm signal from the cas. You just have to hook up the yellow/red wire to the tach, through the white plug by the ecu.

[the tech]

Unfortunately there is only one part of the white plug there and the yel/red wires are connected to nothing.

[DuffMan]

So solder or crimp the wire.

[the tech]

No go on that one since I'm getting no voltage from the #3 pin coming out of the ECU.

[Gladman]

hook it straight up to the tach signal on the coil?

[DuffMan]

There is no tach signal on the coils.

On mine, I saw a voltage signal that increases with rpm (a square wave with a fixed length pulse).

If you dont have that, well I dunno know why you would not.