I have not directly checked the signal at the back of my tach but I will. The thing is, I thought that even if the tach wiring is screwed up, the SAFC 2 would get a signal since it is connected directly to the ECU. There is a value coming through the red/yellow wire which is the tach signal and this wire is connected directly to the ECU. Also, when my friend looked at the S13 wiring diagram, he said that if the fuel gauge signal is coming through, then the tach signal is also coming through.
Oh, and I just checked the SAFC2 manual and its actually In-1 I was refering to and its not the tach signal, its actually the airflow signal.