Re: 1977 175 rpm guage
This engine has an "on/off" switch in each head. When the switch senses 212 degrees, either will trigger the hot horn in the control box. The horn will silence when the head temp gets back down below 175. The factory coiled a grey wire up inside the control box. This is the signal wire for the tach. If someone has used this previously, it may be dangling from the box. You will need a ground and a hot, plus the signal wire to operate the tach.