Which board do you have (v2.2/v3.0/v3.57)? Like Dave asked, are you using MS1, MS2, or MS3? For MS1, the FIDLE pin on the processor can't generate a tach signal like it can with MS2/MS3. You would be better off using JS0, JS1, JS2, JS3, JS7, or pin 10 if you can. You could build the circuit externally, but it would be better to build it inside the enclosure.