You're right, I checked out the
installation guide and there doesn't appear to be a voltage sense mode. Looks like you might need to find another more reliable tach signal. They give you some suggestions along with the required signal levels on page 6 for the purple wire. You could use your multimeter to see if your signal meets/exceeds the minimum requirements. If your signal is correct, you might then want to go through the tachometer programming steps outlined on page 12.
-Rod