Hi David, it will make no difference what way the terminals are pointing as the sensor just rotates round and round, timing is not an issue....... it just sends a pulse (digital) signal to the electronic Tacho.
(unless they copied the old Smiths system like my MK10 Jag, with a little generator on the back of the cam shaft, and the Tacho was basically a volt meter (ie analogue and not digital signal), but i doubt it).
Assuming it is still reading 18 with all power disconnected, The problem with your Tacho (Rev Counter) will be internal, most likely a little rust stuck to the magnet and jamming the needle.
you will need to remove the Tacho and carefully clean the movement (compressed air, or vacuum cleaner).
Then if required reset the Zero adjustment (on the hair spring) to zero the needle.
However, if it returns to zero with the power disconnected (ignition off) but reads 18 when powered up, it is an electrical fault, and most likely a bad earth, or you have the wires crossed over on the sensor.