Tiny Tach problems

[34 FORDOR]

I recently purchased 2 Tiny Tachs , one for my A and another for my flathead v8 . Does the pick up wire normally wrap around the coil wire or a plug wire. On the A , l put it on no4 plug wire and it showed 120 rpm at idle , moved it to coil wire and it increased to 480 - 490 rpm. The one on the v8 started out ok then locked up reading 533. I have emailed the supplier but after a week still no reply.