Tach can problems

I Installed the guts in the can. Purchased a Formula S Cluster from a fellow member last year. Purchased a can that was no good and got the insides to put in. I used the wiring listed on the electronics i will verify my wiring again but it was pretty straight forward on the pcb silkscreen. Grounded out the tach works just very low, as in idle shows about 100 rpm and 2-3k is about half at 1500 readout. noticed when i would bump it with my left foot it would work for a couple days so i pulled the can from the kickpanel. Then running not attached it seemed alot more correct. I am sure i still need to adjust / verify with a under hood tach but non grounded case was better.