Temp guage don't work after engine swap?

I blew my 96 4.0 and put a 98 motor in it.The 98 don't have a temp sending unit so I put a rad flush kit in and drilled and tapped the cap and mounted ithe sending unit in there.I get power to the harness but the guage won't work.It reads at the bottom of guage.I put in another one and still nothing.The guage worked before I blew the motor.What else can it be?Thanks.

All fixed .I didn't have it grounded and do now and it works great.

Just when I was getting ready to tongue lash you for not having it grounded :D