Sending unit question?

I was having a problem with a small leak where the fuel return line goes into the tank,so i got a different(used) sending unit from a buddy. It does not match my current sending unit so i don't know if it is the correct year or not.When I went to checked to see if it was working correctly, it read the fuel completly backwards. My question is, is this the wrong year or did I do something else wrong?
Thanx Digger

After some further checking ,this sending unit is out of a 94 xj, I'm trying to put it in a 90. Can anyone tell me if this will work or do we have a sending unit fuel pump setup we need to sell?

Not sure myself. I just had a new one put in at the dealership. But what I'd do is call up your local dealership, and ask them for compatibility. They'd know if it is or not. I use them as a resource for a lot of inquiries and questions about these types of things. Some of the parts guys are really helpful giving me print outs of what I'm asking about. Worth a shot!

I do not believe that those are compatable. I think I read on here someplace that the sending signals (or something to that effect) are opposite on newer ones.