Sending unit inop

Well I got the gas tank leaks fixed by having several small pin holes welded. After getting the tank back from the shop, now the gauge reads full (when empty). The shop said they welded around the stud for the sending unit, because it was leaking. If I disconnect the chassis power wire from the sending unit, the gauge goes to empty, whcih tells me the gauge is still working. I dropped the tank and removed the sending unit, cleaned all connections (inside the sending unit too), but still have the same results. I tried ohming the sending unit, but couldn't get anything other than .000. Didn't seem to matter what postion the float lever was in, still had .000. Anybody have any thoughts on what's going on? Gauge worked fine before the tank and sending unit went to the shop.