100 Series subtank guage

Hi all, have recently purchased a 2002 gxl (2uz). When switching to sub tank the guage drops to empty and low fuel light comes on, filled to the brim, same behaviour. It does run off the stub tank no problems.

Pulled out the sender, between two pins resistance changes between 180ish ohms and 0. Between another pair it goes from open circuit to closed. Assuming that's for low fuel light. Seems to me this is working? Have checked these values at the harness where the subtank meets the rear body harness, all seems ok there too (about 90ohm). Have tightened the screws in the cluster where the wires come in. No dice...

Assuming something is kaput in the cluster? Any other ideas appreciated!