z32 harnesses get very brittle overtime, that is probably why you ran the wire straight to the guage cluster...but are you positive you ran it to the right wire? just double check, i've seen 3 colors of nissan temp wires, blue/black stripe, yellow, or yellow/white stripe. for your Z should be blue/black. but anywho you could just do that with an autometer.

it's too frickin hott outside for it to read cold for a long ass time, or i'd inquire if the car had a thermostat in it. anywho, didn't see any replies, figured i'd add my .02
good luck
Dave