Good tip on the cluster.? I believe that my cluster has bad connections also.? When I was working underneath the dash there, my cluster display went crazy due to minor movement of the harness cable.? I was eventually able to fiddle with it (was on the road) and get everything showing correctly again without repair but I believe it is a weak spot that wants attention.??
I guess that an easy way to check the coolant level sensor would be to pull the connector and check continuity across the pins on the orb while full and then drain, siphon or suction out (turkey baster/syringe) enough fluid to make it low and check again.? If continuity is different from full to say half full, then it's not the orb/sensor.
Oh - now I read that you did test it.? Good.? Easier to fix than the cluster.? I would definitely try cleaning everything first.
