I have overheating problem. 1996 Didco 1 3.9i AUTO. Cleaned radiotor at Thermo Radiators locked viscus fan cut theromstate. Drive in town with aircon on OK no problem but take open road 6 kilometers it overheat with aircon on , constant in the middle of guage at 100km / h. If 100 km/h is exceaded it overheats?

 

Radiator may need to have calcium rodded out by a rad shop. Builds up, blocks flow in lower tubes, you have half a radiator eventually and it can't keep up with heat load. Shows up as bottom rows of fins more than 10F cooler than top rows. Viscous fan has little impact at motorway speed. You can run without thermostat for testing, or cook thermostat in water on the stove to check operation. If water pump is to be replaced, consider after market version with larger bronze impeller.

Fan belt route is important - please double check.