Originally Posted by Mike D

You might have a bad fan relay. Tap the metal can relays while you have the over heat sensor (not the temp sensor) grounded. Pretty common for those old mechanical relays to stick. You can pull them apart and clean them or replace them (my recommendation) with a newer plastic case Bosch relay. You have to do a little re-wiring of the ends but it's well worth it. I mean, it's all nice and well to have your car original but to have your car original with the pistons melted to the cylinder walls is not all that keen.