My radiator fan decides on its own to run high speed.

I pull the plug off the radiator thermo switch and the fan keeps running.

I remove the new high speed relay and the fan stops. I replace the relay with the old one and the fan runs high speed again without the radiator thermoswitch connected.

This happens even when the temperature guage is showing a moderate temperature and the air being pulled through the radator is fairly cool as well.

If the car stands overnight the fan does not come on when I start the engine. But as soon as the engine gets a bit warm the fans run high speed. Even with the radiator thermoswitch disconnected.

What else can trigger the high speed relay?
I've looked at wiring diagrams until I'm squint!

are you sure it is wired to the resistor correctly?

Checked resistor wiring but anyway that had not been touched.
The radiator fan functioned perfectly until a couple of days ago.