Opel Corsa 1990 160iE coolant fan doesn't come on - car overheats

SOURCE: 95 grand am radiator fan only runs with AC on

Cooling Fan Temp Sensor is the First thing You Check (with what you have described) Cooling Fan Should ALways come On When The A/C is turned On as A/C Adds A Load to Engine, If Temp Sensor is Faulty it Will Not Give GROUND Signal to ECM >For COOLING FAN >RELAY to Give ground and turn ON The Fan.
There ARE 2 temp Sensors that Serve Different Purposes 1 is for Dash READING GAGE & Or TEMP LIGHT the Other is For ECM or Fan Relay (SAME Thing) you Said>""can disconnect the sensor and fan continues to run" < This Leads me to think you Changed the Wrong One. Not Questioning your Skills Just Here to Help.
TEMP SENSOR FOR 3.1L V6 is Located upper RH rear of engine compartment, near exhaust system. Temp SENSOR for 2.3L Engine is Located in LH rear of engine compartment.
I Hope this Helps Lead you to a NON-OVERHEATING Pontiac.
Good Luck and Have A Great Evening. rejakwilson

SOURCE: when the car gets hot,

Start by checking the fuse panel under the hood to see if there is a cooling fan fuse and relay...after checking these components and determining that they are good...make sure the fan motor is good by hot wiring it to the battery...if the fan is ok then the sensor may be at fault..it is usually located on the housing where the thermostat is located or where the top radiator hose connects to...there may be two sensors though..one being for the temp gage and the other for the fan....you could disconnect the sensors one by one...start the car to see when the gage dies then you'll know which one is for the fan