Fans run only with coolant sensor unplugged

My 1990 (3.1 liter) overheats because the fans won’t run – they don’t run even when the temperature goes red. I have tried turning on the AC (it’s never worked since I owned this car) and turning on the defogger and still no fans. Both fans work IMMEDIATELY if I unplug the (brand new) coolant sensor – same problem with old coolant sensor. So I drive with it unplugged, and thus keep both fans running whereby the engine stays at a cool 120 to 160 degrees. The thermostat is also brand new (same problems with old thermostat), and the upper hose gets hot, so I believe the thermostat is opening properly.

My fan relay was really banged up (I think a mechanic did so accidentally, and never told me), so I replaced the relay. The fans started working properly again. Now, just one week later, the fans no longer work (unless I unplug the coolant sensor).

If you have unplugged the sensor and the fans come on then you either have a bad new sensor or the computer is not responding to the temp. overheat. There is a resistance check for the sensor and i believe if it reads between 1000-5000 ohms when at ambient (approx 80 degrees) its good. At 212 degrees it should be about 177 ohms, and at 32 degrees would be about 10,000 ohms. If the sensor is good the computer could be bad. Sorry for the bad news.

It is possible to replace the sensor with another temp switch that would open the circuit when temp reached 205 degrees. just an idea.

Check the grounds for the circuit. Something wierd there.

You don't have a load of teflon tape on the sensor's threads do you? That sensor has to be grounded.

if your sensor is a two wire, it may be a bad harness to the sensor. If the two wires are touching somehow, it would ground and turn the fans on. when you plug the sensor in, it causes an interruption in the polarity, and kills the signal to the relay that turns the fans on.