Hey all i have a 98 S90 with about 156k miles and the cooling fan will stay on up to 5 minutes after a drive, even if its a short drive with cold weather. My battery finally died this morning from the fan staying on after each drive (Duh). Why is the fan staying on and how do i fix it?

 

Is your temperature gauge working?

Try unplugging the pressure sensor on the A/C dryer bottle. It's a 2 pin connector. The dryer is under the hood, right side, forward of the firewall. It looks like a keg and has A/C pipes attached to it.

 

That is a common problem on the white block 850's. It's the ECT, engine coolant temp sensor going bad.

 

The instrument cluster in my car seems to get the temperature from the ECU. So, if the temperature gauge is not working, I'm sure the ECT is the problem.

I was mistaken about the A/C pressure sensor on the dryer bottle. This sensor will prevent the A/C from starting, it shouldn't activate the cooling fan.

The A/C pressure sensor near the condenser is one thing that controls the fan. It's got three wires: Green/Orange, Green/Grey, and Brown/Black.

This pressure sensor runs the fan as follows: if pressure is over 1800kPa, runs at low speed; if pressure is over 2400kPa, runs at high speed.

Unplugging this and running the car will probably set your check engine light.

When you start the car the fan should be OFF and stay off until it warms up. Even on the warm days (70-80F) I only notice the fan cycling at idle when stopped. It seems to come on when the water is 93-94C and shut off about 91C.