My engine fan turns on when I turn the ignition on but not the engine. Why does it do that?

Asked by May 19, 2015 at 10:18 PM about the 1999 Toyota Corolla VE

Question type: Maintenance & Repair

I have a 1999 Toyota Corolla and when I turn the ignition switch on the motor fan turns on without the engine being on. The engine is cold and everything. I have researched and found out which relay switches are for each fan and replaced them, I have replaced the engine coolant sensor next to the thermostat, and I have replaced the computer. Granted all replacement parts were used. Relays were taken from a junk yard but had 6 total relays of the same numbers, and 2 sensors. I am playing off of the odds that there is no way that all relays and sensors are bad, and the computer I ordered was used as well. I do not believe its the computer but wanted to try. The car does not overheat or have any other malfunctions. My only concern is that the fan may eventually burn out from constantly running, and that its something that I want to know what is wrong with it. Do you have  any idea what it could be?

8 Answers


somebody could have wired you fan straight to the battery so when the car is turned on the fan could come on. they could have done this because maybe the other fan is broken (my corolla has two fans). also you do know that the plug that connects up to there thermostat housing if not connected will turn your fan on all the time this is a safety measure just incase the thermostat locks closed.

8 of 8 people found this helpful.

See attached photo. The Engine Main Relay (7) energizes when the ignition is in ON position. The Radiator Cooling Fan Relay (8) energizes when both the Engine Main Relay (7) is energized and the Engine Cooling Fan switch (10) is closed. The Radiator Cooling Fan Relay Contact (8) is normally closed, so when the Radiator Cooling Fan Relay (8) is energized, its contact is open. When the engine is cold, the Engine Cooling Fan switch (10) is closed. Therefore when the ignition is turned on and the engine is cold, both relays are energized, and the Cooling Fan Motor (9) does not receive any current as the contacts in Radiator Cooling Fan Relay (8) are held open. When the engine coolant comes up to temperature (90C), the Cooling Fan switch (10) opens and this breaks the coil current in the Radiator Cooling Fan Relay (8) which causes it to de-energize and "drop out". The contact in the Radiator Cooling Fan Relay (8) is normally closed, so it closes when the relay drops out. This provides a current path to the Cooling Fan Motor (9) and the cooling fan operates. Diagnosing: If you discover that the Cooling Fan Motor (9) is operating whenever the ignition is in the on position, then either the relay contact is permanently shorted (Closed), or the Cooling Fan switch (10) is stuck in the open position. In either case, the failure was a "safe" failure in that the fan operated in the event of the failure. In the cases where the Cooling Fan switch (10) becomes stuck in the closed position, this is an example of an "unsafe" failure as explained in this video: Fixing: Try the click test with the relay like in the video above. Then remove the relay and measure for voltage across the relay coil contacts. If there is no voltage, and you hear no click, the Cooling Fan switch (10) is stuck in the off position. If there IS voltage but you do not feel a strong click (but there is a weak click) then the relay contacts are welded together by arc-current. If so, sometimes, giving the relay a sharp rap on a flat surface, once on each side, will temporarily cure the problem by breaking the contacts free. If on the other hand you pull the Radiator Cooling Fan Relay (8) out when the engine is cold and the ignition key is in the on position and the cooling fan is still running, then there is a short between Engine Main Relay (7) power contact and the Cooling Fan Motor (9) The video above is for an older Corolla. This Video shows the layout of the 99 Corolla Relays. The green Relay (the one the guy is calling a "Condensator" ROFL) is the Radiator Cooling Fan Relay (8) and the Grey relay on the top right is the Engine Main Relay (7). Please post your experiences here after trying this diagnostic procedure as well as any additional findings, so others can learn.

50 of 50 people found this helpful.

The cooling Fan switch in 98-2000 Corolla is located in the thermostat housing behind, and below the alternator on the passenger side. You will need to remove the alternator to get at it. It has a single wire. A sender in the engine water jacket with TWO wires is the temperature sender NOT the cooling fan switch. Signed: Father with daughter attending school in another state...Yay!

8 of 8 people found this helpful.

If the cooling fan switch appears to be stuck in the off position, but then you measure a short circuit (0-10 ohms) between the cooling fan switch terminal and the engine block (indicating the cooling fan switch is closed), then there could be a fault (open) in the wiring harness. With the ignition OFF, measure the resistance between the cooling fan switch receptacle on the harness (you will have to unplug it to do so) and the cooling fan relay coil socket. One of the coil socket receptacle contacts should be connected directly to the cooling fan switch receptacle on the harness. Remove the cooling fan relay, then measure between contact 2 on Item 8 in the drawing and wire L or wire L/B on item 10. (should be zero-ten ohms). If not, the wiring harness has an open between those two points. Since you have already swapped relays and switches, this is likely the case. The most likely failure point is the end of the harness that connects to the cooling fan switch (item 10) at or above the cooling fan switch receptacle, since this receptacle is on the switch (item 10) which is on the motor and vibrates with the motor, which could fatigue the wire or the crimp connection at the receptacle. You could try cutting the receptacle off of another vehicle as far up the harness as possible (so you have the cooling fan switch receptacle on the end of say, 1.5 feet of wire), doing the same on your vehicle, and splicing in the "new one" (with BOTH solder and electrical tape, for sure) at the point where you cut off the old one. Let us know what happens!!!!

6 of 6 people found this helpful.

Here is a wiring diagram for a 98 from the library. I think it applies from '97 to ~2000 or 2001

14 of 14 people found this helpful.

The first drawing I posted disappeared. The first long post above refers to this drawing.

13 of 13 people found this helpful.

Mine because the expansion valve blocked(clogged) (not cool) so the pressure triggered by the AC pressure switch and make the fan always on...after change the drier and expansion switch all back to normal...(Toyota AE101 year 95) TQ

2 of 2 people found this helpful.

I have a 01 Toyota Corolla my engine fans do not turn on the car is running they work I hooked the wire to intestinal the battery must fans work and when I unplug the temperature sensor both fans turn on but my car will not start and I just replace the sensor any ideas

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