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Cooling fan comes on as soon as ignition is on


badbrakes
07-10-2009, 08:18 AM
Just had a problem with my 96 corolla (200 000 kms) and I thought I would post how I repaired the above - hopefully could be useful to someone else.

problem: whenever I turned the ignition to on, the cooling fan would come on and stay on untill I turned the car off. The car was behaving this way regardless of whether it was cold start or hot start.

In my case, the problem was the cooling fan switch (or some say sensor). It should test for continuity when engine cold. When the engine is hot, it should have no continuity. In my case, I had no continuity with the engine cold... so therefore the sensor 'tricked' the fan relay into thinking that the engine was too hot, hence the fan was always on.

Note that on this Corolla, there are two sensors (at least) for the cooling temperature. There's the cooling fan switch sensor and there's the sensor the sends the temperature to your temp gauge. That had me scratch my head a bit.... cause at first, I connected the car to an OBD2 and could see it was reading the temperature ok.... so I had initially ruled out the sensor... but a bit more research and I realized the fan has its own sensor.

Note that some posts point to Fan Relay - however I read that these are typically reliable.

How to fix it:
very easy.
the sensor is right underneath the distributor. Here's what I did:
1) empty about 2 litres of rad coolant into a clean container (you don't want coolant to leak when you take the sensor out.)
2) Unplug the sensor.
3) Pull the coolant overflow container out of the way. it's a slide in... just pull it up and out of the way.
4) use a 17mm deep socket, remove the old sensor, pop the new one right in.
5) reconnect sensor, put overflow container back on. Pour coolant liquid back in. make sure rad cap is on.
6) you're set. 30 minutes max.

here's a picture of what the cooling fan switch looks like.

http://s36.photobucket.com/albums/e37/badbrakes/th_IMG_2345.jpg

jdmccright
07-10-2009, 11:24 AM
Thanks Badbrakes. I'll add a tidbit to say that sometimes these fan temperature switches will fail the other way where they will prevent the fans from coming on at all. You can bypass the switch by unplugging the connector before turning the ignition on so that the circuit is open, causing the fans to run until you can get home or to a repair shop. Thanks again!

badbrakes
07-10-2009, 12:20 PM
Excellent point JDMCCRIGHT. This was all new to me - but part of my periodic 6000 kms maintenance will now include testing whether the fan works nicely... 30 second test... as you say unplug the cooling switch, turn the ignition on --- if fan doesn't come on - something's wrong.

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