what determines when the cooling fan comes on ?

[vibebob]

I did do a search on "cooling fan" and "fan" in the engine forum and came up with squat.so, what determines when the fan kicks in ? last week, it was cold (38F) on the morning drive to work, which is 7 miles, mostly city with about 2 miles of highway (speed 60mph) when I got to the gate the radiator fan was running. (removed), it is cold, why should the fan be on ?the next day, same path and speeds and about 10F warmer, this time the fan wasn't on. Now I am really confused, so hence the question ? I have an 05 base, auto with 4500 miles on it.

[Mavrik]

I believe its controlled by an engine temp sensor that decides when the engine over all reaches a certain degree... it kicks in to cool the engine back down to desired temp. Probably not even detectable on the temp needle we have on the dash. Now my old Z24 cavalier I could watch the temp rise then drop when the thermostate opened or the fan kicked in heh... but these Vibes are a little more high tech. To be honest, I can't ever remember hearing mine or seeing it on. weird... being -8F right now, I doubt I will for a while lol.

[joatmon]

matrix repair manual has this inspection procedureQuote »COOLING FAN SYSTEMON–VEHICLE INSPECTION1. CHECK COOLING FAN OPERATION WITH LOW TEMPERATURE (Below 83C (181F))(a) Turn the ignition switch ON.(b) Check that the cooling fan stops.HINT:If not, check the cooling fan relay and water temperature sensor, and check for separated connector or severedwire between the cooling fan relay and water temperature sensor.(c) Disconnect the water temperature sensor connector.(d) Check that the cooling fan rotates.If not, check the fuses, cooling fan relay, ECM and cooling fan, and check for a short circuit between thecooling fan relay and water temperature sensor.(e) Reconnect the water temperature sensor connector.2. CHECK COOLING FAN OPERATION WITH HIGH TEMPERATURE (Above 93C (199F))(a) Start the engine, and raise coolant temperature to above 93C (199F).HINT:Coolant temperature is the detected value by the water temperature sensor on the water outlet.(b) Check that the cooling fan rotates.HINT:If not, replace the water temperature sensor.3. INSPECT COOLING FAN(a) Disconnect the cooling fan connector.(b) Connect battery and ammeter to the connector.(c) Check that the cooling fan rotates smoothly, and check the reading on the ammeter.Standard amperage:Approx. 8.0 – 12.0 A at 20C (68F)(d) Reconnect the cooling fan connector.

[vibebob]

thanks for the input guys and the trouble shooting tree joatmon. this is kinda like the hard to start problem, it happens so infrequently it could be hard to find. I'd hate to blow this off and something happens to the engine, it is under warranty though. Then again, I'd hate to waste all that time for them to find nothing. I'll stop by and see what the dealer says.

[joatmon]

It could be that there is no problem and the system is working normally. I think the fan gets kicked on when the coolant temp hits a threshhold. It could be that on the colder day, the coolant temp had just reached the fan threshhold, so when you stopped the fan was running, but on the warmer day it reached the threshhold sooner, the fan came on, it cooled down the coolant and had switched off by the time you stopped and noticed it wasn't running. As long as you are not seeing any overly high readings on your temp gauge, or seeing it noticeably change while driving after it has warmed up, then I'd say it is working as designed, switching on and off to keep the coolant temperature in the desired range.

[djb383]

I believe the cooling fan also operates whenever the a/c compressor is on. The two windshield defrost positions on the left dash selector knob will engage the compressor, which engages the cooling fan.