By on February 22, 2010

Scott writes:

Why is it that the radiator fan turns on when I switch the heater knob to “Defrost” and not in any other mode? I have a 2001 Subaru Outback that does it even with the engine off. Turn the key to “On”, set the vents to blow on the windshield, turn the fan speed on any speed except off, and the radiator fan will turn on (and I’m not confusing it with the heater fan. Open the hood and it’s one of the radiator fans spinning). My 1999 Toyota Camry doesn’t do it with the engine off, but does when it’s on.

When I sit with it idling and have the vents pointed at the windshield, you can hear the radiator fan kicking in intermittently as needed. In both cars, with the selector in any other position, the fans don’t run nearly as often. Both cars are the base four-cylinder models with the typical three-knob HVAC layout (both lack automatic climate control). Thanks in advance.

Sajeev answers:

Scott, the answer is simple and complicated.  The simple part: the radiator fans kick on because the A/C compressor is running. You need the fan to make the A/C condenser (that radiator thing that’s next to the engine’s radiator) more efficient.

The more complicated part?  You need air conditioning when you use the defroster.  Dehumidified air quickly removes dew/fog from the windscreen. Think of your HVAC control panel as a Triage department at your local hospital. Cold feet in need of heat?  Tough: wait for the heater to get to work. But running the Defroster is an all hands on deck, like a head injury patient coming through the door. This isn’t a hangnail, you have impaired vision and the risk of an accident goes up high. Way high.  Which is why a failing HVAC vacuum control unit always defaults to the defrost position.

I don’t know why your Subie runs the fan when the car is OFF and the Camry does not.  Perhaps it’s concerned with blowing off the turbo’s extra heat, or perhaps Toyota engineers like wiring everything to the ignition circuit. Not a big deal, just be glad it works when the motor is running.

Bonus!  A Piston Slap Nugget of Wisdom

Let’s talk drag racing.  No really, you’re gonna love this. Heat is the enemy to any motor on the drag strip, especially turbocharged applications. Cooling down between runs ensures you won’t abuse your ride for diminishing returns.   Do yourself a favor in the pits: run the engine with the hood open, set the HVAC to defrost, turn the temperature to full heat and run the dashboard fan at its highest speed.

This does two things: with the A/C running and the fan on, you force cool air through the radiator and out the open hood, lowering engine coolant temperature.  And with the heat on, a second radiator comes online: the dashboard’s heater core dissipates heat like the front radiator, doing a great job if the HVAC fan runs at full tilt.

Sound silly? This trick kept my trap speeds level and my temperature gauge down after repeated laps, but don’t take my word for it: hit the drag strip and see for yourself.

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16 Comments on “Piston Slap: Going Out With Fans Blazing...”

  • avatar

    In a word, Ding! Sajeev nail;ed it.

    As to why the fan will run with engine off but ign on, the system simply isn’t smart enough to know that IGN-ON does not always=Engine Run.

    Your Toyo apprently does monitor a run signal, not just an ign switch position signal.

  • avatar

    Many air conditioned vehicles will run the fan with the ignition off if the temperature of the coolant exceeds some value. I have walked in parking lots by vehicles that were off but the radiator fan was running. I assume they run until the temperature reaches a preset value or for a fixed amount of time. (The coolant temperature rises for a time on an engine that has just been turned off – and maybe running the radiator fan lowers that temperature rise)

    • 0 avatar

      Permitting the fan to come on after the engine is switched off and the key removed is a safety concern, and I don’t think cars do this nowadays (if someone could confirm?). It’s also a nuisance, like on my parents SAAB 9000, when the coolant temp switch failed, keeping the fan on and running down the battery.

  • avatar


    If your “‘roo’s” fan turns on as soon as you start-up – even w/o the defroster or the A/C selected – take it in for service. There was a recall on that a couple of years back.

  • avatar

    Makes for a good case of the belt driven fan – always running.

    I actually had an old late 80’s Chevy Cavalier that I wired up so the fan would always run when ignition was on, rather than wait for sensors to trip the fan to cool things down. Sure, it took longer for the heat to come up in the winter (no clutch like the belt driven kind), but come summer, the coolant temp was always nice and low. Didn’t seem to cause any residual damage and got well over 100k on the engine before handing it off to someone else.

  • avatar

    This reminds me of the advice I was given loooooooooong ago back when a first generation Mustang was barely considered a classic (you know the dark ages of the 80s) that you should not forget to run your defrost every once in awhile during the winter to keep the compressor seals from drying out on the AC unit. I now know this advice was a little silly, cause I’m constantly using the defrost in the winter and I now live in the desert Southwest. Even in Ohio where I grew up, defrost was petty much a fact of life during the winter.

    • 0 avatar

      That’s probably not a bad idea for today’s cars. The refrigerant (R134a) may have changed, but the system is still sealed up by little o-rings.

    • 0 avatar

      But that only applies in places where you almost never use defrost and stop using AC at some point right? Like maybe… Palm Springs, CA?

    • 0 avatar

      Correct me if I’m wrong, but many cars will not run the A/C compressor at all when outside temps are below a certain set point (I believe my dad’s G35 manual said below 25 degrees F?).

      Reasoning is that the refrigerant will freeze up completely in cold engine bay with such cold air coming through. Also, the mere act of heating air coming through the heater core removes enough humidity for defrosting purposes (and colder air is generally lower humidity).

  • avatar

    “the dashboard’s heater core dissipates heat like the front radiator, doing a great job if the HVAC fan runs at full tilt.”
    It works too. I had a Ford Escort which had a broken fan switch… or fan motor… or both, can’t remember. Anyway, the only way I was able to keep the engine cool in long summer traffic jams was to open all the windows, stick the heat and fan on full and roast the occupants of the car. Did a remarkable job of stopping the engine blowing up, but caused everyone to get very hot and sweaty.

  • avatar

    The late 70’s Rabbit was the first car that I remember where the fan can come on at anytime, regardless of ignition position.

    The idea has multiple benefits. The main one is just heat soak under the hood when the car is shut off immediately after driving. It gets warm enough under the hood when you’re driving, when you stop so does the airflow. All that heat is now just being moved out of the engine compartment much more slowly. The fan being able to kick on moves heat out of the radiator, and the engine compartment.

    Less temperature stress is just better for everything under the hood.

  • avatar

    A question and an additon:

  • avatar

    how do you run the compressor when they usually lock out below 45 degrees?

    On our Honda’s, through a sequence of HVAC button pushing and ignition key use, we can disable the AC compressor coming on in bi-level mode or we can now chose to turn off the compressor in defrost.


  • avatar

    Some cars it locks out, some it doesn’t.

    Personally, my idea of HVAC control perfection was achieved in the early 80’s Honda. One slide for temp, one slide for output control, a/c compressor on a button.

  • avatar
    Greg Locock

    Opening the bonnet will change the airflow patterns
    around the engine. The CFD engineer who signed off on your car’s cooling system may raise his eyebrows at your conclusion.

  • avatar

    In the old days, most Japanese cars had the A/C button that would disable the compressor no matter what setting the rest of the HVAC system was on. Most American/European cars gave you the compressor on Defrost no matter what. I believe most Japanese have adopted that process today.

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