When are the front fans supposed to come on?

Discussion in 'Gen 3 Prius Technical Discussion' started by Higgins909, Apr 7, 2021.

  1. Higgins909

    Higgins909 Member

    Apr 15, 2018
    2010 Prius
    Just wondering when the two front fans are supposed to come on. It seems they only come on when I have the AC on. I feel like they could be moving more air, but maybe not. Is this how they operate? I had my engine temp at 195 and pulled over and they only turned on with the AC. Some other components were much hotter. The inverter coolant temp apparently hit 140max but averaged 130, but when I went out just a few it was still at 120 after sitting for a while. Can't remember other temps atm.

    Normal? Using Scan Gauge 2.
  2. ChapmanF

    ChapmanF Senior Member

    Mar 30, 2008
    Indiana, USA
    2010 Prius
    They should be able to run at low or high speed, controlled by the FANL and FANH outputs from the ECM.


    From the previous page in the manual, power is coming in on "A" from the "CDS" fuse, on "B" from the "RDI" fuse, and on "C" from an ignition-switched circuit (that's just the negligible power to the coils of the three fan relays).

    When you look in the fuse box, CDS and RDI are not separate fuses, they are together in a weird three-fer fuse you probably won't find at the local auto parts store. :) (The third one in that three-fer is for the heater blower.)

    When the ECM wants a little bit of fan, it pulls down FANL, which closes the FAN No. 3 relay. Power from the CDS fuse (only) flows through the sub fan (I'm not sure which one is left or right, but it's the one with the green wire), from pin 3 to 4 of the FAN No. 2 relay, through the main fan (the other one, with the blue wire) and returns to ground. Because the two fans are stretched in series across twelve volts, they see about six each, and run at a low speed.

    When the ECM wants a lot of fan, it keeps FANL pulled down and also pulls down FANH. That switches the FAN No. 2 relay and also closes FAN No. 1. Now the power from the CDS fuse is going just through the sub fan and straight to ground through pin 5 of the no. 2 relay, and power from the RDI fuse is going straight through the FAN No. 1 relay and the main fan. Because each fan on its own is receiving a full twelve volts, they both run at high speed.

    If that's not what's happening, that diagram should be enough to start figuring out why.

    Although normal operation is always one of those three states (off, both fans low, both fans high), you can also see on the diagram how some abnormal patterns could also be seen, such as one fan running by itself, in cases with one blown fuse, one flaky relay, etc.

    About the ECM's control strategy, the manual just says this:

    StarCaller and Higgins909 like this.
  3. PriusCamper

    PriusCamper Senior Member

    Mar 3, 2012
    Pacific Northwest, USA
    2007 Prius
    Much like the hybrid battery cooling fan, the radiator fans almost never come on unless something has gone wrong... Even when I've driven in 110 degree weather and pulled over for gas, when I check the dipstick I'm always shocked at fact engine compartment isn't running hot.