Help! Fan keeps coming on

Discussion in 'Trucks [ Eighteen Wheelers ]' started by heavyhaulerss, Nov 17, 2011.

  1. ZippyNH

    ZippyNH Medium Load Member

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    Jakes are usually setup so they can activate the fan...one more wire to check.
     
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  3. The Truckist

    The Truckist Medium Load Member

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    Heavy, he ain't talking about that air dryer. There's a canister in the A/C system.
     
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  4. fortycalglock

    fortycalglock Road Train Member

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    Usually on or near the firewall. A foot long black cylinder with a/c lines going in and out and a sensor on top as well.
     
  5. peterbilt46

    peterbilt46 Bobtail Member

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    i have had this problem before check you intake air temp sensor it will make your fan cut in and out .
     
  6. heavyhaulerss

    heavyhaulerss Road Train Member

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    my fan clutch was leaking bad couple years ago, so I had it replaced. though when it leaked, the fan never came on. had masterbuilt, or master craft one put on. my air pressure is good, cycles on/off correct. I am leaning towards a sensor, wire e.t.c. I just don't understand why the first sporadic engaging of the fan & now always on. going out loaded, fan does not come on, going back empty,the fan kicks on as soon as I start up a hill, then goes off as soon as I top the hill & begin descend. done this all the way home. 8 hills. 8 times. then on the 12 mile stretch home fan never came back on. started truck up at home, off/ on/off/on. manual switch on, fan goes on, manual switch off, fan goes off. put new fan solenoid on, Fan now stays on????
     
  7. heavyhaulerss

    heavyhaulerss Road Train Member

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    new solenoid is identical to old one. put on same way, thought maybe the new solenoid could have air lines on reverse of what they should, so I DID take new solenoid off & put back on old one & still fan runs all the time, mind you this one that was on the truck already, would just come on sporadically, until I took it off & then put it back on, now fan stays on. with old or new solenoid. so I don't see how it could be solenoid or the air lines to it.
     
  8. bender

    bender Road Train Member

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    Don't Kid Yourself
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    Sounds like a possible broken wire/poor contact in fan solenoid harness near the solenoid.
    Keep in mind that fans use a timer to keep the fan off for 2 minutes after they cycle off.
    Is your radiator core and air charge cooler clean?
    Problems like this can be caused by multiple contributing factors so don't want to overlook anything.
     
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  9. Mr. Haney

    Mr. Haney Road Train Member

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    The A/C high pressure switch is on the refrigerant line that leaves the A/C compressor and goes to the A/C condenser in front of the radiator. If it isn't in this line then it will be somewhere in the line from the A/C condenser in front of your radiator to the evaporator coil in the dash. The switch will be located somewhere along these refrigerant lines or as some else said on the A/C dryer that in these lines. The A/C dryer is usually 3" in diameter and 10-12" long mounted in the A/C refrigerant liquid line to the evaporator coil

    This switch is usually on a grounded circuit(not +12V), so a wire that has rubbed through the insulation will make a ground and bring the fan on. The A/C system doesn't have to be operational for this to happen.
     
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  10. heavyhaulerss

    heavyhaulerss Road Train Member

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    where is the air temp sensor?
     
  11. Sparkz

    Sparkz Light Load Member

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    Jul 10, 2011
    New Zealand
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    Horton fan is spring applied. It requires air to release.
    The fan solenoid is normally closed, ie blocks air flow with no power applied.
    To release the fan, the solenoid must be powered up to supply air to the fan clutch.
    This makes the system "fail safe". No air or no electrics and the fan will lock in to keep the engine cool.

    The fan solenoid has 12v ignition power on one side and grounds out through the ECM to release the fan.
    The ECM monitors coolant temperature, oil temperature, and air inlet temperature. If any of these reach pre set values the solenoid circuit will be opened to lock the fan in. Also if the ECM detects a fault with any of these sensors it will lock the fan in and turn the amber Check Engine Light on.
    The dash mounted fan override switch can be wired to;
    1/ cut the ignition to the solenoid or
    2/ ground out an ECM input called Fan Override.

    There is also an ECM input(Auxiliary Fan Control) from a high pressure switch in the ac system. This input is grounded to release the fan. If the pressure in the ac system gets too high, this switch will open and the fan will lock in. To stop the fan cutting in and out repeatedly, the ECM will lock the fan in for a set time, usually 3 minutes. This circuit is prone to problems.

    The easiest way to diagnose the fan circuit is by using a reader or laptop that can show the temperatures, input switch states, and the output state.

    If you give me your engine serial number I can tell you what terminals the inputs and outputs are on.
     
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