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  1. #1
    Bobtail Member
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    09 Cummins isx fan running alot

    I have an 2009 Pete 386 with the ISX 450, and it seems the fan kicks on for no reason at all, the coolant temp is normal, air is not on, but it still keeps kicking the fan on. This is a real pain when off road in a dusty situation. Any ideas, the dealer says this is normal, but they can't answer why. I'm about ready to do a air by-pass to keep the #### thing off! They said there are other sensors kicking it on, but my question is, when the coolant temp is normal, what good is it for the fan to run? It seems to me it can't cool anything other than the coolant which is cooling the egr, and the dpf is under the truck. I understand that the AC will make it run, but it's off and not on defrost. So why is it doing it?


  2. #2
    Light Load Member AgLaw's Avatar
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    It's not normal for the fan to come on w/o the A/C system operating and the engine below 205 degrees F. I have two 389's--one a 2009 w/ a 600, the other a 2010 w/ a 500V--and neither do what you describe...not even in 100+ summer temps in stop-and-go Houston traffic, which BTW, is their area of operation.

    I did have issues with the "minimum fan on timer" which comes from Peterbilt/Cummins with a default setting of 60 sec. I reset mine to 30 with no rapid cycling of the engine fan. Have the dealer check your fan control parameters, but understand that some such as "Manual Fan Switch" will be set to "Disabled" even though the truck is so equipped. That is because certain functions are controlled trough Multiplexing parameters interacting with the Peterbilt/PACCAR CECU (Cab Electronic Control Unit). In particular, make sure the "Fan Vehicle Speed Interaction" value is set to "Disable". That parameter keeps the fan on at low vehicle speeds once the engine receives fan request from a sensor. The only way then to get the fan to turn off is to get the vehicle speed above 30 mph or cycle the key switch. That parameter will bring havoc on dirt roads--these new fans are like tornadoes even at low engine RPM's.

  3. #3
    Bobtail Member
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    I had a Cummins 475 ISX turned up to 500hp and I had the same type of thing happening to me. It seemed that it would come on when I had mash the fuel and started to pull real hard. Well back to the dealer it went and they checked everything and found nothing. It drove me crazy. I was checking things one day and on top of the motor there was a wire harness and one of the wires was just a little to tight so by chance it was the fan wire and when I would mash on the fuel the engine would move and the fan would come on. Moved the wire and it went away!!!! Don't know if you have the same wire on your truck but the wire is connected to the manual fan switch on the dash. Hope it helps.

  4. #4
    Light Load Member
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    In the case of the 2009 the wire for the fan switch goes to a controller in the cab and gets multiplexed to the engine. They also kick the engine fan on when you have low air pressure (to keep the clutch from wearing out by cycling). I forget the value that happens at though, I think its a relatively high psi value.

  5. #5
    Road Train Member
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    On an engine the fan will come on for coolant temp, AC preasure high, and CAC intake tempature, some will also come on for oil tempature. Just a thought!

  6. #6
    Road Train Member Heavyd's Avatar
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    If you have the heater set to "defrost" or mixed "defrost/floor" the A/C will be on too. The A/C is turned on when selecting defrost automatically to help dry out the air hitting the glass surface to aid in clearing them faster. In defrost the A/C will work just same as a hot muggy day in the summer.

  7. #7
    Bobtail Member
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    Figured out the problem, this is a tri-axle with a liftable tag, when you put the tag down the pressure drops so fast in the secondary that it kicks the fan on at 90 psi

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