Help! Fan keeps coming on
Discussion in 'Trucks [ Eighteen Wheelers ]' started by heavyhaulerss, Nov 17, 2011.
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heavyhaulerss Thanks this.
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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.
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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 .
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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.heavyhaulerss Thanks this. -
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.heavyhaulerss Thanks this. -
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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.heavyhaulerss Thanks this.
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