QuestionHello,
many times a week when my truck is started and seems to be at idle, no ac or heat on. I get this noise which sounds like a fan coming on for about 8 seconds and go off. This is about 8 times per week. When I open the door I can hear it some where under the hood. Could you help determine what could be the cause.
AnswerHi Steve,
hard to tell from where I'm sitting.
Here is some general information concerning the AC and heating - perhaps this is the area that is causing the trouble...
While the system is on, the ATC ECU operates the refrigerant system and the inlet air, blower speed, air temperature and air distribution functions to produce the conditions requested on the control panel. When the system is first switched on, the ATC ECU resumes the control outputs in use when the system was last switched off. If conditions have changed, or a different mode is selected to switch the system on, the control outputs are then changed to produce the required new settings.
The system operates in automatic, economy and defrost modes, with manual overrides of the inlet air source, blower speed and air distribution. The air temperature is automatically controlled in all operating modes.
In the automatic mode, the ATC ECU operates the system to warm-up or cool down the cabin to establish and maintain the temperature selections on the control panel, while directing the air to those outlets most comfortable for the occupant(s). If a difference between the LH and RH temperature selections causes a conflict of the required inlet air source, blower speed or air distribution settings, priority is given to achieving the temperature requested on the driver's side of the control panel.
The ATC ECU enters the economy mode when the refrigerant compressor is selected off while the system is in the automatic mode, which reduces the load on the engine. Economy mode operation is similar to the automatic mode, but without the ability to cool the cabin if the ambient temperature is higher than the temperature selections made on the control panel, or to dehumidify the air in the cabin.
In the defrost mode, the ATC ECU sets the inlet air source to fresh air, the blower to maximum speed, the air distribution to windscreen and side windows, and outputs signals to the BCU to operate the rear window heater and (where fitted) the windscreen heater. The BCU starts or, if the heaters are already on, resets the heater timers and energises the rear window and windscreen heaters for a complete on cycle.
Air temperature control
To determine the amount of heat or cooling required by the cabin, the ATC ECU uses the sensor inputs and the temperatures selected on the control panel to calculate target air outlet temperatures for the driver's and the front passenger's side of the heater assembly. The ATC ECU then signals the servo motors controlling the respective blend flaps in the heater assembly to move to the flaps to the appropriate position. The target temperatures are constantly updated
and, in the automatic mode, also used in further calculations to determine the inlet air source, the blower speed and the air distribution.
Compressor control
To engage the compressor clutch, the ATC ECU outputs a compressor clutch request to the ECM, which then energises the A/C compressor clutch relay. Compressor operation is governed by the evaporator outlet air temperature, at one of two settings, dependent on the amount of cooling required. When more cooling is required, the compressor clutch request is output if evaporator outlet air temperature increases to 4 °C (39 °F) and cancelled when it decreases to 3 °C (37 °F). When less cooling is required, the compressor clutch request is output if evaporator outlet air temperature increases to 11 °C (52 °F) and cancelled when it decreases to 10 °C (50 °F).
Engine cooling fan control
While the A/C system is on, operation of the electric engine cooling fan, to assist refrigerant condenser operation, is determined by a combination of vehicle speed and external air temperature. When cooling fan operation is required, the ATC ECU outputs a cooling fan request to the ECM, which then energises the cooling fan relay. The cooling fan request is output if vehicle speed is 80 km/h or less while the external air temperature is 28 °C (82 °F) or more. The request is cancelled, and the cooling fan switched off, if either the vehicle speed increases to 100 km/h, or the external air temperature decreases to 25 °C (77 °F).
-end-
So, your trouble could be somewhere within these components and their sensors. Best to have an expert do the search to locate the fault rather than swapping parts - serendipity is not good when it comes to LR repairs ($$$$).
Best of luck,
JohnMc
