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No. The engine temp sensor affects engine idle speed, and timing. There is an in car temp sensor that controls the position of the blend door and the fan speed if the system has automatic temperature control.It also cycles the AC compressor clutch and the radiator cooling fans.
1. Engine Coolant Temperature.
If the thermostat in the cooling system is getting stuck in the open or partially open position rather than opening and closing as needed to keep the coolant at the required operating temperature then you will get the results you have described here with the coolant temp dropping as you drive. The radiator is providing optimum cooling when the car is moving and there is plenty of cold air flow through it.
I am assuming the temp gauge in the vehicle is reading correctly.
Thermostats in the cooling system do become faulty in time and it may well never have been changed (or only changed once) since the car was built and is possibly gummed up with debris. With those symptoms I would definitely change the thermostat. Make sure a full flush of the cooling system is done at the same time and refilled with fresh coolant.
2. In Car Heater.
I would not worry about the interior heating until you change the thermostat on the cooling system. However if you continue to get cold air from the heater then there will be an issue with hot coolant from the cooling system being restricted in flowing through the heater core.
Thermostat stuck open?
Faulty temp sensor or gauge?
If heat comes out of the heater, engine thermostat is ok. That will isolate it to probably the temp sensor in the temp indication system (sensor & gauge).
The cooling system should be full. Temp sensors cannot read the engine temperature of steam or empty cooling galleries.
If the temp gauge shows normal, you would not expect the fan to be on much. It is controlled by the coolant temp sensor. If the fan will not come on when the car gets really hot, it is possible this sensor has failed.
It is normal for both AC and cooling fans to be on when you turn on AC to cool setting.
This sounds like a bad coolant temp sensor that give computer false reading, check the coolant temp sensor connector and if it is good with no bad wires to connector then replace coolant temp sensor , when the computer gets a signal from temp sensor that engine is hot it turns fans on high speed to cool radiator and displays a/c turned off , if this truck does not have electric cooling fans then they wont run just explaining what happens depending on engine and cooling set up. replace coolant temp sensor and check connector.
When you say you checked the pressure did you connect gauges to the high and low sides to get both pressures, this sounds like it may be low. also what is the temp coming out of the vents. note on really hot days and humid the ac system doesnt cool as good unless the truck is moving down the road or highway and does take a while to cool down. at a stop or sitting or in traffic the ac temp will rise as the condensor needs to be cooled like the engine does, if its not cooling then the temp will not be cold, check the front of condensor for blockage, also make sure the fan is running with ac on. this fan should run when ac compressor is on,maybe clean condensor with water hose blowing from radiator outwardto front bumper, then make sure the hot/cold blend door is closing all the way,a quick test is to turn a/c on, turn temp control to hot and see if it gets hotter then back to cold and it should get colder. this sounds like the system may just be a little low. have an a/c tech check it, you may find its low or nothing is wrong. good day.
Very correct the ECT is a 2 wire thermistor,when coolant temp increases, the resistance andsignal voltage decrease,If you disconnect the sensor the fan goes on, you just trouble shooted the vehicle cooling fan switch, If anything the signal to the gauge itself may be non-op and not getting to the indicator, try this with vehicle at warm up temperature if you turn the A/C does the fan turn on.it should if it doesnt the power juction box (fuse and relay in engine compartment should be checked, for any opeen fuses, By the way the guage information is from the PCM
The cooling fan as in the one in front of the radiator? This fan also comes on when the AC is turned on, It sounds like your ac is turning on (the knob is probably on the defroster setting) and your blend door is probably stuck in the cool position.Try changing the air distribution from to a few different positions and see if it helps, this will help you figure out if you have a stuck blend door or another fault.
indicates gas pressure and condenser cooling problems
check the condenser fan is working properly and have the gas pressures checked by gauges
check that the evaporator fins are clean and not blocked as ice build up will cause the same indications