I noticed the A/C compressor turns on automatically in DEF mode regardless if the A/C button is pressed or not. Why have a "manual A/C" button then force the Compressor in DEF mode?

 

It runs in defrost to dry the air and cool it back down some.. without the compressor on you will just get a hot humid cab... Any air conditioing system with dehumidification control will do this.... It is not a defect or design flaw....the cooling "wrings" the moisture out of the air... thts why you see water running out from under the vehicle when the a/c runs in the summer and even more water when the humidty is high

 

^ x2

 

I've had trucks with true manual A/C before and understand that you need to dehumidify during rainy weather. What I don't like is there is no indicator letting you know the A/C is running.

Does the truck "secretly" run the A/C in other modes or is it just defrost?
Does the truck run the A/C when you're trying to melt the ice off your windshield in the winter. On older vehicles low temperatures would prevent the low pressure cycling switch from activating.

 

Just the defrost cycle, weather its all defrost or a partial defrost (on the ****). I may be wrong on your second question, but I think if the system senses moisture (even in winter) the A/C will cycle to keep condensation off the windshield.

 

The system has no way to sense moisture. It simply runs off its programming (if automatic) and the operator settings which should be described in the owner's manual. No "secrets" here.

 

While we're discussing the A/C operation what is the difference between "max A/C" vs hitting the recirculate and A/C buttons while in the vent mode?

 

Max A/C closes the outside air intake and recircs the air 100% for max cool effect... A/Cb rings in outside air and mixes it with cab air... In Texas... in summer we leave it on Max A/C

 

Modern a/c systems can use a thermister inbeded in the the Evap coil to prevent the evaporator from geting too cold.. I am not sure about auto systems but have seen it in some systems such as european mini split systems... it does work in preventing the evap coil from freezing up by preventing the compressor from running when the evap coil is below freezing/dewpoint.
It's all about enthalpy of vaporization and condensation. Wiki actually has some good articles on the subject.

 

Im not sure of your problem... The compressor will run based on the program/design to help in drying the air even when in defrost in the winter.. It is not a problem and I do not think the mfg wants to keep any secrets in this regard.. If you really want to know every sequence of operation take an auto a/c course and you will get all the info you can stand.. I work commercial a/c in telecom/data facilities so I have enough on my plate and could give a rats behind not trying to insult just my two cents...

 

This a/c with defroster on is nothing new. I remember hearing a new Frieghtliner owner complaining about it back in 1995. He thought there was something wrong. I explained to him that it was normal then. He didn't believe me and called the dealership complaining. This is the way they have been making vehicles for decades and nothing to get upset about.

 

Come to think of it my 99 F150 did the same thing, even though there wasn't a button on the dash to turn the A/C on/off. Somebody came up with a mod that you could take the switch apart and add your own A/C button. I do think the A/C should be driver selectable in the floor + def mode instead of being forced on. If it only forces A/C in those 2 modes I can live with it, especially if the evap sensor prevents the compressor from coming on when there's ice on the window. I just didn't want the A/C cranking when I'm trying to melt 1/2" of ice off my windshield.

 

Ford doesn't use any electronic sensor in the evaporator to sense core temp.

They use the low pressure cycling switch to infer evaporator core temp instead to enable/disable compressor clutch operation.

 

Why is this ?
The AC is drying the air on the inside of the truck to make sure it does not fog or frost up on the inside.

The heat is what is melting the ice on the outside of the windshield.

You seem to have some disconnect that AC means cold, where it does not. The temp of the AC is controlled with the temp **** or EATC temp setting.

The AC can run with hot air blowing.

Not too sure why you are thinking this is in error, it is as designed for a reason ( as not to have fog on the inside of the windshield ).

 

The A/C compressor running makes the evap coil get cold, this cools the air? Are you saying this is not true? I thought the evap cooled the air and when you turned up the temp it simply reheated the air.

I just would prefer a button to turn on/off the compressor in any mode, if the windows fog up I can hit the button. So the "MAX A/C" does the same thing as pushing the Recirculate & A/C button while in the "dash vent" mode?

Another thing I've noticed is speeds 1-3 seem a a lot lower than the 4th speed. It's almost like they used too high of a resistor value for the lower speeds. I hope this is normal and not an indication of a potential resistor problem.