I have a 94 ram 5.9 auto and even with the heat in when I turn the defrost on it kick the ac compressor on I hear it clicked on and off. So I understand plugged it so it wouldn't kick on with it turn the heat to the dash. Is that gonna hurt anything? I didn't get a check engine light yet and put about 200 miles on it. But I don't want to hurt anything

 

Won't hurt a thing. (not to mention, the PCM doesn't care if the compressor engages or not, it doesn't even pay attention to that part of it, it just 'commands' the compressor on. PCM doesn't know if it is running or not.)

The idea there, is refrigerated air (a/c compressor running...) carries less moisture.... Of course, I figure that if I need the defroster, ALL the air is ALREADY refrigerated..... so, is unnecessary. Now, on those cool (not frigid) damp mornings, when you want to just clear the fog from the windshield, having the compressor running is a good idea, and makes the system much more effective.

 

The rumor I have heard is that, many moons ago, a/c compressors were seizing and manufacturers were replacing them under warranty. They were seizing because they sat idle for so long (usually in winter), not being used. So, everybody decided to have the compressor cycle with defrost-foot mode, thinking that this was the most common setting they people used. Since, all of my vehicles are such, whether American made or foreign born are that way. There was a mod to just switch off the compressor on my jeep, I just haven't bothered and/or gotten that far yet with the ram (not to mention the tacoma either).

 

I'll plug it in occasionally just to keep it from seizing up. But should help gas milage and alittle power when on defrost

 

It's working as designed. Defrost uses AC as a dehumidifier. There is a reason why it's called "air conditioner" and not "air cooler".

 

AC is definitely a dehumidifier which is why if you have AC at your home you also have to have a way to drain off the moisture collecting at your AC's evaporator coils (often via a condensation pump).

Here's a psuedo-scientific bit of research done to identify the best way to defog your cars windows: Defog