Hello, I am new to the site and new to the Dodge world. I recently bought a used '99 Ram 3500 and it is in pretty good condition. As the weather here in Northern Utah is getting cooler, the need for AC is decreasing. On the air direction selector control (I have no idea what the legal name is for the control) the majority of the vent positions turn the AC system on. The selections are OFF, AC Max w/upper vents, Normal upper vent, Lower and Upper vent, lower vent, Lower vent w/Defrost, Defrost only. (I think I am forgetting one?) Any way, this is the first vehicle that I have owned that has not had a AC power button, so the AC system in this truck, and verified in my the owner's manual, runs in all modes except upper vent only and lower vents only. In the winter, why would I want the AC to be running if I am trying to heat the cab? Would the extreme cold outside adversely affect the AC system, causing it to freeze up and damage itself?I assume that I could pull a fuse, maybe reroute the serpentine belt--or maybe none of this is a big deal and I should just leave it. The OCD in me would like to just shut it off when I don't want to use it. Thanks for your help.

The heating system will override the a/c, so to speak. I believe the a/c system is used in all the selections to help remove moisture from the air. That is what a/c truly does.

The heating system will override the a/c, so to speak. I believe the a/c system is used in all the selections to help remove moisture from the air. That is what a/c truly does.

Yep. The windows would stay mighty foggy in the winter if the a/c wasn't working. I found that out when one winter I was hearing my a/c clicking on and off repeatedly. I was low on 134a and it was cycling on the low pressure switch. After I got her running right I asked around and found out it ran in defrost mode in the winter too. That surprised me a little, but makes sense.

Like others have said a/c is used with defrost to dry the air. I owned a Toyota pickup w/out a/c and the windows were almost always fogged in adverse weather.

the a/c will turn off on its own, unlike the 3rd gen trucks. its nothing to worry about if you keep after it.

the a/c will turn off on its own, unlike the 3rd gen trucks. its nothing to worry about if you keep after it.

My 3rd gen cycles the compressor???

My 3rd gen cycles the compressor???

IIRC mike says that the 3rd gens don't cycle the compressor, or maybe it didn't have a lower pressure shut off switch? Now im wondering. because my 2nd gen the truck has a low pressure shut off, maybe that is what mike meant.

From my 2005 Service manual. The powertrain control module (PCM) cycles the A/C compressor clutch off and on as necessary to protect the A/C system from evaporator freezing and optimize A/C system performance.The 3rd gens (at least my 05) don't have a low pressure transducer, as you mentioned. The PCM uses the high pressure transducer for high/low readings and shuts down operations accordingly.

Keep the A/C on to keep the window clear. When air first come into the hvac unit it's drawn in by the fan and is blown across the a/c evaporator to dry and cool it. The air is then routed by a blend door which will either send it to the vents of through the heater core and to the vents. The blend door reacts to the position of the heater control knob. The a/c is automatically turned on when the defrost mode is selected to dry the windshield.