ac compressor coming on when switch on hot and other switch on defrost??

Lots of systems will have the AC compressor cycle in the defrost mode.

 

Absolutely normal. When you are in defrost mode the ac is turned on to dry the air in the cab to take away any moisture. Therefore clearing the fogged up windows and drying your wet jacket you just hung up in the cab.

 

Normal compressor kicks on when in defrost to pull moister out of the air helps keep the windows from.fogging up

 

Regardless of demand your pump will cycle every so often anyways. I believe it is programmed into them, something about keeping the compressor from seizing.

 

I always pull my a/c fuse the beginning of every winter to save fuel. as when on defrost, compressor will come on, just like in summer weather. I never noticed any difference in more or less fogged windows, when my truck is warm, & defrost on windows get defrosted quick with out compressor on. this also saves on the life of the compressor clutch. if there is any downside to this, I dont know of any. been doing this for 13 years.

 

That might work on a Prius but I really doubt that a 450 HP diesel engine gives a darn about a lowly AC compressor cycling.

 

Still uses the same amount of HP. As a percentage of total engine HP you are correct - the truck engine doesn't mind it so much. But the engine DOES notice that it's running. Switch the A/C on at idle, and you'll hear the throttle compensate to maintain idle speed. That means more fuel is being consumed with the compressor running. 5 HP to run the compressor will require roughly the same amount of fuel whether you're powering it with an engine capable of 45HP or one capable of 450HP. Engine efficiency and fuel types aside, a larger engine can't magically create energy; it has to convert the fuel to energy.

 

Minor point; many systems switch the A/C on when set to defrost; they may also set the cab air intake to 'recirc'. If you disable your A/C compressor you may eventually build up moisture inside your cab that was usually removed by said A/C. This may make it harder to keep windows from fogging.

 

Leave the fuse alone... Your system is designed to have the ac come one to remove excessive moisture. Besides, its good for the compressor to operate every so often to keep things lubed up and flowing freely.