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Does Defrost work better with A/C Compressor?

jeff in co

jeff in co

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Can't seem to find it specifically in the search......Does the defrost work better if I have a working A/C system? I thought the A/C "dried" the air thus helping in the defrost mode.

Driving in my side and a portion of my dash started fogging up even though I had my defrost on. Just wondering if I had my A/C system up and running, if it would help?!?
 
Well, the A/c gets cycled when teh defrost is on....so i would say yes.

That being said, properly working blend doors and sealed ducts make a huge difference. Check all your linkage/vac pots on the heater box, and make sure the defrost duct is properly fitted to maximize effect.
 
defrost with a/c works better yes.

reason is the a/c runs with the defrost to pull out the moisture from the air to keep the windows from foging over inside the vehicle.

is it life or death with out no. but sure make driving much better not fighting the damp foged up glass inside the vehicle.

if you dont have it try some anti fog stuff like rain-x anti fog or somthing llike that. and keep the glass clean and free from dirt and normal junk build up. if your a smoker the smoke crap will build up bad on the glass and make it real bad.
 
I kinda thought it would help and now do remember the compressor use to cycle on and off while in defrost. thanks all.

Although it takes, I'm guessing $1,000 to fix by a shop, could I just buy all new a/c parts and then fill it with the r134 stuff? I've asked about a/c stuff before and the general concensus was, get all new (ac delco preferred) gear. So if I got a new condensor, compressor, hoses, accumulator, orifice and evaporator, I'd be in about $650 plus refrigerant. If everything is new, no need to flush....just add refrigerant and go. Am I missing something or could this work?
 
While the A/C will the help defrost a bit, the other thing that will help is heat and airflow (or just airflow). If you roll the windows down, for example, your fog will go away. You will also probably freeze to death ... If you crank up the heat, it will go away *if* you also have some air flow.

Personally, I *hate* that new cars turn on the A/C when you put the defrost on. Why? Simply because then your car does not get warm inside! If you have the heat on with the A/C it drops the temps a good 10-20 degrees. You know what ... it's 20 degrees outside, I don't need the A/C on! :rolleyes: I want 100 degree hot air coming out, thank you very much!

To top it off, when you turn off the car, moisture will condense on the exchanger under your dash. The next time you start your car you now have a blast of wet air which instantly fogs up your windows.

I recall that moisture isn't the best thing to breath in all the time either.

Come winter, I unplug my A/C pump from the system so I can enjoy my heat, and I don't have any problems with fog.
 
When you turn on the AC in my car to help with the defrost it blow as hot as ever. Warms it up almost exactly the same.

My heater is hot enough in the Jimmy that I usually crack a window just cause I like fresh air coming in. Never makes me cold
 
jeff in co said:
I kinda thought it would help and now do remember the compressor use to cycle on and off while in defrost. thanks all.

Although it takes, I'm guessing $1,000 to fix by a shop, could I just buy all new a/c parts and then fill it with the r134 stuff? I've asked about a/c stuff before and the general concensus was, get all new (ac delco preferred) gear. So if I got a new condensor, compressor, hoses, accumulator, orifice and evaporator, I'd be in about $650 plus refrigerant. If everything is new, no need to flush....just add refrigerant and go. Am I missing something or could this work?
Click to expand...


Pretty sure you need to pull a vaccuum on the system before adding refrigerant.
 
Thanks for the tips.

I'm slowly putting this truck back to working order. The PO had the wrong heater fan in it, didn't ground it propertly so it wouldn't work for the most part. Also, when he somehow removed or replace the motor, he tore up the heater box on the firewell. Not really repairable with much success. I picked up a new one at the junk yard for $12. The real fun began when I had to drop the inner fender wheel well to access the last 2 bolts!

Long job but finally got it fixed. Also found a reciept for the motor (warranty!!). Went to Autozone and traded it for a new one, and also the right one! Now, I at least get good, solid heater air flow.

A/C is not critical now but may be nice later on.

Last 3 major items, new dash, new instrument bezel and new A/C.
 
You will need to pull a vac on the system, before you put in your refrigerant.

1 to confirm no leaks

2 to remove all air/contaminants
 
The AC in my ford needs to be recharged and I drove it for 2 winters with no defrost problems due to lack of a working A/C system.
 
Would be nice to have heat at the windshield, everytime i switch to defrost, the air moves to those vents, but the heat goes away :doah:
 
Defog is much better with AC. On some muggy days it is almost impossible to defog without it. You can defog without any heat at all much of the time as long as your AC works.

Replacement of ALL the parts is not necessary to convert to 134a but the retained parts must be thoroughly flushed.
 
wasted wages said:
negative ghost rider...
Click to expand...

I am sorry but it does confirm that the system is sealed, if you can't reach a certain vacuum level it means the system is not properly sealed.
If you work in the field and you still stand firm on the statement I will not comment again.:D
 
i'm in the process of fixing my whole system too. the AC pump hasn't worked in years and my friend hated driving her without it working. wonder what those cost now a days.the whole heater/AC system is what i really want to work in my trucks, i spend a lot of time in there. i found a broken vent door connector and they finally showed up at the dealer today...... i need time to fix it, then i'll find a short in the horn/lighter circuit, fukkin goody! but i'm also glad it hasn't snowed yet.
 
Last edited:
imiceman44 said:
I am sorry but it does confirm that the system is sealed, if you can't reach a certain vacuum level it means the system is not properly sealed.
If you work in the field and you still stand firm on the statement I will not comment again.:D
Click to expand...


I agree. It needs to hold 30 inches of vacuum for four hours. On some real problem cases we held it over night. Now to check for leaks, I use nitrogen. Put 150 psi on the system then use Snoop to find the smallest leak. Then use vacuum pump to pull the nitrogen out and clear the system. Without nitrogen, a vacuum is a must to rid the system of moisture.
 
How we do it on the semi's we work on is pressure test at 350 psi with nitrogen for 30 minutes to check for pressure loss. If thats good we move the vacuum and hold it for 30 minutes after a 30 minute vac.
 
British Slave said:
I agree. It needs to hold 30 inches of vacuum for four hours. On some real problem cases we held it over night. Now to check for leaks, I use nitrogen. Put 150 psi on the system then use Snoop to find the smallest leak. Then use vacuum pump to pull the nitrogen out and clear the system. Without nitrogen, a vacuum is a must to rid the system of moisture.
Click to expand...

The better way is what you described because sometimes vacuum will suck in a connection and keep it sealed while pressure from nitrogen will push it open, I agree, but vacuum test only is something I can do cheap and has worked for me.
I have seen some people do the pressure test with just a compressor from a fridge, that introduces moisture and even though vacuum will remove most of it, there is no guarantee it removes everything so I don't do that.
I worked in A/C mostly automotive for a few years, and learned the good and the bad ways.:whistle:
 
Any refrigeration compressor with a shaft seal will leak. Period.
ALL shaft seals leak...It is by design...it is just a controlled leak with the final seal being a film of oil on the carbon ring/polished steel surface.

Under a vacuum....it will pull air in. So unless you have service valves on the compressor and can isolate it from the rest of the system your arguments do not hold water..( no pun intended )

Inches of vacuum is not the gold standard for leak testing.

Why ? because unless you are using a micron gauge you WILL have a rise in microns on a shaft seal and never see it on a vacuum/low side gauge.

1 inch hg vacuum = 25,000 microns.

Takes a lot of rise in microns before that needle on a gauge even thinks about moving...

Now...if the system is a completely HERMETIC system... yes, vacuum can be used as a method of leak testing...in fact I have pulled 50 micron vacuums on entire grocery store rack systems and held them for three days at 50 microns with the vacuum pumps turned off and disconnected from the system.

50 microns.....72 hours.....That my friends,,,is a tight system.


Nitrogen and R-22 Trace is the best way to leak test a system.

Oh,, and 350 lbs of pressure ??? you are really testing the bursting limits of the evap coil.... I have seen the float balls in compressor oil floats and coils inside of heat exchangers crush internally under those pressures.

150 psi is all you ever need..

think about it this way...say the internal surface area of an evap coil is 100 square inches....at 350 lbs per square inch...that's 35,000 pounds of force stored up in that little coil...just food for thought...:whistle:
 
wasted wages said:
Oh,, and 350 lbs of pressure ??? you are really testing the bursting limits of the evap coil.... I have seen the float balls in compressor oil floats and coils inside of heat exchangers crush internally under those pressures.

150 psi is all you ever need..

think about it this way...say the internal surface area of an evap coil is 100 square inches....at 350 lbs per square inch...that's 35,000 pounds of force stored up in that little coil...just food for thought...:whistle:
Click to expand...

Yeah but working pressure is about 275~325 depending on weather that day. The over pressure plug on the back of the compressor is supposed to pop at 425~450 ish. So using 150 on a system that leaks down every 6~8 weeks won't get me to fix it right.

A trick with a compressor is to put the system under pressure or vacuum and wiggle the drive head. Just a little shake or turn it back and forth to see what the gauge set does. If it moves the seal is toast and you need a compressor.

Oh one note, the high pressure fan switch on most semi's is set for about 275 or so. So when the high side hits the magic number it turns the engine fan on. Almost every truck on the road has an engine fan that freewheels until needed. Helps cut down on noise and better fuel economy. So if you test at half the working pressure you may never find your leak.
 
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