Most Likely A Dumb Question

[jpdriver1]

Had to use the defroster this morning and found the aux fan would turn off after a loud click sounded from the passenger foot well. After a few mins the click would sound and I could hear the fan begin running again.  It also did this with the A/C this afternoon. This guy is very confused as my 90 didn't do this (as far as I can remember) 

 

thanks folks 

[HOrnbrod]

Low freon.

[Eagle]

Low freon.

 

Normal operation. The a/c runs in defrost mode. It's supposed to cycle.

[gogmorgo]

 

Low freon.

 

Normal operation. The a/c runs in defrost mode. It's supposed to cycle.

Even with the heat on?

[Oyaji]

 

 

Low freon.

 

Normal operation. The a/c runs in defrost mode. It's supposed to cycle.

Even with the heat on?

 

 

Yes - the air is first cooled to drop the relative humidity by condensing the moisture out, then heated. It does a much better job that way.

[87manche1]

Yes on defrost, point of it is to dehumidify the air that's hitting the windsheild. So it defogs quicker

[jpdriver1]

Low freon.

so a recharge should allow it to stay on eh?  --- great thanks 

[Comanchemodder]

I'm not saying BS on the a/c before heat, but it's darn close. I grew up, in fact I bet most here, grew up witH a dad too cheap to buy a/c in his cars. The heat in the winter worked just fine and any noticeable "condensate" was negligible. Typically in winter RH is low anyway, so a/c the air before won't do much. In Minnesota a guy doesn't turn the heat on until the temp is far below the A/c temp.

I'm sure there are lots of charts and graphs and no doubt I expect to be flamed, but heat worked just fine with out A/c pre-charge thank you very much. Now, if some people need heat when it's above 35 deg, well then you can't be helped! Lol.

[Oyaji]

People don't get flamed here - folks are generally pretty darn nice. But you may run into argument from time to time... and that can be a good thing. It is a fine way to learn, presenting and comparing ideas and arguing their merits.

.

The A/C in vehicles so equipped generally comes on automatically when the defrost function is selected (some vehicles allow the option of choosing defrost with or without A/C). You are right about relative humidity (RH) being lower at colder temperatures, but that is exactly the principle by which running the A/C with the defroster takes advantage: it drops the RH of cabin air even further. The condensed moisture then drains outside the cabin space, where otherwise it would remain in the cabin air, free to condense on a cold surface again. When the conditioned air goes through the heater core, the RH drops even more (because warm air can hold more water vapor than cold air), allowing the heated dry air to evaporate and hold more moisture. Vehicles without A/C still have a defroster function, but it just doesn't dry the condensation on the inner side of the windshield as well. If you live in a climate with a lot of fog and high humidity, you come to appreciate the difference.  :)

[mvusse]

I've also heard the alternate theory that it is to make sure the compressor gets used during the winter to keep the oil circulating.

 

As far as a dumb question goes, I've been taught the only stupid question is the one you never asked.

[87manche1]

U may not need to charge. U need to understand an a/c refridgerant pressure scale.

Which gets way more involved than I can type. Its better to charge it when its nice out. If u charged at 40f and didn't weigh it it from an empty circuit. U may actually overcharge it. They are meant to run off of the low pressure switch in winter.

[Eagle]

I'm not saying BS on the a/c before heat, but it's darn close. I grew up, in fact I bet most here, grew up witH a dad too cheap to buy a/c in his cars. The heat in the winter worked just fine and any noticeable "condensate" was negligible. Typically in winter RH is low anyway, so a/c the air before won't do much. In Minnesota a guy doesn't turn the heat on until the temp is far below the A/c temp.

 

I'm sure there are lots of charts and graphs and no doubt I expect to be flamed, but heat worked just fine with out A/c pre-charge thank you very much. Now, if some people need heat when it's above 35 deg, well then you can't be helped! Lol.

 

If there's any sure path to getting yourself flamed, it's jumping in after multiple people who KNOW how the system works have answered a question correctly, and telling them that what they wrote is close to BS. Before being so quick on the trigger, you might in the future stop to consider that some of us on this board are automotive professionals, and others of us have been serious automotive hobbyists for over fifty years.

 

I also grew up before a/c in cars was commonplace. I think I was 35 yes old (or more) before I owned my first car with a/c. I understand that heaters will defrost the windshield without help from the a/c. That doesn't mean that a/c won't make it happen faster, and it certainly doesn't mean that the Jeep engineers didn't design the system to cycle the a/c when running the defrost.

[HOrnbrod]

Had to use the defroster this morning and found the aux fan would turn off after a loud click sounded from the passenger foot well. After a few mins the click would sound and I could hear the fan begin running again.  It also did this with the A/C this afternoon. This guy is very confused as my 90 didn't do this (as far as I can remember) 

 

thanks folks 

 

Getting back to the original post, the A/C cycling frequency in all modes, including defrost, is controlled by several variables. These include the ambient temp, relative humidity, proper HVAC system operation, and the correct amount of freon charge. The latter is the most common cause of excess compressor cycling. You mentioned that in the defrost mode that the recycling occurred over "several minutes". This sounds normal. Then you mentioned it happened again in the afternoon in A/C mode. This may or may not be normal, depending on the variables mentioned above.

 

If you are concerned and want to know if your compressor cycling is normal or not, go to a reputable A/C shop and have the Hi and LO sides gauge pressures checked to verify you have the proper freon charge for your system.

 

Are you still using R12 refrigerant or have you converted to R142? I'm still R12 because it works better. Whenever the compressor recycling gets excessive during the A/C and/or defrost modes in my opinion, or the A/C is not cooling as I know it always has, I throw in another can of R12 and all is well. I have to do this about every third year or so.

[jimoshel]

To steal the thread, but is is on the same subject. Can R12 and R142 be mixed or should you flush out all the R12 before charging with R142?

[gogmorgo]

Don't know if it's available south of the 49th, but I've topped up r12 systems using r12a, which is a product manufactured by RedTek, adding it directly to whatever's still in the system. I've only used it with r12 systems, but RedTek says it can also be added directly to r134a systems as well. It's pretty cheap because it's basically just a can of propane. They also sell it mixed with compressor oil and leak dye.

 

Don't recall ever coming across an r142 while searching for r12 alternatives.

[jpdriver1]

thanks hornbrod, the system had been converted to 142 prior to my ownership according to the sticker under the hood --- no jim I don't believe you can mix em -- I do know the fittings are different between the two systems 

[dasbulliwagen]

Being an auto proffesional, I too have never heard of r142... maybe you guys are guys thinking it wrong? R134a conversions are commonplace. All it needs are the new fittings, and a long vacuum time to make sure all the old stuff is out. And just FYI... R134a is on its way out as well. It is being replaced with R1234yf. The pressures are similar and older systems should be easy to convert to the new stuff when the time comes. There have been questions of its flamability during a crash, but the only people that got it to catch fire were the same ones who had developed their own refrigerant system and were trying to get it certified... Merceded Benz. Their refrigerant is CO2, and the pressures it takes to get it to cool are MUCH higher and put more drag on the system.

 

Sorry... off topic.

[mvusse]

I'm with Gogmorgo and Dasbulliwagen. R142 (HCFC-142) does exist, but it's an even much more potent ozone affecting molecule than R12, and it's manufacture was prohibited 3 years earlier than R12.

 

It was never commonly used as a refrigerant. It was common for a few other uses including a propellant in aerosol cans.

 

Read that sticker under the hood again to see if maybe it says 134a instead of 142. All newer AC systems use R134a, and the only conversion kits I have seen for older units using R12 use R134a as well.

[HOrnbrod]

You guys are right of course - I meant R134a. Typing too late at night I guess................ 

[jpdriver1]

nice catch guys -- thanks -- gotta love those long hours eh hornbrod?  

[Comanchemodder]

If there's any sure path to getting yourself flamed, it's jumping in after multiple people who KNOW how the system works have answered a question correctly, and telling them that what they wrote is close to BS. Before being so quick on the trigger, you might in the future stop to consider that some of us on this board are automotive professionals, and others of us have been serious automotive hobbyists for over fifty years.

 

I also grew up before a/c in cars was commonplace. I think I was 35 yes old (or more) before I owned my first car with a/c. I understand that heaters will defrost the windshield without help from the a/c. That doesn't mean that a/c won't make it happen faster, and it certainly doesn't mean that the Jeep engineers didn't design the system to cycle the a/c when running the defrost.

 

I do apologize if I offended anyone on this board in regards to their expertise on Jeeps.  That was not my intent.  I do believe the a/c turns on when defrost is used as pointed out in this thread.  This is not peoples imagination.

 

My issue is why the system was engineered that way. I have heard a couple of theories:

 

1) To cycle the a/c compressor in the off season - these seems reasonable and seems like a good reason the system turns on.  Perhaps automotive companies had a lot of warranty claims on A/C compressors and found the off-season was a potential cause?

 

2) To reduce the humidity during a defrost cycle.  This is the reason I find sketchy.  I reasoned it out and I can't seem to get there on how it would increase defrost effectiveness.  But I am open to anyone who can give an explanation.  Assuming outside air is used on defrost, the need to de-humidify the air would be marginal at best.   Outside air, drawn into the heater core is heated without a change in absolute humidity (amount of water vapour constant since water is not introduced). As a result heated air forced onto the wind shield inside the cab has a much lower RH than the surrounding air of the cab.  Only in that short period when the engine heat is cold (not warmed up yet) would the defrost be blowing unheated air on the wind shield.  Since the wind shield is not heated and equal to outside air temp, the defrost air will not condense onto it.  In that case, if there was any residual condensate already on the wind shield (from heavy occupant breathers), air may not be able to defrost the wind shield.

 

Obviously, if inside air (recirculated air) is used, it would be like a rain shower.

 

But with all that said, I may be missing something obvious.

[gogmorgo]

When you decrease the temperature of the air the amount of water vapour it can hold goes down. The water falls out of the air (like rain but on a much smaller scale) and if you have an issue with the a/c drain tubes, into your lower vents and onto your feet when you go around a corner. Same principle that leads to water droplets appearing on the outside of a cold beverage container.

When the air is heated, it can hold more water, so it pulls the water off your windshield. Also, the heat melts the frost on the windshield and the warm windshield in cold air prevents water from condensing on either side of it.

If the air isn't dehumidified, you're relying mostly on the heat to do any defogging/defrosting. This morning, as on many occasions, with the defroster blowing at full blast I observed my breath freezing onto the inside of the windshield before the engine was warm enough that there was enough heat in the coolant to warm the cab. Having some kind of dehumidifier might have helped prevent my breath from condensing and freezing, but since the a/c compressor won't operate below something like 35°F, and it was about -20°F outside, I just had to sit and wait a few minutes like I always do. Your a/c will get cold a lot faster than your engine warms up, so if it's warm enough outside for it to safely operate, it'll be faster to use the a/c as a dehumidifier than to wait for the heat.

 

While I suppose either or both the lubrication or dehumidifier theories could be correct, I personally thought both were crocks of $#!&. Running the a/c in the "off season" up here for either purpose would destroy the compressor, as it would be trying to compress liquid freon that wasn't evaporating in the evaporator because it was just too damn cold for it to be a gas, especially not at the added pressure of the a/c system. In fact there's a temperature switch that prevents exactly that from happening, too low a temp and no a/c. I even went out last week and stared at my a/c compressor clutch for five minutes with the truck idling and the fan set to full defrost and it never kicked on. Then the other day I had the truck running in a nice warm environment with the defrost going by chance, and noticed that sure enough, the a/c clutch was engaged. Who woulda thunk, right?

[Comanchemodder]

hen the air is heated, it can hold more water, so it pulls the water off your windshield. Also, the heat melts the frost on the windshield and the warm windshield in cold air prevents water from condensing on either side of it.

If the air isn't dehumidified, you're relying mostly on the heat to do any defogging/defrosting.

 

I am on-board with everything you described.  However, the point I quoted above highlights my original comment.  The outside air is heated thereby lowering the RH relative to the outside air (since water is not added to the air in the heater core).  This air is then forced onto the wind shield and will defrost since it is both heated and low RH.  You pointed out something all us cold blooded guys have had to do:  wait for the engine to warm up and either hold our breath or open the window (I would usually exhale into my unzipped coat).  By opening the window, the cab air remains at the same RH as the outside air and therefore no condensation on the wind shield (unless you blow on it with warm, high RH breath) since wind shield is the same temperature as the outside air (since no heat yet).  

 

I agree if someone turned on the a/c it will lower the RH, but I think the effect is so minimal in a defrost situation (since the heat in combination with outside air is already doing most of the heavy lifting), it would not drive auto engineers to implement such a feature.  My feeling it was something else.  But I could be wrong.

 

EDIT:  I just ran quick conversion for RH:  Assuming a cold wet day At 30 def F and 100% RH, if the air is heated (with no water added) it will result in a RH of ~10% at 100 deg F (defrost temp?).

[gogmorgo]

In addition to the engine taking a short time to warm itself up, the windshield also doesn't warm up immediately, so even that 10% humidity would still deposit itself onto the windshield somewhat.

I do agree with you that the effect is minimal, especially since at 30°F (an atypical and extremely warm winter's day up here) the compressor shouldn't even be running. Typically the safety switch shuts them off around 35-45°F. A 100% RH at 45°F would still work out to ~15% RH at 100°F. But what if you were trying to defog the windsheild when it's warm out, say at 90°F and 95% RH? The effect of dehumidifying the air first would be much more noticeable. You could end up in this situation if you were in a warm humid environment and it started raining. The rain would cool down the windshield slightly, leading to the warmer air in the vehicle condensing on the windshield.

 

With this in mind, I'm pretty sure that the a/c compressor running in defrost is really only for dehumidifying and defogging in warm weather, and "lubrication in the offseason" is a happy coincidence that only affects those in nice, warm climates. You don't benefit from the a/c compressor running when it's not running. ;)

[Comanchemodder]

In addition to the engine taking a short time to warm itself up, the windshield also doesn't warm up immediately, so even that 10% humidity would still deposit itself onto the windshield somewhat.

I actually disagree with this point.  Outside air is drawn in.  Even if the outside air is 100%RH (non-condensing water at that specific temp).  It is heated up to 90+ deg F and approx 10% RH.,  If the wind shield is the same temp as the outside where the air came from, it will not condense since the wind shield temp is above the dew point of the air.  Exhaled air is what typically is deposited on the wind shield since the dew point of exhaled air is quite high.