defrosting windshield

[BigPapa]

When I put my defroster on the ac automatically comes on, along with fresh air (the recycle option won't work while defrosting).

That's the way I thought it worked, too.

 

[KATOOM]

Mmmm thangs gents. The radiator is full full expansion tank is full but I'll keep an eye closely peeled for a core leak.

I did run the compressor for a while but not extensively so I'll do that going forward and see if that helps.

Meanwhile I wired up some muffin fans to augment the blower on the dash

The AC compressor automatically runs when the HVAC selector is on defrost and floor+defrost. If you dont hear the compressor kick on when you turn the defrost on then you should find out why its not.
But again, if you smell antifreeze in the cab then there's a core leak.

 

[brucejohnson]

The AC compressor automatically runs when the HVAC selector is on defrost and floor+defrost. If you dont hear the compressor kick on when you turn the defrost on then you should find out why its not.
But again, if you smell antifreeze in the cab then there's a core leak.

IIRC the AC will not run once the outside temperature reaches a value near freezing. He mentioned 20*F, snowy boots and heat. If the mode is in recirculation the scene is set for a moist time in the cab. Then there is the strong possibility of an elusive leak - somewhere. He'll find the solution to his issue with our collective suggestions.

 

[brucejohnson]

Another thought just popped into my organic calculator. One of my son's had an 06 Mega cab. I seem to remember a couple of grilles on the back panel which led to some on the outside of the back of the cab. I think they were the exhaust ports for the flow through ventilation. Do you have a bunch of stuff stored behind the rear seats that may be obstructing the air flow out of the cab? Maybe, just maybe...but I could be wrong...

 

[AH64ID]

IIRC the AC will not run once the outside temperature reaches a value near freezing. He mentioned 20*F, snowy boots and heat. If the mode is in recirculation the scene is set for a moist time in the cab. Then there is the strong possibility of an elusive leak - somewhere. He'll find the solution to his issue with our collective suggestions.

That's not something I have noticed, even sub zero, but I do think that battery temp is used for some of the logic and it will warm up with the engine bay.

 

[petersonj]

Moist boots and wet clothes will add water vapor to the air inside the cab and when that water vapor condenses on cold glass the windows will begin to fog. Usually when this happens, the fogging will form on the glass farthest from the windshield and begin creeping forward because the air from a properly operating HVAC defroster will be flowing very dry air when the outside temperature is at 20 degrees (with or without the AC on) .

Let's just assume the worst conditions - that the outside 20 degree air has a dew point of 20 degrees which would saturate the air with the maximum quantity of water vapor it can hold and would have a relative humidity of 100%. This relative humidity will fall to well below 5% when that parcel of air is heated to 120 degrees, so that air will easily keep a windshield clear. Unless, of course, water vapor is being added before the air arrives at the defroster vents (such as water collected around the evaporator drain area, a leaking heater core, or a recirculating door that is not fully closed to ensure that only outside air is being drawn in).

- John

 

[brucejohnson]

Moist boots and wet clothes will add water vapor to the air inside the cab and when that water vapor condenses on cold glass the windows will begin to fog. Usually when this happens, the fogging will form on the glass farthest from the windshield and begin creeping forward because the air from a properly operating HVAC defroster will be flowing very dry air when the outside temperature is at 20 degrees (with or without the AC on) .

Let's just assume the worst conditions - that the outside 20 degree air has a dew point of 20 degrees which would saturate the air with the maximum quantity of water vapor it can hold and would have a relative humidity of 100%. This relative humidity will fall to well below 5% when that parcel of air is heated to 120 degrees, so that air will easily keep a windshield clear. Unless, of course, water vapor is being added before the air arrives at the defroster vents (such as water collected around the evaporator drain area, a leaking heater core, or a recirculating door that is not fully closed to ensure that only outside air is being drawn in).

- John

John, You hit that nail square on the head with the explanation. Thanks.

 

[brucejohnson]

That's not something I have noticed, even sub zero, but I do think that battery temp is used for some of the logic and it will warm up with the engine bay.

In the 2002 Service Manual, Page 24-21, A/C Low Pressure Switch - Operation: "... Lower ambient temperatures, below about -1* C (30* F), will also cause the (Low Pressure) switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system."
Similar event happens with the refrigerator out in my garage when the temps go below freezing. The ice cream goes from solid block to milk shake consistency, but still cold. The fudge bars flex right off the stick into a blob.

 

[AH64ID]

In the 2002 Service Manual, Page 24-21, A/C Low Pressure Switch - Operation: "... Lower ambient temperatures, below about -1* C (30* F), will also cause the (Low Pressure) switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system."
Similar event happens with the refrigerator out in my garage when the temps go below freezing. The ice cream goes from solid block to milk shake consistency, but still cold. The fudge bars flex right off the stick into a blob.

But this isn't a discussion on a 2002. The fan in the 3rd gen is ECM controlled with a viscous clutch, and can be used for the AC, where it's pure mechanical in a 2002.

 

[petersonj]

But this isn't a discussion on a 2002. The fan in the 3rd gen is ECM controlled with a viscous clutch, and can be used for the AC, where it's pure mechanical in a 2002.

While this is quite true, I think what @brucejohnson was getting at was that the evaporator core must be kept above 30 degrees or so to prevent the evaporator core from icing and blocking the air flow. Since 20 degree air is already is 10 degrees below that threshold and must first pass through the evaporator core, it is not likely that the AC would even cycle even if it was turned on. If it did cycle, it would probably be very brief.

I certainly will admit that there is plenty that I don't know about the newer HVAC systems, but I think that the minimum temperature of the evaporator core hasn't changed much, regardless of how the components of the HVAC system are controlled.

@brucejohnson, if this was not what you were thinking, please let me know.

- John

 

[BigPapa]

AllData has this on the ‘06. I think they get it from the FSM.

The A/C pressure transducer signal to the PCM/ECM will also prevent the A/C compressor clutch from engaging when ambient temperatures are below about 10°C (50°F) due to the pressure/ temperature relationship of the refrigerant. The A/C pressure transducer input to the PCM/ECM will also prevent the A/C compressor clutch from engaging when ambient temperatures are below about 10°C (50°F) due to the pressure/temperature relationship of the refrigerant.

 

[brucejohnson]

John, Scott, you're both right on. While I am not by any means an HVAC expert there is a point where the pressure/temperature relationship gives little to no benefit to the system. It may even create undesirable results as I mentioned in the refrigerator example. In essence the refrigerator consumes energy with little output compared to the designed results. It is cold, but not frozen to the anticipated level. Think air in your tires at 70* then at -10*. It's holding the rim off the ground, but not safely or efficiently.

 

[brucejohnson]

But this isn't a discussion on a 2002. The fan in the 3rd gen is ECM controlled with a viscous clutch, and can be used for the AC, where it's pure mechanical in a 2002.

AC works pretty much the same, regardless of the AC juice or other components which affect the efficiency. The fan is used to assist in heat transfer from the A/C juice to atmosphere thru the condenser, especially at low to no forward speeds. The compressor restores the A/C juice to liquid state so it can again do its magic and turn to gas, cooling and dehumidifying the interior via the evaporator...or something like that.

 

[trust]

wow, lots to digest, thanks guys. Yes I have a lot of stuff behind the back seat, in fact the back seat is down for the dog in his kennel but alot of stuff in there regardless so I guess I will clear it out and make sure the discharge vents are clear and functional. Thanks Bruce, I wouldnt ever have gotten to that as a contributor.

The windshield fogs first and then the fogging moves aft from there for whatever thats worth.

Im not sure how to rule out the heater core leak, coolant level remains constant in both the radiator and the coolant expansion tank, I guess I will take it in for a pressure test and make sure it holds pressure. That makes the most sense I guess

 

[brucejohnson]

wow, lots to digest, thanks guys. Yes I have a lot of stuff behind the back seat, in fact the back seat is down for the dog in his kennel but alot of stuff in there regardless so I guess I will clear it out and make sure the discharge vents are clear and functional. Thanks Bruce, I wouldnt ever have gotten to that as a contributor.

The windshield fogs first and then the fogging moves aft from there for whatever thats worth.

Im not sure how to rule out the heater core leak, coolant level remains constant in both the radiator and the coolant expansion tank, I guess I will take it in for a pressure test and make sure it holds pressure. That makes the most sense I guess

You say it starts at the windshield? Does that mean you go out to a cold truck and everything is clear on the glass all around inside the truck? Then when you start the truck the fogging begins at the windshield? Leads to heater box as suspect. HVAC setting to recirculate or stuck in recirculate mode? Check your floorboards, especially at the low spots around the entry sills. Pull the sill trim and poke your fingers around the low spots there. Windshield seal leak? Have you checked your third brake light for leaks? The rear slider window for that series of Mega Cab has a habit of leaking and dripping to the low spots and migrating forward to the front via the door sill depressions. Put a strip of paper towel in the window slides and see if they get sopping wet when it rains. Do you have clearance lights on the cab?
Then, back to the heater box, or maybe start here... I don't remember where the heater fan is located on the 06, but if it is an easy access item, remove it and see if there is a lot of foreign matter at the heater core and what is visible of the heater box floor. In other words, is the drain plugged from years of stuff falling into the airbox intake. Have you been able to locate the AC drain coming from the firewall and does it have an extension hose to clear the drainage away from the body? Does it drip moisture on days that the AC is in use? Since you aren't loosing coolant and you don't seem to detect antifreeze odors we can only assume, but not fully rule out, no issue with the system's integrity. I would recommend a timed pressure check of the cooling system.
Whew! There is a lot to consider. Look at the overall picture and sit back, analyze, what are the symptoms and when do they occur, and think simple. Pick a point near the issue then systematically look at and touch each suspected area, rotating outward from the area...what could cause the issue you are experiencing.
Good luck.

 

[petersonj]

The windshield fogs first and then the fogging moves aft from there for whatever thats worth.

That is a good piece of information. It indicates that water vapor is being added to the airstream before it exits the defroster vents and when that air contacts the cold windshield, it immediately cools and condenses. The three logical places to get water vapor into the airstream are the bottom of the evaporator core housing (holding water), a leaking heater core, or a recirculation door that is broken or is not sealing properly. Another possible source of moisture could be the first place of entry - that is, is there melting snow or ice entering the cowling behind the hood?

When in defrost mode, outside air enters the into the cabin by first passing by the recirculation door (that is blocking cabin air from entering the airstream), then through the blower housing, then through the evaporator core, and then through the blend door which directs all or part of the air through or around or both, through and around, the heater core.

You indicated that you do not smell coolant in the cab and you don't seem to be losing coolant (radiator is full and overflow container level isn't dropping). And, you have verified that the evaporator core housing is draining properly.

I recommend checking the condition and operation of the recirculation door. When in the defrost mode, if that door is not closed and sealed properly, then you will be recirculating some or a lot of cabin air which will continue to gather water vapor from any source, such as wet floormats, wet clothing, every breath you exhale, etc. and condense that moisture repeatedly onto your windshield.

I think you can get access to inspect the recirculation door fairly easily by removing the blower motor.

- John

 

[Jihn]

I think you can get access to inspect the recirculation door fairly easily by removing the blower motor.

On an 06 set aside a full day to get at the blower motor and heater core, it's not a easy job, it's the same as an 08, as far as I know(looked it up at Geno's same part).

Jihn

 

[AH64ID]

AllData has this on the ‘06. I think they get it from the FSM.

That is out of the FSM, but I can tell you that the a/c compressor does, and will, operate below 50°F.

It's based on pressure with the 3rd gens, so there is not a hard line drawn in the sand.

 

[trust]

No clearance lights and I haven't noticed any leakage at the rear slider but haven't yet looked in earnest. I will today when I empty the cab. The truck has a topper on the back FWIW.

I've checked the recirc door for operation in the past but not lately. So I'll get on that today as well as a more thorough clean out of the drain, is there also a unique drain for the cowl intake or does it share the ac drain?

Thanks again guys

 

[brucejohnson]

Judging from the crud we removed from one of my son's heater box anything making it through intake will try to make it through the drain. Leaves and seeds remain to reduce heating and cooling effects and possibly draining if enough pieces parts get past the cores.