Climate Control Question

[agc]

Hey guys, I notice something yesterday about my climate control. Here is the sequence of events (I have a 2000 LS400)

1. Turn on car (climate control is currently off

2. Turn on the climate control, by selecting AUTO

3. Turn up the temperature all the way to HOT, notice that the AC light is ON

4. Turn OFF climate control system

5. Turn ON climate control system, with the temperature set all the way up to HOT, notice that the AC light was still ON.

My question is why is the AC light ON even when the temperature is set all the way up to HOT

Thanks all

AGC

[Lexusfreak]

Hey guys, I notice something yesterday about my climate control.  Here is the sequence of events (I have a 2000 LS400)

1. Turn on car (climate control is currently off

2. Turn on the climate control, by selecting AUTO

3. Turn up the temperature all the way to HOT, notice that the AC light is ON

4. Turn OFF climate control system

5. Turn ON climate control system, with the temperature set all the way up to HOT, notice that the AC light was still ON.

My question is why is the AC light ON even when the temperature is set all the way up to HOT

Thanks all

AGC

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Should specify in your owners manual. I'm pretty sure mind does the same thing. When you hit 'auto' & the AC light s on (regardless of the temp setting), if you hit the A/C button, does it go off & your still getting air blowing into the cabin (if on hot I assume from the heater ducts)? B)

[RFeldes]

In auto mode the a/c is on to rid the cabin of moisture. I keeps fog from building up especially when it rains. If your windows fog up during a rain storm (hot or Cold) turn on the a/c and watch how fast it clears while maintaining the set temperature. If you turn it off on a humid day (hot or cold) you will observe the air is a bit musty or damp.

[Lexusfreak]

In auto mode the a/c is on to rid the cabin of moisture. I keeps fog from building up especially when it rains. If your windows fog up during a rain storm (hot or Cold) turn on the a/c and watch how fast it clears while maintaining the set temperature. If you turn it off on a humid day (hot or cold) you will observe the air is a bit musty or damp.

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I just finished reading that in my owners manual. So it's perfectly normal operation..........No worries agc! B) ;)

[wwest]

Do a google search for:

Denso + demist + exclusively

[branshew]

As mentioned above - when the incoming air passes across the AC condenser coils (before it goes to the heater core) it removes a fair amount of condensation from the air. This is turn cuts down on the amount of fogging of the windows that can occur - especially in the winter when warm moist air comes in contact with cold glass.

Try defogging the windows w/o the A/C on and you'll find that its difficult.

[agc]

Thanks guys, I just wondered whether that was normal operations. In my other car (Honda) when you turn up the temp the AC does not come on. It only comes on when you do one of the following.

1. Turn on front window defrost

2. Lower the temp all the way down to 60 degrees (cold)

[nc211]

Here is what I don't understand about mine. When I turn the car on, and say it's like 50 degrees outside, but the temp setting is in the 60's, why does is default to the recirculate setting ALL the time? I don't use the auto button, don't like the drain the ac pulls when it's not needed. But why does is always default to the recirculate setting, even in manual mode???

[RFeldes]

In auto the system will not turn on the blower until the car reaches operating temperature. This is to prevent the cold air circulating in the cabin before it can be warmed. A good idea if you ask me. I assume the recirculate in manual default is the same thing. Not allowing cold air in until it can warm it.

[nc211]

That makes sense....but, what about when the sun has heated up the inside of the car to like a 100, but it's 60 outside and the temp setting is set at 66? That's what I don't get. I get the part about keeping it warm inside when cold out. But keeping it hot inside when it's nice out, that baffles me "which isn't a hard thing to do by the way "

[wwest]

"Try defogging the windows w/o the A/C on and you'll find that its difficult."

NOT! NYET! NO!

The absolute best way, and the ONLY reliable way to defog your windshield and/or windows is to lower the relative humidity of the cabin atmosphere. That means bringing in FRESH outside airflow, heating it as high as possible, and moving as much of it to the interior surface of the windshield and windows as the MAX blower speed can accomplish.

Heating the airflow will DRAMATICALLY lower its RH, increasing the vapor pressure on the condensate, and the heat will also help to quickly raise the temperature of the interior surface of the windshield to well above dewpoint. Please note that we humans have absolutely no way of sensing or detecting whether or not the RH is high enough for the A/C to be effective in any given instance.

So, the ONLY safe thing to do, ALWAYS, is to use HEAT as the primary resource for defogging a glass surface.

I do not disagree that in many cases the A/C can be used as an assist for lowering the cabin atmosphere's RH, but absolutely cannot be relied upon even for minor assist unless the outside climatic conditions are "correct".

If you check your owners manual or ask your dealer service manager you will find that in COLD weather your A/C compressor is completely and totally disabled, by factory design. Some vehicles do this at ~47F and lower while others don't disable the A/C until the OAT declines below ~35F.

In COLD weather (coinsidentally, that's also when your windshield and windows are most likely to be subject to fogging) depending on the ability of the A/C to efficiently dehumidify the incoming airflow is comparable to playing a game of Russian Roulette with a loaded pistol.

In COLD, or even COOL weather, the relative humidity must be unusually high, one might even say extraordinarily high, for the A/C to be efficient enough to QUICKLY defog your windshield.

Yet Lexus and Toyota both use NipponDenso, Denso US, automatic climate control designs, as do many if not most asian car manufacturers. NipponDenso's design is horribly FLAWED and has been since the introduction of the Lexus product line.

This design relies EXCLUSIVELY on the ability of the A/C to dehumidify the incoming airflow although its ability to do so is highly questionable below ~47F and absolutely NON-EXISTENT below ~35F wherein the A/C compressor, by design, is completely disabled.

Now, I should point out that this flawed design will work quite well unless the cabin atmosphere is "upset" by some event. Like picking up two sweaty teenagers, wet clothing and all, after a day of skiing. Or maybe even a life's partner after work who has been standing waiting in the rain for a few minutes.

Or worse of all, having the system itsself cause the UPSET. Regretably a fairly common circumstance.

Common Circumstances:

Its a fairly cool morning as you drive your Lexus out of the garage and head down the road to work. The Lexus A/C system operates continuously, all year 'round, unless the OAT declines enough, you manually turn it off, and of course last night when you parked it in the garage.

So last evening when you parked it in the garage the ~10,000 square inches of the A/C evaporator cooling vane surface area was still covered with a thin film of condensate, WATER!

Now this morning about 5 miles into your drive the engine water jacket will reach the point, 130F, at which the system blower motor is activated.

POOF...!

Your windshield is suddenly covered with condensation.

Unless you remember, night after night, to leave the windows slightly lowered in your garage so that thin film of moisture can evaporate and via convection airflow leave the car's cabin area, or at least most of it can. And by the way, for those of you with the A/C mould and mildew odors you can avoid that via the same method, let the car dry out each and every night in the garage.

So, beware of using the A/C temporarily, say as an aid to defogging/demisting your windshield and windows and then turning it off. Within just a few moments of having cycled it off the vane surface area temperature will begin to rise to that of the incoming airflow and that previously condensed moisture on those vanes will begin to evaporate into the passenger cabin.

Also beware of driving from a reasonably warm area, say above 47F, when the A/C continues to operate, and into an area of cooler or colder climate, say a mountain pass. As the OAT declines the A/C will beome disabled at ~35 and that will result in the evaporation of the thin film of moisture on the evaporator vanes.

POOF!

Your windshield has been CHILLED as a result of the COLD impinging 60MPH outside airflow and now the cabin's RH is rising rapidly due to the airborne water vapor from the A/C evaporator vanes.

When, if, your windshield fogs over and you happen to be driving a car of asian manufacture then you should IMMEDIATELY:

1. Turn the heating demand to the highest extreme, MAX HEAT.

2. If the blower motor should happen to not rise as you execute step one then turn it to MAX Speed.

3. Now move the climate control system airflow mode to defrost/defog/demist.

4. Lower a rear window slightly so moisture being evaporated from the windshield and windows can quickly exit the cabin thereby helping to lower the cabin atmosphere's RH.

That last, #4, is required because almost all modern day vehicles, and Lexus most especially, are well sealed and insulated to reduce the escape of previously conditioned air and thereby increase the fuel economy.

I would like to say that it would do no harm to activate the A/C for use as an aid in removing condensation, but the aftereffects, windshield suddenly fogging over yet again, even more quickly than before, can prove to be deadly.

So, unless the OAT is above 47F, AND you expect it will remain there for the duration of your drive, NEVER allow the A/C to operate, even in defrost/defog/demist mode.

As of 2004 the RX330 has three C-best options, as likely do other Lexus models, that will be an aid in these efforts.

1. The A/C on/off control can be set such that if you turn it off manually it will remain off, indefinitely, multiple restarts, until you turn it back on.

2. The A/C can be unlinked from operating automatically (it does this without an indication to you) in defrost/defog/demist mode.

3. The climate control system can be "set" such that it does not automatically switch to COOLING mode once the cabin atmosphere rises to, or nearly to, your temperature setpoint.

In COOLING mode there is NO warming airflow routed to the interior surface of the windshield to (hopefully) keep it above dewpoint. So, not only is cooling mode, cool and dry airflow to our face, discomforting to our human metabolism during the colder climatic periods, but due to the lack of warming airflow to the interior surface of the windshield in this mode our demise from an accident resulting from sudden and unexpected windshield fogging is made more likely.