As cooler temperatures arrived here in NC, I moved my "Climate" controls from Auto A/C to just running the heater. I turned off the Auto, turned off the A/C and moved the fan to just blow on my feet. I then decided to try running the heater in Auto mode at 72 degrees and noticed when I hit Auto the A/C light came on (I would estimate the interior temperature at below 60 degrees. I turned off A/C (Auto stayed on set to 72 degrees) and went about my driving.
Now the problem. I started to notice that my computer mpg reading was considerably lower than anticipated. When I switched back to "Climate" the A/C control light was on. I switch off the A/C control again. This has happened many times now with most of the time the climate control turning the blower to the defrost/feet setting and the A/C light is on. I have not touched the climate controls, let alone moved the blower control. The only way I can keep the A/C light from coming on is by turning off the Auto setting. The change appears to occur when I power up-but I am not sure of this.
My last tank was at 42.2 mpg, down from about a 48.6 mpg average, and I tried running Auto most of the time. I am now running with Auto off and almost 50 mpg on the computer. Any ideas as to what is going on?

 

Use the auto setting and don't worry about the A/C light. All it means is the car will use the compressor if it gets hot enough for it. I doubt the climate control system is what is affecting your mileage anyway.

Also keep in mind when you select "demist/defrost" the A/C compressor is used to remove water vapour from the air, whether the A/C light is on or not (I think it turns the light on when you press that "button").

 

During wintertime driving the very BEST way to operate a Toyota or Lexus climate control system is to turn the A/C compressor off(***), totally and completely, switch the airflow mode to footwell and windshield combined air flow' and then set, maintain, your temperature setpoint to your own personal comfort level, or that of your passenger(s).

Saturday morning I drove home, Portland to Seattle eastside, ~200 miles, temperature hovering around 34-36, overcast and light snow flurries the full way.

The significant level of warm airflow diverted to the windshield surface helped to keep mine and my wife's upper body warm. Otherwise NipponDenso, Denso US' unique dual layer airflow would have left our upper body subject only to the radiant cooling effects of the surrounding landscape amd more likely than not an encounter, an episode, with sudden and unexpected windshield fogging.

*** In earlier times I resorted to unplugging the a/c clutch circuit.

In my '92 LS400 I added a "winter mode" operational switch so the OAT sensor always indicated sub-freezing conditions during the winter months. Lexus disables the A/C entirely if the OAT is below ~35F.

My '01 AWD RX300 has two c-best optional settings that address this issue. one of those allows you to disable the A/C indefinitely by simply switching it off manually one time. The second unlinks the A/C from operating automatically (with no A/C "on" indication) in defrost/defog/demist mode.

Later Lexus model have additional C-best options, one that prevents the system from automatically switching into cooling mode when the cabin temperature rises to within a few degrees.

I notice that '08 models automatically switch to heating, footwell airflow with slight flow to the windshield, once OAT declines to the point of the A/C being disabled.

Previous models are prone, HIGHLY prone, to instances of sudden windshield fogging when the A/C is automatically disabled as the OAT declines below ~35F.

'90 LS400 to '08 models, 18 years and they still haven't gotten it "right".

That's what they meant by...

"Relentless Pursuit of Perfection"

Relentless = NEVER getting to your goal(s).

Do you suppose that's why they changed it too....??:

"Passionate Pursuit of Perfection"

 

Hi,

I agree, don't use or enable the A/C during the winter unless you are dealing with severe window fogging. You might check your air distribution switch and make sure you're not selecting either of the 'defrost/defogging' settings on the extreme right. I hate the way this will enable the A/C to dry out the air but doesn't give a good indication that the A/C is on.

GOOD LUCK!
Bob Wilson

 

It was my understanding that leaving the HVAC system on the "Auto A/C" didn't mean the AC was on but rather that the computer was controlling the cabin air temp based on the temperature level setting. Is my understanding incorrect? Does the Auto A/C actually mean the A/C condenser unit is operating, even at low winter temperatures?

UTpiper

 

You are correct, the "A/C" light just means that the system can control the compressor. If the light is off, then the compressor will NEVER be used/available. -- If it meant that the compressor was operating when lit, then you would see the light cycling on and off during normal operation.

I need to add the disclaimer that this is the way it works with the '07 TCH and that YPMV (Your Prius May Vary).

 

No, many modern day systems have the A/C linked into the defrost/defog/demist mode and will operate automatically in that mode, typcially with no actual indication to the driver, and provided the OAT is above ~34F.

The absolute BEST way to proceed to most QUICKLY remove interior windshield or window condensation is:

Turn the heat up to MAXIMUM, if the blower speed doesn't automatically follow then turn it up to MAXIMUM also, then switch to defrost/defog/demist mode while assuring that the system is in FRESH inlet airflow mode.

With severe fogging it will often help to lower a rear window slightly.

Twenty years ago I would have been an advocate of using A/C in defrost/defog/demist mode but no more.

Up until the late 1980's only upscale/luxury class vehicles used the A/C all year around to provide a higher comfort level for passengers winter and summer. Clearly a market class that wasn't especially subject to the extra fuel cost taht resulted from continuous use of the A/C.

So what changed...??

A) Fuel prices...

B) Government mandated CAFE and emissions standards

C) Lexus and Infiniti.

D) NipponDenso, Denso US.

If climate control comfort using the A/C was to really appeal, marketed to the "masses", then the FE aspects HAD to be addressed.

See:

http://www.sae.org/technical/papers/1999-01-1199