medialexus

I was just wondering about my climate control in my Lexus, 1998 LS400. Whenever I turn the ignition, it defaults to "auto" mode, and almost always the airconditioning comes on, even if it 65 degrees outside. It seems that heating is unaffected by the auto mode switch, and is controlled by the digital temperature setting in the car. Why isn't there a manual "on-off" switch for heat, like in almost every other car?

Thanks for any clarification on this.

blitz

hmmm, mine doesn't....

medialexus

I own a 1998 LS 400, "base" model.

emus1

Push the off button, which puts the system in manual mode. You can then control the fan etc. manually.

willard west

Toyota/Lexus climate control systems are supplied.designed by Denso USA, once NipponDenso.

Denso engineers have never understood that over half of the contribution to human comfort is due to the radient cooling or heating effects from our surrounding environment. All of that cold, snow covered countryside you see through the car's windshield and windows has an adverse, chilling effect on your body's comfort level.

Lexus/Denso thinks that if the interior air temperature within the car is at or near the setpoint then the occupants would be more comfortable with the system in "cooling" mode, system airflow routing to the upper body and face, as much bare skin as possible.

But that leaves you with no choice. In the wintertime, or actually anytime your body comfort "meter" tells you that you need "heat", not cool dehumidified airflow in your face, you will have to manually change the mode of the system to "footwell".

Another comfort point. Lexus/Denso has come up with something they call bi-level airflow.

Most modern day (non-denso) climate control systems pass the incoming fresh airflow through the A/C evaporator to cool and dehumidify it, then a mixing vane is used to devert a portion of this airflow through the heater core. The evaporator temperature is usually very close to freezing and the heater core is usually at 180F or above.

Downstream of the heater core ALL of this airflow is re-mixed such that the system outflow temperature is within a comfortable range, just above or below the setpoint enough to keep the cabin climate near the setpoint.

In the Lexus/denso system the airflow to the upper portion of the cabin, windshield and dash outlets, flows through the A/C evaporator, but does not flow through the heater core nor the mixing chamber or plenum, it flows directly to these outlets.

So on a cold winter day your feet will be kept comfortably warm while your chest, neck, arms, and face will be chilled (to the bone) with airflow that is as much as 20 degrees F below the footwell outlets. The two airflow masses are re-mixed within the cabin resulting in a net cabin temperature very near the air temperature setpoint.

Why would any engineer, or engineering team do such a stupid thing?

Well, first, the effects of human body comfort with regards to radient heating and cooling are not widely known, not widely taught and certainly not to automotive engineers.

Second, Denso says that by routing cool and dehumidified airflow to the interior surface of the windshield there is less likelihood that it will fog over due to increase humidity.

We are our own worse enemy...

All modern day automotive heating/cooling systems share one major flaw. If we are driving along, say four of us, on a winter day and it is really cold outside we want to stay warm and cosy in our nice automobile.

But pretty soon, due to our own body functions, or maybe we're returning from a ski trip, the cabin humdity starts to rise. And not too much after that the interior windshield surface (60MPH COLD impinging airflow) begins to fog over with condensation.

So what do we do, roll down the windows so the stale humid air can exhaust?

No, we turn on the defroster. If the outside air temperature is nearing freezing then the A/C compressor cannot be used to dehumidify the incoming airflow, and most systems automatically disable that circuit. So we are left to crank up the heat and the blower speed so as to heat the windshield surface and remove the condensation and keep it from reforming.

Is that very discomforting to the driver and his front seat passenger? You bet! Been there, done that, many times.

What this all comes down to is the fact that in these circumstances and with the restrictions of modern day automotive heating/cooling systems these IS NO HAPPY MEDIUM!

With the sole exception of Lexus of Canada, there is no modern day automotive heating/cooling system that can both accomodate the comfort needs/desires of the human occupants, and prevent and/or remove condensation from the interior surface of the windshield.

The Canadian LS430 has an electrically heated windshield.

The rest of us will still have to panic when we see condensation forming on the interiro surface of the windshield and if you are a Lexus owner you had better have burial insurance or know how to turn up the blower spped and the setppoint temperature simulatneosuly with activating the defrost function.

Lexus/Denso climate control systems DO NOT automatiucally increase the heat level of the air flowing to the windshield in defrost mode, if the cabin has already reached an air temperature comfort level of 71F then the airflow to the windshield might be as low as 51F.

And now think about what MIGHT happen if the A/C compressor was shut down automatically by the system as you drove up a cold mountain pass and the outside temperature declined to freezing.

Oh My! All that previsouly condensed mositure, literally saturating the evaporator vanes, and now nothing to keep it from evaporating into the airstream headed directly for that cold interior windshield surface.

Be careful out there...