Defog/demist Fix!

[wwest]

Just completed the stage 1 mod to my 2001 AWD RX300.

Have had too many instances of sudden and spontaneous windshield fogging with no ability to quickly remove condensation.

In defog/demist/defrost mode the Toyota/Lexus automatic climate control system relies EXCLUSIVELY on the ability of the A/C to dehumidify the incoming airflow. If that airflow is already warm enough or heavily laden with moisture the operation of the A/C will dehumidify the incoming airflow and thereby create enough vapor pressure over the interior surface of the windshield to rapidly evaporator the condensation.

The problem is that the ability for the A/C to efficiently perform this function is highly dependent on the level of moisture, Rh, relative humidity, of that airflow. In most northern, colder in the winter, regions of the US Rh actually declines with temperature and so the ability of the system to perform the defog/demist function also declines precipitously so below about 47F.

As a matter of fact even though the design itself is such that the A/C is disabled below ~35F Toyota/Lexus STILL relies on the probability that fresh incoming airflow will be dry enough to rapidly defog the windshield, a new form of Russian roulette.

As of ~2001 they added a new C-best option that allows RX300 owners, and maybe others, to unlink the automatic enabling of the A/C compressor when the climate control system is put into fully automatic mode. A second option will also unlink the A/C from operating automatically, without the driver's knowledge, in defog/demist mode.

While I remain an advocate of the use of the A/C as an aid in defogging the interior surface of the windshield the manufacturers need to recognize that in too many instances this method alone will either be far too slow or completely non-functional, whereas FRESH incoming HEATED airflow to the windshield's interior surface will ALWAYS work. Heating the airflow can dramatically lower its Rh, raising the vapor pressure substantially thereby resulting in virtually immediate vaporization of interior windshield surface condensation.

Additionally, and this is REALLY IMPORTANT, applying heated airflow continuously will raise the temperature of the windshield surface such that it moves substantially above the dewpoint of the "local" atmosphere.

If you own a new RX400h you will find a note within the owners manual indicating that the climate control system will automatically switch to floor/footwell outflow mode once the OAT drops below ~35F. That's because they just disabled the A/C compressor and now unless the windshield receives at least some level of heated airflow it will soon fog over.

When your system is in cooling mode, dash or dash/floor airflow paths enabled, there is NO warming airflow to the interior surface of the windshield and therefore it can, and may, chill to the dewpoint of the local atmosphere.

I fully expect that Toyota and Lexus will now begin to see not just a few lawsuits from accidents, injuries, and even deaths due to instances of sudden windshield fogging during wintertime operations of the RX400h. Automatically routing even a small portion of system airflow to the interior surface of an already thoroughly CHILLED windshield, at the same instant (OAT declining below ~35F) the A/C compressor becomes disabled, is likely to dramatically increase the number of incidents of sudden and spontaneous (to the driver) windshield fogging.

But the main problem in all of this is what happens to the remaining this layer of condensate (too thin to accumulate into droplets and run down the drain) on the chilled evaporator vanes, 10,000 square inches thereof, once the A/C compressor is disabled automatically as the OAT declines below ~35F. Or you simply "exit" the defog/demist mode not knowing, with no indication thereof, that the A/C was operating and now you have just turned it off, potentially releasing a huge MOISTURE BOMB that will soon quickly coat your already chilled windshield.

The C-best options in my RX300 have been set such that only I control my future lifeline. I only enable my A/C if the air temperature and/or the MRT require it. That means that it is never enabled during the winter months. Beats having to disconnect the A/C compressor clutch power lead each fall and then reconnect it the next spring.

What we have done with my 2001 RX300 is add an ECU, microprocessor, that manipulates the climate control ECU's input signals such that when I activate defog/demist/defrost mode the system will ALWAYS provide a high level of HOT airflow to the interior surface of the windshield.

When activating defog/demist/defrost there will be a slight delay(***) and then the reheat/remix apportioning vane will automatically move to the max heating position and then the blower speed will go to full "high".

*** The new ECU detects that the system is in defog/demist/defrost mode by "watching" the system outflow routing servomotor's feedback potentiometer. When defog mode is activated and the servomotor has moved the airflow routing to the windshield outflow position the new ECU takes over control of the reheat/remix servomotor and moves it to the MAX HEAT position. Once it has reached the max heat position the blower motor speed is raised to maximum.

The new ECU will remain in control of the system for a minimum of 45 seconds and then it will relinquish control back to the parent system's OEM ECU.

In my 1992 Lexus LS400 I was able to accomplish this with a simple switch that added a resistance in line with the IAT (cabin atmosphere temperature sensor). The added resistance caused the climate control ecu to "think" the cabin had instantly gotten a LOT colder and it would therefore quickly raise the heating level and the blower speed.

Somewhere between 92 and 2001 the engineers had added some digital signal processing for the various sensors and now the climate control ECU does not react to a "sudden" decline of the cabin temperature until about 20 seconds have elapsed. And even then it does so in increments.

The new ECU design still forces a cabin COLD signal from the IAT sensor to the OEM ECU. But since that ECU will not react to such a dramatic, quick, (INSTANT)temperature change in the cabin atmosphere we have added new capability. After the 45 second delay the OEM ECU will have recognized, fully, the COLD state of the cabin atmosphere.

After the 45 second delay the AMKA ECU will return control to the OEM ECU, EXCEPT for maintaining the FALSE cabin cold signal from the IAT sensor. That will be removed, negated, only by leaving the defog/demist/defrost mode.

Once the defog/demist/defrost mode is deactivated it will take ~25 seconds for the OEM ECU's digital signal processing algorithm to adjust to the now correct cabin atmosphere temperature indication. So for that 25 second interval the driver will notice the blower speed and the airflow heating level decline incrementally.

Hopefully that will be a good reminder to keep the damn thing out of cooling mode.

If there is anyone out there with good technical knowledge and electronic DIY skills that wishes to participate in a Beta test of this product let me know.