Important Custom Parameters.

[wwest]

The RX400h and HH have several custom setup parameters which owners should be aware.

The A/C compressor can be disabled indefinitely so it does not operate automatically in normal automatic mode or in defrost/defog/demist mode.

With these settings the A/C can be turned off manually and will remain off indefinitely until it is manually turned back on. Once turned on it will operate continuously just as it otherwise would. As shipped the A/C will also operate automatically with the defrost/defog/demist function with no indication to the driver of same.

Operation of the A/C during the winter months is needless except for the remote possibility of it being able to dehumidify the incoming airflow to a level which prevents windshield fogging. Since most occasions of windshield fogging is the result of system "upset" (loading a couple of cold, wet, and sweaty snow-skiiers with their soaked clothing, etc.) or in many cases the result of A/C operation being discontinued due to declining OAT.

Intermittent use of the A/C only during defrost/defog/demist mode is a catch 22 of itself. In the rare case wherein the incoming air's dewpoint is such that the A/C can provide a reasonable level of dehumidification when the normal operation mode is resumed the A/C operation will stop. That will result in all of the previously condensed moisture remaining on the evaporator vanes to evaporate into the incoming airstream often resulting ain a reoccurance of windshiedl fogging.

Another new feature is that the dealer can change a custom setting wherein the system will not automatically switch from footwell, heating mode, to dash/panel outflow, cooling mode(***). The cabin temperature control only responds to air temperature or thye sun's radiant heating effects. The human comfort equation is much more complex than can be rsolved by just those two parameters.

The effects of radiant heating (or lack thereof) from the exterior and interior "landscape" surrounding your body also play a very great part in establishing human comfort. On a cold dark night with the landscape covered with snow the system, driven only by its measurement of interior, cabin air temperature, will almost always shift, first, into mixed footwell/dash outlet mode, and then into full cooling mode, dash airflow outflow mode, once the cabin temperature closely approximates your manual temperature setpoint.

As many of you have already no doubt noticed, that can be quite discomforting to you and your passengers.

But there is yet another important aspect of this feature, heating/cooling mode parameter setting. In full cooling mode, which the system would otherwise almost always default into as the cabin temperature reaches your setpoint, no warming airflow reaches the interior surface of the windshield.

It can become extremely dangerous to allow the windshield interior surface temperature to decline due to the onrushing COLD outside airflow during wintertime operations. Keeping the system in footwell, heating mode will result in a significant level of airflow being continuously routed to the windshield thereby keeping it well above the dewpoint of the cabin atmosphere.

You may note that in your owners manual is a new (as of 04?) note of caution. Since the A/C will shut down automatically if the outside temperature should decline to or below the freezing level the newer Denso climate control systems will automatically switch a portion the system outflow to the interior surface of the windshield just as the A/C is disable.

A bit of shutting the barn door after the horse has galloped off down the road, that is.

Or actually it can be very much worse.

When the A/C is disabled in thsi manner any previously condensed moisture remaining on the evaporator vanes, 10,000 square inches thereof, will begin to evaporator into the incoming airstream. Absent your having had the dealer make the parameter change from the factory default the windshield interior surface may very well have already been chilled to well below the dewpoint of the super-saturated airstream now routed, automatically, in its direction.

But never fear, there is yet another parameter that the dealer can set for you that will help to defog/demist the windshield. The dealer can set a custom parameter which will cause the blower speed to automatically increase when you select the defrost/defog/demist mode.

Now if we could just get them to recognize that the airflow heating level to the windshield should also go up simultaneously.................

*** I would be very interested in the results if anyone choses to have this parameter set to prevent an automatic switch to cooling mode. I assume one can still manually switch to cooling mode. My question is does it operate in the same way as the A/C disable custom parameter?

If I manually switch to cooling mode will it thereafter automatically switch back and forth as it otherwise would until I manually turn cooling off?

[monarch]

Toyota has an ongoing TSB that applies to all models and all model years that recommends running the AC compressor a minimum of 30 seconds once a month trhoughout the year (and preferably 5 minutes or more once a week) This is necessary to keep the compressor properly lubricated and to prevent the system seals from deteriorating. Owners who don't follow these rules risk premature compressor failure and loss of refrigerant due to leaks. I followed the rules and have been rewarded with 469,000 miles of troublefree AC system performance (on my '92 Toyota pickup)

[wwest]

Toyota has an ongoing TSB that applies to all models and all model years that recommends running the AC compressor a minimum of 30 seconds once a month trhoughout the year (and preferably 5 minutes or more once a week) This is necessary to keep the compressor properly lubricated and to prevent the system seals from deteriorating. Owners who don't follow these rules risk premature compressor failure and loss of refrigerant due to leaks. I followed the rules and have been rewarded with 469,000 miles of troublefree AC system performance (on my '92 Toyota pickup)

For more years than I care to remember, count, I have always disconnected my A/C compressor clutch during the winter months. Granted most of those car were fords, highest mileage of any one being in the range of only 275,000. But then even my 1992 LS400, at 100,000 plus, has survived 14 winters with the A/C disconnected and no problems and no apparent ones.

But those all used freon....

Not by any means advising against running the A/C to help lubricate the seals, etc, just against using it as an integral part of the HVAC during the winter months.