Heater Settings

[apatrules]

When I turn on the heater and go to the automatic settings, there is a setting indicating that the A/C is on. I assume that means the compressor is engaged even though it is 20 degrees outside. Heat works fine but I've noticed on the other vehicle (GS 400) that when I punch the a/c off in winter, the car idles at a lower rpm.

I always turn the a/c indicator off in winter assuming that I will get better mileage and less wear on the compressor and engine. Anyone else deal with this issue? And why does one need the a/c engaged in winter. Something to do with defrosting?

[wwest]

Your vehicle has three c-best options that the dealer can set concerning the most efficient, proper, and SAFE use of the A/C.

1. Option one is so you can disable the A/C for an indefinite period, infinite restarts, simply by manually turning it off. It will not operate again in automatic mode until you turn it back on manually.

2. Unlink the A/C from operating automatically in defrost/defog/demist mode. Below ~45 OAT the A/C's ability to efficiently dehumidify the incoming fresh airflow is highly dependent on climatic conditions. Conditions which rarely exist north of the snow line where the defrost/defog/demist mode is most needed. Additionally there is NO A/C system that can operate efficiently below ~35F and most factory designs disable them at 32F and below.

3. Prevent the system from automatically changing the outlet airflow mode from heating (footwell airflow, with substantial "leakage" warming airflow to the interior surface of the windshield) to cooling (dash outlets).

With factory default settings the system will automatically switch to cooling mode as the cabin temperature rises, approaches, your temperature setpoint. In cooling mode there is literally nothing to prevent the interior surface of the windshield from declining to dewpoint as a result of the COLD 60MPH airflow impinging on its exterior surface.

Since Toyota and Lexus' climate control system's defog/demist capability relies EXCLUSIVELY on the sometimes non-existent ability of the A/C to dehumidify the incoming airflow you must be fully prepared to interact with the system in order to quickly defog/demist your windshield should the need arise.

To most quickly defog/demist the interior surface of the windshield turn the temperature setpoint to max heat and then put the system in the defog/demist mode.

Many years ago, say prior to 1990, allowing the A/C to operate in conjunction with substantial HEATED airflow to the windshield in defog/demist mode was a no-brainer, sometimes it helped and if it didn't, no harm done.

"no harm done...."

No longer true.