Strange Thing With A/c

[sainikai]

This morning when I started my car and while I was waiting to get it warm, A/C got ON by itself. This thing happened earlier also but did not paid attention to it. Do you think this is some sort of symptom for some A/C service or so?

[90LS400Lexus]

Did you have it on "defrost" mode? I think that the A/C comes on when that feature is used, or did you push no buttons at all?

Off to work I go..... <_< :)

[sainikai]

No, I did not use any buttons and also defrost was not on when I started the car.

[wwest]

All Lexus, at least up until 01, default with A/C compressor on each time you start the vehicle. After 01 C-best option allows the dealer to change it.

[Lexus Envy]

The reason the defrost comes on is that AC removes humidity, normaly you can manualy depress the ac button and turn it off. This has a good side effect though, it keeps your compessor from becoming dormat durning the winter months. I don't know if you know this but it is not good to leave a compressor in the same position for extended periods.

Jimmy

[wwest]

I think you mean defog or demist, defrost is usually an indication of ice on the outside of the windshield and lots of HEAT, MAX HEAT and high blower speed, is the only way I know to quickly solve that.

Defog/demist:

The A/C can ONLY dehumidify the incoming airflow if the Relative humidity is high enough that the A/C can cool it to its dewpoint.

In the wintertime the air is nationally drier, lower overall relative humidity.

So below about or below 45F the incoming atmosphere must have an unusually high moisture content in order for the A/C to substantially dehumidify it.

Additionally, as the blower speed is raised above minimum the efficiency of the A/C for dehumidification will be decreased accordingly.

So, if you have condensation on the interior of the windshield the very best thing to do is turn up the heat and the blower speed and leave the A/C OFF!

And by the way, the A/C compressor is disabled automatically, and cannot be restarted although the indicator will come on, if the OAT declines to or below about 35F.

If you should use the A/C for defog/demist on a cool day be aware that when it turn it off the A/C evaporator will be CLOGGED, virtually saturated, with the moisture just condensed from the airflow. With the A/C now off that mositure will be released into the cabin to radically increase the RH even about what it was before you used the A/C.

Keep that windshield warm and avoid all of these problems. In the wintertime be careful to keep the system airflow to the footwell so that some of the airflow is toward the windshield to keep it warmed.

In automatic mode ALL Lexus will switch to cooling mode(***) once the cabin's atmosphere approaches, rises to, the temperature setpoint.

As of 01 the RX300 could be optioned via C-best to not automatically activate the A/C in normal mode and separately not in defrost/defog/demist mode. My 01 RX300 is set that way. Apparently the 04 RX has a c-best option wherein the system can be prevented from automatically going out of HEAT (footwell) mode when the cabin temperature setpoint is reached.

Maybe other new Lexus' have the same feature.

*** airflow from dash outlets or combined dash and footwell wherein NO airflow is directly routed to keep the windshield warmed and thus above dewpoint.

[Lexus Envy]

^^^^^ Thank god for people who really know what they are talking about

Thanks for the info wwest

Jimmy

[flanker271]

All Lexus, at least up until 01, default with A/C compressor on each time you start the vehicle. After 01 C-best option allows the dealer to change it.

Is there a reason for this setup?

Also, has anybody experienced that the two side vents blow air 10 degrees warmer than the two center vents when the a/c is on during a hot day?

In addition, occationally (not always, randomly) I hardly have any air coming from all of the vents, even though I set the fan speed to high. I can hear the fan very loudly, but there is still hardly any air coming from the vents. What could be the cause?