Hvac Blend Door

[katie10543]

I've got an 99 RX300 with 167K miles on it and running great! The only issue I have is a longstanding one that's been getting worse over time. The blend door (so I'm told by a Lexus mechanic) oscillates back and forth rapidly when the outside temperature is high. It's gotten to be quite noisy recently and I'm wondering if there's an easy way (i.e. something that I can do myself -- I'm pretty good at minor car repairs) to quiet this down.

The AC and heat work fine as does the blower fan, temperature regulation and all vents and valves to those vents.

Thanks,

Katie

[wwest]

The "blend" door is the reheat/remix control. All of the system airflow is first cooled via flowing through/over the chilled evaporator vane surfaces and then a selected portion is routed through the heater core for heating and then both portions are remixed to attain a temperature outflow level that cools or heats the cabin without undue discomfort, not too hot nor too cool.

If the blend door were oscillating slowly that would result in periods of too cool and/or too hot airflow. Rapid oscillation and you likely wouldn't notice except for the sound of the blend door servomotor running.

When I need the use of the A/C in my 2001 RX300 I always use the recirculate mode, turn the temperature control to max cooling which "parks" the blend door for NO reheat airflow, and then I use the blower speed to regulate, adjust, my comfort level.

That procedure increases the efficiency of the A/C rather dramatically sometimes gaining me as much as 1 to 2 MPG in extra fuel economy.

Later models of theh RX300 and 330 have two new c-best option settings. One of these allows you to disable the A/C compressor when the OAT makes it needless and the second unlinks the A/C from the defrost/defog/demist mode when it becomes hazardous for use.

Before those c-best options were available I simply disconnected the electrical connection to the A/C compressor clutch during the winter months.

[mh6]

The "blend" door is the reheat/remix control. All of the system airflow is first cooled via flowing through/over the chilled evaporator vane surfaces and then a selected portion is routed through the heater core for heating and then both portions are remixed to attain a temperature outflow level that cools or heats the cabin without undue discomfort, not too hot nor too cool.

If the blend door were oscillating slowly that would result in periods of too cool and/or too hot airflow. Rapid oscillation and you likely wouldn't notice except for the sound of the blend door servomotor running.

When I need the use of the A/C in my 2001 RX300 I always use the recirculate mode, turn the temperature control to max cooling which "parks" the blend door for NO reheat airflow, and then I use the blower speed to regulate, adjust, my comfort level.

That procedure increases the efficiency of the A/C rather dramatically sometimes gaining me as much as 1 to 2 MPG in extra fuel economy.

Later models of theh RX300 and 330 have two new c-best option settings. One of these allows you to disable the A/C compressor when the OAT makes it needless and the second unlinks the A/C from the defrost/defog/demist mode when it becomes hazardous for use.

Before those c-best options were available I simply disconnected the electrical connection to the A/C compressor clutch during the winter months.

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Well done WWEST, answered some questions I have been meaning to ask about. :)

[katie10543]

Thanks for that good description and the suggestions on how to avoid the noise. Indeed the noise I hear is the servo moving the door back and forth -- often as fast as once a second. I had discovered the fact the MAX COOL will lock the door to one side, but I'm not sure that I can attain good temperature control by just using the fan speed setting when it's in that mode -- I'll try it. I'm surprised that the mileage would improve in MAX COOL mode as the AC compressor would be on all the time, right?

[wwest]

Thanks for that good description and the suggestions on how to avoid the noise.  Indeed the noise I hear is the servo moving the door back and forth -- often as fast as once a second.  I had discovered the fact the MAX COOL will lock the door to one side, but I'm not sure that I can attain good temperature control by just using the fan speed setting when it's in that mode -- I'll try it.  I'm surprised that the mileage would improve in MAX COOL mode as the AC compressor would be on all the time, right?

←

"...right?"

No, wrong.

Your A/C system has a reservoir to store liquid refrigerant. The A/C compressor only operates to refill the reservoir when the "store" of liquid refrigerant is low.

Bypassing the reheat cycle of the system results in more cooling efficiency and thereby fewer operational cycles of the A/C compressor.

[katie10543]

Wow, that's quite sophisticated, I guess you do learn something new every day!

Thanks again!

-Katie

(Sorry I posted this in the wrong spot. I'd move it, but I don't see how that can be done.)