filmteknik

2008 RX350 with NAV. 211K miles.

For some time now, the ventilation (HVAC) blower would change speeds on its own even though I'm in manual mode. Lately it mostly stays slow. Occasionally off entirely. Sometimes, turning the system OFF, waiting a moment, then selecting a manual speed or hitting AUTO will revive it. Sometimes that gives a burst of faster speed but then it settles back to slowest. Naturally, at this many miles I don't want to put too much into the car.

I do not think it's a problem with the motor itself nor dampers / blend doors sending air the wrong way. (But nothing is totally ruled out.) And the cooling (provided I keep it charged; it does leak slowly) seems fine--when it runs.

So most likely a computer board somewhere or a sensor or corroded connector. Or maybe a leaky vacuum hose.

So what should I look at? Is this going to show up in codes?

FWIW I have not noticed anything odd with any other features with the NAV or other features on that display.

Also, is there a place where I can get at the actual wire feeding the motor? I would certainly consider rigging something where I send 12V directly to the motor. Maybe with a rheostat. Either as a temporary solution or long term if this repair requires a costly board that requires taking apart half the dash to get to.

spinellib

I doubt it's a vacuum issue. Not likely to show up in codes (DTC's), but if you have Techstream you might be able to delve into the HVAC System and see if there are tests in that section for the operations of commands. I don't know what you'll learn, but maybe it will point you in the right direction.

I found these two threads that might help:
https://www.clublexus.com/forums/ls-...l-problem.html
https://www.clublexus.com/forums/ls-...or-broken.html

DennisMik

There are 4 parts that could be causing this:

1) the HEATER relay. The relay is simply an on/off switch for the 12 volts that powers the motor. If the contacts are corroded, it will not allow full power to the blower motor. It is located under the hood on the passenger side. Probably the least likely of the 3 suspects.

2) the blower motor. The bearings could be going bad and binding. When the motor is cold, the motor will spin faster. As things get warm, the parts expand and the binding becomes more of a problem. If you wanted to put some type of external rheostat on the blower motor, it will have to be able to handle 50 amps @ 12 volts. The blower motor has 2 wires going to it, 12 volts and ground. The motor works by controlling the ground wire for speed. The 12 volts is direct from the fuse and only the relay is involved. It is the ground wire that is connected to the blower motor control unit for speed control. A rheostat would be connected to this wire. On the motor, the red wire is 12 volts and the yellow wire is the ground/speed control.

3) the blower motor controller unit. This printed circuit board is the thing that directly controls the speed of the blower motor. The a/c amplifier printed circuit board sends a signal to the blower motor controller unit telling it how fast to run the blower motor. Not much you can do other tan replace the controller. But you could check that the ground wire to the blower motor controller unit has a good ground. The ground wire is a white with black stripe wire.

4) the a/c amplifier printed circuit board. This is the heart and soul of the HVAC system, the brains. It controls every last part that makes up the HVAC system. As stated above, it sends a signal to the blower motor controller to spin the fan and how fast to spin it. I don't think this is your problem, but it is possible.

With the exception of the relay, all the components are behind the dash on the passenger side, close to the outside of the car.

pulprack

I have a 2004 RX 330, 169K miles.

For a few weeks now, my AC will automatically switch to outdoor air. I normally switch off the AC and open the windows (the outdoor temperatures lately have been in the 90s, so moving air from the windows is better than hot humid air blowing in from the vents of a closed-in vehicle).

When I turn on the AC again after a few minutes, it usually blows cold again. Not always, but usually. Or sometimes it'll just switch back to cold after a few minutes.

Is this an issue with a sensor or thermostat?

Thanks,
Duane

Margate330

Yeah, definately not a vaccum issue. All the flaps are servo controlled these days.

Margate330

The post above by Dennismilk has the voltage path and control for the blower motor.
However, until you can have consistent symptoms to report it would be difficult to establish a path forward.

Using Techstream to controll the AC could answer a lot of questions fast and rule out a bad navigation unit, AC amplifier or other failure.
If it was my car I was thinking I'd start with the fan resistor until you just mentioned the ac switching modes too so hard to say without more consistent info.

spinellib

It sounds like the Auto function is fighting it out with another function/sensor that wants to do something else.
I wonder if Techstream can disable certain functions so you can rule them out...

pablo87

Why not replace the blower first? I have a 2008 RX350 now. My blower was unacceptably noisy at 1 or 2 setting so I bought a budget one at Rock Auto for $50 and my son replaced it for me. Took him less than an hour (sorry, don't have a step by step). The replacement is noisier at settings 4 or 5 than a factory blower (though acceptable) but those speeds aren't used too often.

Margate330

My blower motor was noisy and I ordered a generic replacement on ebay for like $20 and it was a 30-60 minute job replacing it.

When I dropped the motor the blower fan was full of leaves.