Air conditioning / blower surging

[BubbaFL]

I have a 2003 QX4 that is doing something that seems odd:

On hot days, when I start the car, the climate control will crank the blower up to full (four blades in the graphic), and then the blower speed will seem to surge irregularly for a few minutes or until I start driving and get the RPM's up to 2000-3000+.

It's almost acting like a fan that is losing a bearing, but there is no bad bearing noise.

My voltmeter says voltage at the cigarette lighter is dead stable 13.9-14.0V, so I don't think it's an alternator problem.

Is this normal behavior on hot days (95+F) when starting the truck with climate control cranked down (set on 60F) or do I need to dig deeper?

Thanks in advance!

[Towncivilian]

Check for debris in your blower motor squirrel cage. It is located behind the glove box, remove the glove box and unscrew the motor's 3 screws and out it comes. Maybe change the cabin air filers while you have the glove box off too.

[ARKQX33V6]

If you have access to a DC amprobe, check the current of that motor.

[BubbaFL]

Thanks for the suggestions!

The cabin air filters were disgusting - I washed them out and will order replacements ASAP. I didn't see any large debris in the air filter housing, and didn't pull the blower motor yet.

The A/C system was slightly undercharged, from what I gathered from the FSM - only 25 psi low / 250 high on a 91+ degree day.

If it happens again I'll pull the blower motor and test it.

[ARKQX33V6]

Blower motor have a negative pressure on the incoming side compared to a positive pressure on the downstream side. A plugged incoming side compared to a plugged downstream side makes a difference as to motor load and motor amps. The FLA, full load amps of the motor indicated on the motor is the highest amps to run, at higher amperage the motor is working too hard, at lower amps the motor is working less.

A plugged downstream usually makes the motor work less but it gets louder as the air and the fan, a forward leaning vane will fight and create noise. A rear leaning fan does not ususally overload the fan motor.

A plugged incoming cause less air to meet the fan and the fan can speed up as in a free wheel and use more amps.

These small fans are made to deliver their CFM of air within a very specific pressure or ESP, external static pressure and they must stay within their loading of CFM, and amps otherwise weird noises emanate especially if the radial fan touches anything other than air. The motors by design are not very torque producing and the fan rubbing dried leaves will cause an over load.

Be sure to clean the air vanes around the fan wheel, unbalances makes for more weird noise.

[BubbaFL]

Good info, thanks.

Where is the external opening for air to enter the cabin?

[Towncivilian]

[BubbaFL]

I took a look behind the blower motor - all clear there. Also changed the cabin air filters. It hasn't done it since I washed the old ones last weekend.

Thanks for the photos and diagram.