2004 LeSabre Limited HVAC weak airflow

[Maximum Turtles]

Hello, I'm new here and to Buick, nice to meet all of you.

My partner was driving around one day in Sacramento with the AC on, then stopped for lunch, and after coming back the HVAC fan is suddenly running very poorly. The fan is working, and responds to commands to change fan speed, but the airflow is only about 30% of what it should be, from all vents.

I tested the connections to the fan, finding 14.8v to the center pin, ground on left and ground with resistance on the left pin. The resistance changes according to desired fan speed, as I would expect (0 ohms at full speed), and only about 0.2 ohms on the ground side. It seems the resister and wiring is OK.

Next I removed the blower motor from the HVAC housing and ran it, the motor seems to be running at a pretty good speed, about as fast as a desk fan. However, when I reinstall in the housing and run, the speed sounds slower than when installed.

I read on here about blend doors, so that was my next guess. My understanding is that I have 4 electrically operated actuators in the system, one for the driver's side, one for the passenger's side, one for mode selection, and one for re-circulation. Both drivers side and passenger side airflow are equally poor, I can select modes, and I confirmed that the re-circulation door is opening and closing by removing the air filter and watching.

Anyone have any clues? It was very sudden and makes no sense to me.

TLDR: just read the bold print.

 

[DonP]

Is the air flow equally bad when calling for heat vs. cooling?

 

[Maximum Turtles]

Is the air flow equally bad when calling for heat vs. cooling?

Yes, and it blows the appropriate temperature.

 

[DonP]

Then it is not likely the blend doors or a blocked a/c coil. You may have a weak blower motor. (It will sound somewhat slower then installed because it is under load and not in free air, but the highest speed setting is rather noisy.)

 

[Maximum Turtles]

That's what I've been figuring more and more. Can anyone think of a good way to bench test a blower motor? My partner is unemployed in Sacramento, so fixing this on the cheap is very important, don't want to throw parts here.

 

[DonP]

The FSM indicates that the blower motor speed is controlled by the duty cycle of a switching pulse from the IPM to the Blower Motor COnol Module (not resistor taps), and if that control circuit is out of calibration it could result in low blower output. But calibration requires a special scan tool that can command the HVAC system (not a common item). I know of no simple bench test for the motor beyond comparing its torque with a know good motor. Maybe you could find a low cost used motor at a pick-and-pull to try? New blower motors start at around $35 in line, well over $100 for Delco.