HVAC, when the A/C or heat is run for a while the output of air decreases slowly down to zero

Hello,

It sounds like the blower motor or the resistor is overheating and slowly stops blowing air.
It probably has too much internal resistance and as it heats up the resistance increases and slows down the fan and stops.

The blower motor resistor is located on the HVAC housing behind the dash (see diagram).

Here is a helpful video:

https://youtu.be/uouVsBuSOzc


Please let me know of any questions.
Thank you.

I'm sorry if I wasn't clear in my description of the symptom but the blower motor continues to blow. I can turn up the speed and I hear it get louder but the volume of air coming from the vents slowly decreases to near zero. Changing the air flow to floor or anywhere else does nothing to increase the volume of air.

So, the fan will still be blowing air, but it will not come out of the vents, right?
Do you hear any clicking noise under the dash?
I am wondering if the air door actuator might be faulty and acting strange when it is at certain temperature.

Yes, I can hear the blower running and it gets louder if I turn up the blower speed. I've had the door actuator replaced so I understand the clicking your talking about and there is no clicking. It's just that the air flow from the vents begins to diminish to almost nothing. This occurs regardless if the airflow is set for the floor or the upper vents. It may take several minutes to diminish but it does happen consistently. When I turn off the system, not the truck but the system for about 30 mins it seems to be enough for the system to reset and then it runs fine again.

Just to jump in on this, it sounds like a control module issue. What I would do is remove the actual actuators one at a time when this is acting up and not blowing air and move the door using your fingers or a small screwdriver. You should be able to turn the door so that you can see when the air starts flowing again.

This will do two things. First confirm which door the issue is on. Next confirm that the actuator is most likely being commanded to do this. The fact that it slowly starts doing this and then shutting it off will get it working again, points to a control issue.

What I suspect is happening is the HVAC control module is causing one of these doors to slowly move to cut the air flow off. Most likely it is the blend door so I would start with this one.

Let us know what you find with this, and we can go from there. Thanks

Am understanding you right to mean the control module on the dashboard, may be causing the problem? I had that module replaced many months ago and the air flow problem is fairly recent. Could it be a problem with the HVAC control module?

That is exactly correct. If this was replaced and now has this issue, then I suspect the replacement module is the issue?

Any chance you still have the old module? If so, I would plug it in and see if this issue goes away. Clearly the prior issue will return but at least if you know this is not acting up anymore then we know that was the issue.

I attached the info below on how to replace this if you would like to swap it out.

If you do not have the other module then I would remove the actuator and move the door just to make sure this is the issue.