HVAC blower speed slows after engine restart

[Brucebva]

I have a 21 F 150 Lariat 4x4 3.5 EB.
When the AC, heat, vent, defrost is first turned on Max, blows super high as long as you don’t shut down he truck. Can drive hours and works perfect! Do a restart after a minute or 45 minutes, fan speed drops to 80% and cannot get back to max unless you let the truck sit for over an hour!
Changed the blower/controller - no change! Changed HVAC controls - no change!
Totally stumped now! When I took it to Ford while under warranty, said they saw no problem, so I been living with it until this super hot summer where the temps have been close to 100!
All I can think is that it is a programming issue.
When I first installed the new controls before loading default values, I think did work at max, but nothing else worked until I loaded the original values from factory.
I think it must be a software setting but unsure if with HVAC or APIM for blower speed slow down for phone calls.
Anyone have an idea where to look? What I can try? Any help at all would be greatly appreciated! Been doing this since new!
Can’t find any thread of a similar issue anywhere!

Sponsored

 

[HammaMan]

It may be switching inside/outside air as that sounds different. See if cycling recirculation on/off makes the noise change.

 

[powerboatr]

It may be switching inside/outside air as that sounds different. See if cycling recirculation on/off makes the noise change.

That

And what does the fan speed indicator read when you manually ramp up fan
Or. Is it in auto

 

[Brucebva]

Thanks for replying! I don’t use auto fan at all. I like it cold so I use Max most all the time in 95 degrees! I am using a card to compare the velocity of air coming out of the right drivers vent. Always in recirc mode. There is a definite difference as it is like dropping the speed from 7 to 5 consistently. Does not go back to max until the truck sits for 65 minutes not running, ignition off! Even tried not using air, vent, or heat in a short drive and speed still slows next restart. You can start the truck, let the blower ramp up to max, kill the engine and immediately restart, the the air is down at that moment! If it’s hot and I need to drop an item at the post office, I leave it running, lock it up, and it will stay on max velocity until I kill the engine and restart! Really bugging me!

Strange issue! Must be software controlling this but beats me! I am at a loss on this one!

Oh, fan speed indicator show 7 (max) at max and the downgraded speed!

 

[HammaMan]

Again, you should hear a change going between inside/outside air, if you do not the truck is deciding where it wants air to come from. This may affect the speed of the air at that vent. The blend / vent doors may also be affecting things. It makes you think you're in control but you're not really.

 

[Brucebva]

It is definitely fan speed itself. You can tell not only by sound but air velocity by taping a small piece of paper on top of a vent and see how high it flies by fan speed! I can shut down for 5 seconds, restart and see the difference immediately!

Sponsored