05 lariat with Electronic HVAC control planel. In Auto mode I set the temp to 90 or to 60 and i thought the fan should blast on High. The fan in auto mode never gets any more than half way. i can take the control out of auto mode and manual crank up the blower to high. Is this common? Or normal the truck still has the orginal battery and I noticed it turned over a little slow lately. Could it just need a new battery to give it more power. Just wondering if when it starts it reads what power is available and goes into some limp mode? Please someone confirm if I should have max fan speed in auto mode at the 90 and or 60 settings. At least then I can try to narrow it down a little more. All help is appreciated.
thanks,

 

Has it always done that?

 

Honestly I don't remember. It's been acting screwy for a month or so not heating up until I cycle if from cold to hot to cold to hot. I thought it was the blend door actuator but if the fan is acting wrong too, maybe it's the head unit. Thought I'd ask before I bought some parts to swap out.