If your blower only operates on its highest speed chances are you have the series type of blower motor resistor.
Ford f150 heater fan only works on high.
Fan kicks on about 30 seconds after vehicle is started and is extremely loud.
Articles how to s heater blower fan fix only high speed i did my search and found plenty of help.
2010 ford f150 radiator fan seems to be running on high more than it should.
Most commonly the fan will only run on the highest setting.
My fan was working on high only.
Even kicks on when its a cooler day.
I would like to thank everyone who contributes information.
And yes the resistor is bypassed with the fan is on high.
Any obstructions or hindrances can blow the resistor quickly.
All you need to do is to replace the resistor ford part number.
However most car makers bypass the blower motor resistor completely when you move the fan speed switch to high.
Also stays on when driving down the highway.
My truck ia a 1999 f150 with manual heat ac 115 000.
This is a well documented problem and is caused by a failing blower motor resistor.
The 2004 2014 ford f 150 with manual temperature and fan controls may have problems with the the blower motor only blowing air on certain settings.
4l3z 19a706 aa or napa part number.
This is the most common failure mode.
This happens very often to ford pickup truck or suv.
You should check and make sure the blower motor is working properly not jammed or any obstructions preventing the free movement of the blade fins.
Correcting the problem is straight forward and requires replacement of the small resistor.