As the title states, my cabin heater fan speed pulses faster/slower on its own. Very strange, to me at least. Any insight to why it does this?

 

Mine does it too. 2012 FX4, crew, 6.5 box, eco. I notice it the most during warm up, when the dash settings are full defrost, high fan. It may do it all the time, but with the defrost on high the fan speed changes are more noticeable. I first thought it might be a alternator thing, but it does not appear rpm dependent.

I was wondering about the alternator as i do have a Freestar with a funky alternator that does change voltage quite a bit. The voltage "wander" is documented in the Freeestar, I was curious if the F150 alternator is a similar design.

BJ