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Hello. 2014 E350 AC question, 78,000 miles. When I start the van, the front AC blower switch works on three of the 4 settings (third setting is same blower speed as second). If I'm driving for quite some time (interstate travel), I usually leave the front switch at the lowest fan setting with the air conditioner on. However, after a few hours none of the switch settings give me any more air flow than the lowest setting, with exception of the fourth setting when the max ac kicks in - then I get slightly more air flow, but not nearly what it should be. What's going on here? Thanks.
Blower resistor. It is probably rusted and needs to be changed. Easy job, take the battery and you will see the blower motor mounted on the firewall. This is the best picture I can get. It doesn't show, but it is by the marked spot.
This might also be the dash board mounted speed control switch has failed---its happened to me a few times on various E-Series ranging from '97's up through '05. Then again a failed switch or resistor network tends to not be intermittent. You could also have a wiring harness or connector issue.
A failed switch might look like this, possibly worse:
