2014 E350 Econoline front ac blows on high only, I have replaced the resistor and tested both new and old for resistance (harness on resistor side looks good), switch and harness that connects to switch and still the same issue. When this started happening I smelled a burnt plastic smell and then it worked on high only and finally it melted the switch and connector and the switch was stuck. Again, I replaced all 3 components and still the same issue only works on high. I'm wondering if somewhere down the harness cable may have gotten damaged

 

Something you might consider,
Check the blower motor for excessive current draw. If it is pulling excessive current it will cause wires and connections to overheat and develop additional resistance.
The problem at the sw very likely is the result of the motor requiring excessive current which requires the blower motor to be replaced.

 

Motor was pulled out and checked, spins freely . Thing is that even if the Blower motor was pulling excessive current, that takes a bit to take out the newly swapped switch and harness. Does not happen right away. The relay, (relay harness was checked) switch, and switch harness was swapped and still it only runs on high, which leads me to believe that somewhere down that harness buried who knows where any of those 4 lines going to the relay may have gotten damage, will have to trace it.

 

Have had more than one time when sw required replacement, sw came with wire connectors attached (push on) because if you did not replace the overheated push on connectors they were rusted and did not make great contact with sw terminals.
Keep in mind, Ford system here provides the GROUND through sw for the motor to operate.
I should mention I purchased parts from Ford.

 

Which sw are you referring to ? the relay side under the battery tray or the switch on the dashboard ? I replaced the connector and switch on the dashboard. The relay was replaced and test before replacements for resistance. The connector on the relay side was not replace, it looks good.

 

They were all on the dash.

Blower speeds other than hi, all go through the resistor.
Because of the hi current requirement of the blower motor hi speed is the only one that goes through a separate relay.