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Hi my name is Robert....
As soon as you turn the the thermostat ON the fan should kick on before anything else, if the fan is not coming on then it could be an issue with the thermostat, time delay relay, fan motor or the circuit board. On some older SF models you will have a time delay relay, IF you have the relay you will have one post that is constant hot, one post leading to the blower motor, a ground and the wire from the thermostat which is the trigger for the fan. Click HERE for a service manual and see the wiring diagrams for the SF model. IF you have power on the wire coming from the T-stat check for constant power on the relay, if you have constant power jump the relay to the blower motor wire and the fan should kick on. If the fan does not come on then I would jump direct to make sure it is not the fan. IF you jump the relay and the fan comes on and have voltage on the wire from the t-stat then the relay is bad. If you have NO voltage on the wire from the T-stat bad T-stat.
Some models the relay is built into the circuit board, if you have that style then I would have a RV shop test the board out, most RV shops have a special board tester and can tell if the board is at fault. Look at the service manual it should help out. IF you have more questions or concerns please feel free to contact me, follow ups are included, IF you are satisfied with my efforts please feel free to select a rating... Robert
Also a test light would work to check for power at the relay or board, most auto parts stores sell them for a couple dollars.....
IF you have more questions please feel free to contact me, I am around daily and respond when I can.... Robert
Hi.... Was just checking in to see if my answer helped, IF you have more questions or concerns please feel free to contact me..... Robert