Blower won't come on on "auto" but does with "on"

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Karl messner

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IMG_5590.JPG IMG_5591.JPG IMG_5593.JPG IMG_5595.JPG I have an old oil furnace. And no the blower will not come on in the auto position. It does come on in the on position on the thermostats for the blower motor is fine.

So that sounds like the fan limit switch.

So I bought a new fan limit switch and hooked it up.

And it twists right past the fan off and Fan on text and keeps going to the safety off and then the whole thing shuts down.

There is a transformer and relay mounted on an electrical box below that and the transformer and relay it was replaced last year like an HVAC Pro.

What should I check next?
 

Reach4

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I suspect the fan and limit control. In a classic gas furnace, the fan is controlled by a thermostat that turns on when the temperature rises to some point -- maybe 100F. So while there is still heat, the fan stays on. I expect it is the same with an oil unit.

The over-temperature aspect would be to shut down fuel and probably more if the temperature gets abnormally high.
 

Reach4

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The control closes a contact when the temperature is high. When the temperature is below the limit, there would be a voltage across the contacts. Is the voltage there? If so, what is the voltage?
 
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Hey, wait a minute.

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