Another thread on carb fan. Have trouble shot everything help

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This is the switch. Not sure if one can determine if it is a snap switch or thermistor.

I cannot find that relay. This is a 11/78 rig. No time to pull controller box off last night but will do tonight

think it is worth trying the ohm test as grounding it did not work

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Engineer8000

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You could try checking the sensor. Not knowing the manifold temp, I would expect to see anywhere form 1k to 5k ohms after you heat it up for a few minutes. I would monitor during warm up to normal operating temp and see what you get. Sensor center post to ground. These have a good bit higher resistance when room temp (maybe 30k)
 

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Doing the same troubleshooting in my 78. Sensor appears to be the same so maybe a thermistor. Following with interest and will post on the resistance cold and hot.
 
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sorry for the delay. Here is my carb fan controlling unit and the internals. Anyone see an issue with the board and is this mysterious relay incorporated in the board?

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I would re-flow all the solder pads. I have no idea why there would appear to be something leaking onto the circuit board on the component side below what appears to be a can type relay. If it were a capacitor it would make more sense.
In the first photo what is in the dark area to the left of the green socket, next to the ceramic capacitor ? It looks like a clump of something.
 

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They were sloppy in the manufacture of these things, they are coated with left over flux. And they use a bit of some sort of glue to keep the caps from moving around. I have also seen transistor leads completely rusted away.
 
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78 carb cooling fan systems use a thermistor under the manifold (like you've pictured) so, yes, there is a specific resistance value determined by temperature. I believe it has a wax pellet inside - whatever that is. There is no seperate can relay like that pictured in an earlier post. That officially started in 1/1979. But I have seen other wiring anomallies with very late 1978 vehicles mostly in 12/78 models that have features of 1979 models but with no catalytic converter.
 
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The relay on the the circuit board sends power to the fan. I heard my relay clattering (when the fan was trying to turn on) in an old, tired controller years ago, and that problem went away when I installed a different controller. You might remove the plastic relay cover (if possible) and study the condition of the contacts.
 
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I have been totally tied up with work, kids and summer schedules. Sorry for not checking in. I swapped another control box and still no fan. I attempted to take the thermistor off to find a replacement and it is rusted on. I need to check it with an ohm meter but have not done so yet. I have time this weekend and am hoping to get to it.
I will for sure advise
 

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