A tech here on Appliantology was working on a single evaporator refrigerator that was very cold in the freezer (-8 F) but warm in the fresh food compartment (~46 F), despite the fresh food being set to 34 F. 

Given that this is a single evaporator refrigerator, we can immediately suspect that we have a damper which isn't opening. But that just leaves us with another question: would that be due to a problem with the damper itself, a control issue, or a sensor issue?

As with any problem, there are a number of valid ways you could go about troubleshooting this. We could be direct and check if the damper is getting a good power supply and the signal to open. Except...

...the schematic doesn't give us good specs for what we should find on those lines going to the damper. That makes it hard to half-split between the damper and a control issue.

However, our tech settled on a different tactic that bypasses this issue beautifully. Instead of testing the damper, he went right to testing the FF thermistor, and he did it using a very clever little tool: a resistance substitution box.

This nifty little device lets you choose exactly the resistance you want and then plug it into a circuit. So our tech took note of which resistances correspond to which temperatures...

...and then plugged the resistance box into the FF thermistors circuit.

Once he had this set up, he manually changed the resistance to correspond to being below the set temp and above the set temp. As he switched it back and forth, he saw the damper open and close in response. We 100% have an out-of-spec thermistor.

It's only through understanding how these circuits and their components' technology work that we can come up with clever solutions like this. It's a prime example of troubleshooting smarter, not harder!

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