Frigidaire FRS23R4CW2 fridge no defrost?

Question

we have a Frigidaire FRS23R4CW2 refrigerator manufactured in 2010. Has been working well until a few weeks ago. This is a side-by-side unit with freezer on the left.

Noticed the icemaker was not making ice. Then noticed the freezer section is warmer than normal. Finally got to the point where food is barely frozen.

Google says one of the more common causes of this problem is a failed evaporator fan motor. However, I can hear the fan running at full speed when I open the freezer door.

Removed the freezer shelves and the back panel. The evaporator was completely clogged with ice.

Unplugged the fridge and let it sit overnight with towels in the bottom to capture excess moisture. There was still some ice at the bottom of the evaporator, so used a hair dryer on the lowest heat setting to get rid of it.

Plugged the fridge back in and stuck a thermocouple in the evaporator. Refrigeration cycle is working well - evap coil got down to -24C in a few minutes.

The wiring diagram for this model says that the unit has an electronic defrost control. The procedure to invoke a manual defrost is to operate the light switch on the fridge side 5 times within 6 seconds. I did that but do not see any signs of defrost action. I'm using both a non-contact AC probe (Fluke LVD2) and checking at the defrost heater as well as the defrost termination temp snap switch clipped to the top tube of the evaporator. Tried this several times.

Clip-on AC ammeter also shown no defrost current.

The evap coil is currently sitting at about -43C and is accumulating ice. I'm leaning towards suspecting the refrigerator control board but would greatly appreciate advice on other things I can check.

Answer 1

Problem turned out to be the "Adaptive Defrost Control" module. There are two electrolytic caps on the board. The larger of the two (1000uF / 10V) had the classic bulged top. I replaced that capacitor and checked both the value and ESR of the other capacitor (22uF / 100V) - seems to be okay. I would have replaced it as well but that isn't a value that we stock. But it seems okay.

Fridge has been running for only a short time but long enough for the evaporator to get down to -20. I triggered a manual defrost and it seems to be working.

You can see the bulging top of the cap to the left of the blue rectangular film cap. Replaced it with a Sanyo 1000uf / 16V (very) low ESR capacitor that we use in switch mode power supplies.

I also took the opportunity to test the relay - both NC & NC contacts. It's a DPDT relay and all 4 contacts were less than 10 milliohms each.