This is a followup on a question I asked awhile back about temporarily disabling the auto-defrost circuit on my KitchenAid KRFC300ESS fridge: modify fridge to temporarily defeat auto-defrost?

I believe the solution is to add a method to temporarily break the connection to the defrost heater. I'll use an NC (normally-closed) relay, so that I can bring a low-voltage wire pair to the front of the unit, where I'd put a "disable" switch (I believe the defrost heater operates off line voltage). I want to put the relay in the machine compartment at the rear of the fridge, on the relevant wire that goes to a connector into the cabinet itself (where the defrost heater is located near the evaporator coils).

But I'm loathe to do this without first understanding the circuit. Here is the relevant part of the schematic, showing the defrost circuit near the bottom and some surrounding context:

enter image description here

I believe interrupting the WH/BK wire is a non-starter, because it's connected to a number of other components (perhaps it's a neutral). So that leaves me with the OR and BR wires, both of which appear to have the ability to energize the defrost heater ("DEF HTR"). I suppose I could use a DPST-NC relay and interrupt both, but I'd really like to understand what they do. The BR wire comes from a connector terminal labeled "STAT" - does that mean thermostat ? Except I thought auto-defrost circuits were usually based on timers, counting of door openings and such. If anything is a thermostat, it would appear to be the thing labeled "DEF TERM" ("defrost terminator"); and why does it have that resistor bypassing the switch, is it to leave a trickle current in DEF HTR when it's off, maybe so the heater itself doesn't frost up ? The OR wire goes to a connector terminal labeled "D-SENS" and I have no idea what that means.

To add further confusion, here is a portion of a companion drawing labeled "Wiring Diagram":

enter image description here

Note that in this drawing, the OR and BR wires appear to be going to pins 3 and 5 on a single 9-pin connector, whereas on the schematic they appear to go to two different connectors. You'd think the actual physical connectors are more likely to be in "Wiring Diagram" than "Schematic", but who knows ? I suppose I'll sort that out when I open up the back and locate these wires.

For completeness, here is the entire page from the "tech sheet" from which I lifted the above images.

![enter image description here

  • It would be nice if the full page drawings were more legible. If they came from PDF by way of a screen shot of a PDF viewer, you could try acquiring the image instead through an image editor. For example The GIMP will open PDF directly and allows the the resolution to be set (even larger than your desktop size, if desired). That might make an image with better legibility than the screen shot method can. – Greg Hill Oct 12 '20 at 3:13
  • @GregHill See if it that doesn't look better. – RustyShackleford Oct 12 '20 at 4:49
  • Whoa.. yeah that's more readable. I see there's a mullion heater in there too. In architecture a mullion is a dividing element in a frame. In a fridge.. maybe it's the divider between fridge and freezer compartments? I'm aware of a luxury brand which has heaters behind the door seals to drive off condensation so that the seals don't get mold, mildew, or frost buildup. Maybe yours does some of this too, and maybe it's a power consumer to also be considered. – Greg Hill Oct 12 '20 at 19:02
  • I imagine it's just the "humidity control" that you can turn on/off (typically seasonally) to limit condensation on the outside (and maybe the door seals too, like you say). I don't think it's really a luxury thing though: my old vintage-1987 Kenmore had it, and of course this pretty modern KA has it. Dotted lines though, so I guess some models don't have it. – RustyShackleford Oct 12 '20 at 19:25
  • This explains it pretty well, much as you suggested: appliance411.com/faq/howdefrostworks.shtml. The "adaptive defrost control" described is probably done with the OR wire feedback to D-SENS. So maybe I just feed the BR wire through a SPST-NC relay. Only issue, when terminator trips, D-SENS sees a signal from a voltage divider of the heater and the resistor shown in the terminator. When I interrupt BR, D-SENS is gonna see the WH/BK signal; dunno if that'll confuse the adaptive circuit and weird stuff happen (it'll see the defrost cycle terminated as soon as it begins ?) – RustyShackleford Oct 12 '20 at 19:46

It seems "DEF TERM" might mean "defrost terminator." If you can locate this component it may shed some light on what it does -- I wonder if it might be an auto-resetting thermal switch. It seems that a well-placed switch would experience temperature of 32F (or lower) until the moment when the ice surrounding it has turned to water, and then the temperature of the switch would rise. A control could use that behavior to terminate the defrost cycle at the right moment. A time-based defrost would run the risk of being inadequate by letting ice remain or inefficient by heating the freezer compartment excessively.

"D-SENS" might be an abbreviation for "defrost sense." If the guess about the terminator is correct, this sense line would provide a way for the controller to detect that the defrost terminator had activated. That could have been done by sensing the current flowing through the defrost heater, but maybe in their circuit design they decided a voltage-based sense was better than a current-based sense.

There's definitely some experimentation in your future. I'll guess you could use the NC relay to break the connection between the terminator and the heating element, leaving the sense connected to the heating element. That'll probably interrupt the power to the heater and hopefully still provide the sense feedback so the controller can decide to end the defrost cycle. But you may possibly upset the controller -- it could possibly do some kind of self-test to confirm the heating element is functional and this tampering may cause such a test to fail.

  • Yes, "DEF TERM" is definitely defrost terminator (see 2nd image) and I like your explanation. And, yes "D-SENS" may well be a feedback of some sort. If that's the case, then I need only interrupt the BR wire (with an SPST-NC relay). – RustyShackleford Oct 11 '20 at 21:15
  • Putting the relay between terminator and heater would require breaking into the rear of the freezer compartment where the evaporator lies, which I'd really like to avoid - more difficult, and seems a lot kludgier somehow (though the thing is well out of warranty); putting the relay in the rear seems much cleaner, despite the wires that'll run along the outside of the cabinet to near the front (they'll be low voltage DC). – RustyShackleford Oct 11 '20 at 21:16
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    I think the tolerance for experimentation may be limited by matrimonial concerns, though she is definitely on board with trying to make this work. (As you can see in affiliated question I mentioned above, the goal is to have this be one, and the largest, of several critical loads powered by a 120vac/1000kW inverter connected to our EV). – RustyShackleford Oct 11 '20 at 21:19
  • Experimentation will at least tell me what the contact rating on the relay need be. Stuff I've read suggests that auto-defrost is typically in the 600watt range, but that wouldn't explain why the thing trips the inverter. I've experimented with the fridge, and the inverter handles compressor-startup surges no problem. And yet, another time, the thing was tripping the inverter when the fridge was the only load. – RustyShackleford Oct 11 '20 at 21:20
  • A Kill-a-Watt meter can be very useful for this sort of investigation (i.e. you catch it in defrost and see what it's pulling) though certainly my new fridge actually calls the defrost load out on the device label (and it's all of 200W, but that is a very new, very efficient energy-star fridge, and older fridges will probably have much larger elements, IMPE.) – Ecnerwal Oct 11 '20 at 22:10

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