Defrost Board Wiring Diagram

[Jerica Shilt]

Dependingon which model thermostat you have, it is actually correct that the auxiliary heat is typically wired to the W2/AUX terminal. The real question is, did the technician make changes at the air handler or the outdoor unit at the same time? I asked this question because in a heat pump system, it is the outdoor defrost control board that initiates the auxiliary heat. Between the outdoor heat pump unit, and the indoor air handler, the defrost control board has a W2 terminal that is connected to the W2 terminal in the air handler. A dual fuel system would typically be a heat pump with a gas or oil fired furnace. You employed a workaround when you had it wired as a dual fuel system and the temperature cut over at 40F.

The tech did do something in the air handler. But just to clarify, I have a Nest Learning thermostat (3rd gen), a heat pump, and heat strips in the air handler. The Nest wiring diagram currently shows a white wire into W1. Should it instead be in W2?

GL4, the way heat pumps work between the defrost control board and the indoor air handler is that the defrost control board uses a W2 signaling wire to activate the electric heat in the air handler. Typically a thermostat is wired to the very same auxiliary heat unit using an E wire to enable the homeowner to activate the emergency heat in the case of a failure of the outdoor unit. As for auxiliary heat, when activated by the outdoor unit, the signal wire sends a signal to the thermostat on W2 to allow the thermostat to know that auxiliary heat has been turned on. In some cases, smart thermostats can be programmed in such a way that if it detects that the heat pump is running, and the outdoor ambient temperature is so low that the heat pump cannot keep up with the demand for heat, the thermostat can also activate the auxiliary heat to boost the heating capacity of the system. The nest is one of these types of advanced thermostats.

The goal here is for you to, that is, if you are comfortable, to ensure that your thermostat has the white wire on W2/AUX and in the air handler, the same color wire is also on the W2 of the control board. Do you have access to the indoor Air Handler?

If he indeed connected all three stages together then it should work. The problem is if the control board has indeed 3-stages of heat, and if the air handler does not have 3-stages of heat, it can act erratically. You should not tie W1, W2 and W3 together in the air handler. For me to look up the control system, I need the model number of the air handler. Some systems have multiple banks of heating strips and get activated is a sequence or as Stage 1 and Stage 2.

Sometimes the installer uses differ colored wires instead following industry standards. What ever color is in W2 should be in W2 at the other end. Your Trane air handler can indeed have up to 3 stages of electric heating coils. In order for that to be the case, separate electrical contactors are used for each stage. Additionally, you air handler has an Elecyriclly Commuted Motor (ECM) and can greatly vary the speed of the blower. Can you get me the model number of the outdoor unit?

Your outdoor unit is a single stage Heat Pump. Not sure how many stages of electric heat you have in the indoor air handler unit. The auxiliary heat kits are installed by the contractor and not by the factory. Since the outdoor unit does contain an outdoor temperature sensor, the connection between W1 and W2 and W3 is not a valid configuration. The proper configuration between the Nest thermostat, the air handler and the outdoor unit typically W1 from the thermostat to W1 in the air handler, W1 from air handler to W1 on the outdoor heat pump, W2 from the outdoor heat pump temperature sensor W2 going to the air handler W2 to the W2 of the thermostat.

The AC Cooling Wizard

The Nest App just uses different colors to simply differentiate the wires. What matter is what color wire is in the terminal backplate and the other end. If you think about it, the thermostat is not smart enough to see the color of the wire. So, the software uses the industry standard wire colors for each terminal.

I'm also having a defrost issue with this model. Marcone shows the control board W11321510 to be replaced by W11566851. So, I ordered the updated part number and that's what shows on the box. However, the board inside is the older part number but with a different revision letter than what's already on the refrigerator. Any insight as to whether those revision letters mean anything?

Does anyone have anything else to add on this refrigerator model with defrost problems? I've been called to check this refrigerator for the third time with the same problem....evap freezing over. All components test ok. Replaced the control board and defrost thermistor anyway. I even ziptied it directly to the suction line without the plastic clip. Door seals are in great shape and sealing correctly.

Did you check defrost heater molex connectors? Especially the one with Green&Black harness. I had service call today for model WRS315SNHM06 with exact same issue. During the Forced defrost cycle, I noticed it was not drawing any amps, as soon I touched that green&black harness, amps started drawing.

In my case, I was not able to figure out what was causing the machine to freeze up or not initiate the defrost cycle. I could command defrost in the diagnostic mode and the heater would operate fine. Give it a week or so and the evap coils would be full of ice.

Is it not going through defrost at all if not you need to check the over temp cut outs for continuity. If it got too hot from over defrost it'll open those up. Even before you do that you should put it in diagnostics and see if it'll run the defrost heater while it's in diagnostics. If not check Loz power to element and if getting power then you should be able to test the over temp cut outs. You can also check in diagnostics if the thermistor/DF sensor on the evaporator is closed while it's frosted over.

Maybe check if the drain tube under the evap unit is OK and not blocked, preventing the defrost meltwater from fully draining away, so that it refreezes and eventually builds up covering the fan and the evap unit.

The tech sheet wiring diagram shows a thermal fuse in the heater circuit. I realize that you said that the heater is working so the thermal fuse must be OK Looking at the parts list (see below), the thermal fuse is part of the evap fan harness cable and it is there as a safeguard to prevent the evaporator unit getting too hot during a defrost cycle.

There's no mention of an "evap sensor", thermistor or thermostat in the parts list for the Freezer compartment or Unit parts section (only a thermistor for the Refrigerator compartment) of the refrigerator, but there is a component called a "heat probe" (p.16 Unit parts, Item #35 part # W10895163 (now superseded by W11568748) which is mounted under the defrost heater so I'm wondering if that is it. Is the "heat probe" used as a temp sensor for the freezer compartment, I don't know? Looking at the image for the part in the link, it seems like it is just a metal bracket of some kind

The tech sheet linked above also shows what lights are showing on the control board in the diagnostic mode if there a problem with the defrost sensor (the tech sheet wiring colours for the "evap" sensor may help to find it.) or the mainboard+the "defrost sensor"

With most other refrigerators where the control board controls the defrost cycle time, the defrost cycle occurs either every 8-10 hours or every 8-10 hours cumulative compressor run time (which is usually longer). The actual length of the time between defrost cycles depends on the make and model.

Cumulative run time means that the control board times how long the compressor has been actually operating, as it doesn't have to be operating continually. It is stopped when the preset lower limit temps are reached and then started again when the temp rises above the preset upper limit of the temp range e.g. depending on the ambient temps, overnight it may not run that often at all because the doors are shut for longer as it isn't being accessed that often like during the day, so the cold air isn't lost so much etc.

Then I read all the forums I could about the MSD2756DEW and how it is prone to frosting problems. I saw that the adaptive defrost control circuit board was anywhere from 40 to 55 dollars, and difficult to test, so I had a discussion with the counter help at Johnstone Supply where I buy AC parts. With the wiring diagram, found beside the defrost control board, I was able to match up the B10-988 mechanical timer and 4 of the 5 wires on the original circuit board. The white with black stripe wire is not needed because it is the sensor wire. Using the mechanical timer we will rely on the sensor to open the defrost circuit, but we do not need that information back at the new fixed interval (non-adaptive) timer.

Once I had the new timer working, I could hear the defrost cycle start with a click and the cabinet lights dimmed, but just for a few seconds. You can make the defrost cycle start by turning the small plastic wheel protruding through the timer back, but you need to do this with a meter attached to see when the contacts close between 2 and 3 (yellow to white wire). A quick check with my fingers and I found that the defrost heating element was still cold. Do this cautiously because when it works it will be red scorching hot and will not be something you want to touch. I moved very slowly toward it to see if the element was hot at the lowest point in the loop.

I tested the thermocouple switch that I removed, by putting it into a deep freeze at 0F. It had high resistance, above 100 ohms, too high to power up a heating element that shows only about 20 ohms cold.

If you know the freezer has electrical power because the inside light turns on, but the compressor won't run, a failed cold control thermostat could be the problem. To determine if the freezer temperature control is working, unplug the freezer and check for continuity from the power cord wire to the compressor wire at the control. If your meter doesn't measure continuity between those wires, then you'll likely need to replace the freezer temperature control because it isn't sending electrical current to the compressor.

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