Sam Brown

[Sam Brown]

How much information about the control board’s algorithm you’re given and in what format it’s presented depends a lot on the manufacturer and model. One way you might find it in a service manual is when you’re reading information about how the control responds to a failing load. Something like this:

When the control board senses no motor speed feedback signal within 10 seconds of supplying voltage to the motor, it will stop supplying voltage to the motor for 5 seconds, then retry for another 10 seconds. This will repeat two more times, and if no speed feedback signal is sensed at the end of the last attempt, the control board will stop supplying voltage to the motor and display an F5 error code.

I made that one up, but it’s a variation on the kind of theme you’ll see.

The reason it’s important to be aware of things like this is because, in the case above, you might measure that the control board isn’t supplying voltage to the motor and immediately assume an issue with the control. But that’s in fact just a symptom of the control board doing what it’s programmed to do.

While it’s very important to know information like this, it’s often not presented in the clearest way. It can be scattered about the manual/tech sheet in descriptions of load functions, error code descriptions, board documentation, etc. So it’s always important to do a thorough analysis of everything relating to your LOI, including algorithmic information.

[Sam Brown]

Dunking a thermistor in ice water is a way to test it at a specific temperature — 32 degrees. Usually, this is only necessary if you do not have a thermistor chart and the manufacturer is instead only telling you the resistance at 32 F. In this case, it’s usually easiest to completely disconnect the thermistor and check its resistance, since dunking it in ice water while it’s still connected to the control is often impractical.

[Sam Brown]

You’re correct that we should never use chassis ground as a reference for any AC measurement. Remember too what was talked about in the video for this unit: how the DC power supply has its own ground that it produces, separate from the AC world and separate from the chassis.

Sounds to me like a DC measurement with respect to chassis ground just isn’t a valid measurement. Is there an answer in the quiz that reflects this?

[Sam Brown]

Are you referring to the fact that we’re not always drawing the red and blue lines up to the drive motor?

If so, that’s only because it’s not the load we’re currently looking at. For example, in case study #3, we’re looking at the heating circuit and timer, and so we only draw out the power supply for those circuits. But if you look at the drive motor’s circuit and compare it with the timer chart, you’ll see that it is receiving power whenever a cycle is running.

[Sam Brown]

Yes, with the switch off, any point to the left of it is electrically equivalent to any other point to the left. In an uninterrupted section of wire (meaning there are no loads or open switches dividing that section of wire), all points within that section will be electrically equivalent to every other point within that same section.

[Sam Brown]

Hi Darrell,

There are two correct answers to that question, and you need to select both in order to get it correct. Overlong dry times is one correct answer, so you’re halfway there.

To get the second correct answer, review the lesson materials and think about how the thermal protector in the heating circuit might react if a lot of heat gets trapped in the machine with nowhere to go.

[Sam Brown]

The key thing here (which is what this question is demonstrating) is that we are not giving a spec within the text of the unit. We are just stating a general range to give you a feel of what a normal fill falls within.

So if you encounter a fill that takes 105 seconds, you can assume that it’s probably within range, but you still need to check the spec for that particular dishwasher, since different models will do different length fills.

[Sam Brown]

I am just curious why BLDC motors are more efficient? Is it because they don’t have the friction from the brushes? Or is it something to do with the commutation?

Depends what exactly we’re saying they’re more efficient than.

In the case of the old-skool brushed DC motors you referred to, then yes, you’re basically correct. Mechanical commutation has a lot of drawbacks, such as the friction of the brush and the need to change it every now and then.

BLDC motors are also more efficient than split-phase AC motors, and this is because split-phase motors suffer from slip losses. Defining what that actually means involves a lot of math, so we won’t get into that, but it arises from inefficiencies due to the induced voltage that’s part of split-phase motor operation.

Shaded pole motors (which are basically just a special type of split-phase motor) suffer from huge slip losses, so replacing them with low voltage BLDC motors (like what all refrigerators are using for their evaporator fans these days) is a big improvement.

[Sam Brown]

Ok so the Samurai says we discover voltage on the relay board at Dlb and PR1
This is great but still in my mind is only part of an answer.

You’re right — there’s still more to the story! That’s why the case study doesn’t end at unit 7. Proceed on to unit 9 to see how the troubleshooting proceeds…

[Sam Brown]

Is there any special technique to hear the fans clearly.

Not really a “special technique” per se — just careful observation. For the evaporator fan, you usually need to both listen in the compartment for a soft fan noise, as well as feel for airflow. Similar thing for the condenser fan: you can put your ear to the bottom of the machine to listen for it, and you can feel for airflow at the bottom. If all else fails, you can slide out the unit and get eyes on it.

Also, if I’m on a call and just arriving and I don’t hear fans running how could I tell if the unit might be in defrost mode or not.

There’s always a chance that the machine is in defrost when you arrive, so it’s important to be aware of that possibility. That’s why using the diagnostic mode to test other functions is important. You can also perform a current measurement on the defrost heater circuit if you ever need to confirm whether you’re in defrost or not.

Another question, do most fans stop working when the door is opened?

The condenser fan is unaffected by door openings, but the evaporator fan usually is.

[Sam Brown]

The 3-phrase power supply that the inverter produces is electronically commutated AC. What that means is that the inverter is putting out DC power, but then using electronic switches to reverse the current flow through the motor windings many, many times a second, effectively creating AC.

We cover exactly this in much more detail in the motors section of the Core course, so I would recommend reviewing that if you would like more information.

[Sam Brown]

Sorry for the delay in getting back to you — yes, it looks like you’ve really got a handle on it now.

If so, I basically feel comfortable closing this topic. Although I do have a question about if the metering device is located between the capillary tubes and the evaporator. Even though I quickly googled it, I do feel unclear about its purpose. Am I ready for that yet?

Could you clarify what metering device you’re referring to?

[Sam Brown]

This cooler gas is now ready to enter the evaporator coils to absorb heat from the warm returning air pulled in from the evaporator fan.

Just a quick clarification here: the refrigerant remains a liquid all the way through the capillary tube, and it only boils into a gas when it enters the evaporator, where the pressure is much lower. It is the action of boiling that absorbs heat energy from the freezer compartment, since the phase change from liquid to gas requires energy. Once the refrigerant has all turned into a gas, it is no longer useful for cooling and is sent back to the compressor to begin the cycle again.

Does the return duct have to be located at a higher position than the damper to ensure the warmest air in the refrigerator is returned rather than colder air below?

No, the location of the return duct relative to the damper isn’t that important. Different manufacturers will put it in different places, including in the bottom of the compartment.

[Sam Brown]

Hi Brandon,

First off, let me address your two questions about the course material:

One:
It was explained that the refrigerant is compressed into the condensor coil would not this simply make the hot boiling gas coming from the evaporator a much hotter liquid? But if the object of the condensor is to make the refrigerant cooler, why does it have to go through this phase change which makes it even hotter? Is it due to volume concerns? Please explain. Thank you.

Remember that the whole point of the sealed system is to absorb heat from the refrigerator by the phase change from liquid to gas. In order to get the refrigerant in a state where it can boil from a liquid to a gas in its next go through the evaporator, it needs to be compressed. Higher pressure = higher boiling point. That’s the job of the compressor.

Now, compressing the refrigerant does make it hotter, but that’s why you have the condenser right after the compressor. The condenser allows a lot of that extra heat to radiate off the refrigerant. This cooling down, combined with the much higher pressure in the condenser compared to the evaporator, causes the refrigerant to change phase back from a vapor back to a liquid. Once it’s done this, it’s ready to exit the condenser, go through the capillary tube, and turn to gas all over again in the much lower pressure evaporator.

Two:
Does the damper exclusively allow a one way flow of air from the freezer to the fresh foods compartment? if so, how does it do this? from positive or negative pressure of a fan, or some mechanical means like in a check valve? Thank you for your time reading and for you responses.

The damper is for airflow from the freezer to the FF compartment. There is also a separate return duct, since otherwise the air pressure would prevent any air from moving from the freezer to the FF compartment. Normally, there doesn’t have to be a separate fan to push air through the damper — the evaporator fan does that.

As for your specific question about your aunt’s refrigerator, that would be a great question to post over at Appliantology. That’s our tech help forum that you have access to with your free MST Student membership. Start a new topic in the Appliance Repair Tech Help forum asking your question. That way, you can get both my advice and that of all the other talented techs at that site.

[Sam Brown]

Clarify where the points are from a supposed potential to the other side of the potential circuit. Otherwise points could mean points on the same side of the load.

Sometimes you do make a measurement between two locations in order to check if, for example, a switch is closed. It all depends on the kind of test you’re making.

When we talk about test “points”, we’re never referring to the pre-existing dots on the schematic. We’re always talking about chosen test points in the circuit — for example, I could call out test points as CN7 pin 1 to CN5 pin 3. As I said, sometimes those line up with the dots on the schematic, and sometimes they don’t. The dots are just indicating terminals, connectors, and tie points between wires.

Don’t get too hung up on the word “point”. Pay more attention to the specific designations, like “CN7 pin 1” mentioned above.

Again, is there a specific video or course unit in our material that you’re referring to? I’m happy to clarify, but it’s difficult to talk about these things as generalities.

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