Your RV Refrigerator, Part 4-controllers and recalls

By Paul Unmack, PE-CSE, ME #116483, www.arprv.com

In the previous installments of this series, I discussed how the cooling unit works. Then I covered how heat is applied in order to drive the cooling process. Like all systems, something has to control the cooling process to make it work in a manner that is useful and safe. Who wants their iceberg lettuce to actually resemble an iceberg? It is the job of the refrigerator controller to insure the temperature within the refrigerator is kept within desirable limits and that the process does not have any errors.

Refrigeration Controllers
All refrigeration controllers have one commonality: They have a temperature sensor that monitors the temperature inside the refrigerator. The temperature sensor itself is monitored by the controller. When the temperature is out of range, the controller turns on the refrigeration process to cool the refrigerated space. You might say that the control of the cooling unit is binary. It is either on or off, and it is the average time that the unit is on or off that determines the temperature inside the refrigerator cabinet. If the cooling unit is on full-time, everything becomes frozen inside the refrigerator. If it is never on, the temperature inside the refrigerator gets too hot and food spoils. You could monitor the temperature inside your refrigerator yourselft with a thermometer and then turn the cooling process on and off to control the refrigerator cabinet temperature, but most of us have better things to do with our time; thus, making the automatic controller essential.

Types of Controls
There are two basic types of controls found on RV refrigerators. The first type is the manual control, a mechanical-type control in the sense that no electricity is needed for power. The manual control is easy to recognize because the operator must manually initiate the heat source when using LP (liquid propane) gas. The operator holds down a button and uses an igniter to light the LP gas flame. Once the flame is lit, the temperature sensor controls the process by making the flame hot enough to drive the cooling unit process. When refrigeration is no longer called for, the temperature sensor puts the burner into a low-flame mode that results in no refrigeration. Also, the manual control is a safety device that monitors the LP flame and turns off the LP gas to the burner if the flame is not lit.
The second type is the automatic control. This type of control is easy to recognize because the operator uses an on/off switch to turn on the refrigerator. The control takes over once the power is turned on. The automatic control is used on a majority of RV refrigerators due to convenience and the cost of manufacturing. Unlike the manual control, the automatic type of control requires a 12-volt power source. The following portion focuses on the automatic control.

Refrigerator Temperature Sensor
The refrigerator cabinet temperature sensor is generally a thermistor-type sensor. A thermistor-type sensor changes resistance based upon the physical temperature of the sensor. This type of sensor requires only two wires for proper operation. The refrigeration controller uses the resistance of the refrigerator temperature sensor to know when to turn on or off the cooling unit heat source. As a result, if the thermistor fails or is not located in the proper location within the refrigerator, the controller will not work properly. In some models, the thermistor is attached to one of the refrigerator compartment cooling fins. If the thermistor is not making good contact with the fin, it can lead to the refrigerator cooling too much and freezing the contents. If your refrigerator runs too cold, no matter what the setting is, check to see that the thermistor is attached to the fins. A failed thermistor can cause the refrigerator to run too cold. If your refrigerator has a refrigerated cabinet temperature readout on the control panel, then it is using the controller thermistor to determine the cabinet temperature. Using the refrigerator temperature readout is the simplest way to determine if the thermistor is working or not. If your refrigerator does not have a cabinet temperature readout, troubleshooting the thermistor requires a table of resistance values versus temperature. If you know the physical temperature of the thermistor, you can measure the thermistor ohms with an ohm meter and compare the reading to the table of values. The thermistor values are supposed to be in the refrigerator service manual. (My wife, Mao, and I post tables on the trouble-shooting section of our Website.)

b2ap3_thumbnail_MainController.pngThe Black Box
The main controller for the refrigerator is usually a black box mounted in the cooling unit compartment. Automatic-type controls require 12VDC power from your RV house battery to operate. There should be a fuse between the RV battery and the refrigerator controller. In addition, there is usually a fuse or two under the cover of the controller. If your controller is not working and you have tested for 12VDC power and ground to the controller, remove the controller cover and check the fuses on the circuit board.
Warning: Before working on any electrical device, disconnect all power sources, including the shore power, until it is safe to perform tests. Consult a qualified RV tech if you are in doubt.

There are a number of tasks that the main controller performs in addition to monitoring the temperature sensor. Similar to the manual control, the automatic control must perform the safety task of knowing if the LP flame is lit or not. If the controller calls for LP gas to turn on, it sends power to the electric gas valve and the igniter. If the flame does not light, the controller senses this situation and turns off the LP gas. This function is easy to test. Turn off your LP gas valve at the bottle and put the refrigerator into LP mode. You can hear an audible click from the gas valve followed by a snap, snap, snap from the igniter arc. After a number of tries to ignite the flame, the refrigerator should shut off the gas valve and alert the operator that there is a problem. (Reference “LP Gas Operation” in the November/December 2015 issue.)
In addition, the main controller has a sensing mechanism that determines if there is shore power available or not. When the control is in the automatic sensing mode, the logic of the controller is set to check for shore power first. If shore power is available, the refrigerator will turn on the electric heaters to power the cooling unit. If there is no shore power, the refrigerator will attempt to turn on the LP gas if cooling is needed. This automatic energy selector (AES) can usually be overridden by a switch setting on the refrigerator control panel that allows you to force the system into the LP gas mode. You can also simply unplug the refrigerator from shore power (at the receptacle in the cooling unit compartment) so that the AES cannot run the refrigerator from shore power.

Error Codes
Some refrigerator controllers can detect if there is a problem such as the cabinet temperature sensor is not working. The result is to store an error code that can be read out for diagnostic purposes. See your operator manual to determine if your refrigerator has error code capability or not. One classic error code is the NO CO error, which is fondly referred to as the NO COLD error and indicates that the cooling system is not operating. If your refrigerator has error code storage capabilities, it is a good idea to know how to access the error codes before you need them. Consult your user manual for more information. My wife, Mao, and I also discuss error codes in the troubleshooting section of our Website www.ARPrv.com.

Recalls and Safety
Around the mid-1980s, RVers wanted larger refrigerators. Due to a number of factors (some beyond the scope of this article), safety issues arose and the manufacturers initiated recalls. These recall campaigns were to minimize risk to life and property, but the recalls did not protect the refrigeration process itself. One of the initial campaigns recalled around 300,000 refrigerators. The fix was a thermal switch that sensed the temperature in the cooling unit compartment outside the boiler housing. Another method to improve safety was the introduction of the controller that produced the NO CO error code.

The NO CO error code is initiated if the main controller calls for cooling by turning on a heat source and refrigeration does not occur. If the temperature sensor inside the refrigerator does not sense a drop in temperature in around two hours, it may be concluded that there is some kind of problem. When the NO CO error is issued, the refrigerator shuts off and then the controller displays the NO CO error code. Both of the aforesaid methods were not found responsive enough to fully prevent failure of the refrigerator, which could lead to safety issues.
There is one last recall we will mention. This recall turns on a red LED light and turns off the refrigerator if the boiler has reached around 800 degrees Fahrenheit. This recall has had a number of complaints and issues that are not addressed in this article because of its extensive coverage on the World Wide Web. The greatest issue with all of the recalls is that they really do not control the refrigerator in the sense that they are not protecting the cooling unit, nor can the recalls restart the refrigerator even if possible to do so. This is one of the reasons that the ARP controller unit was developed.

As discussed in the previous installments to this series, the absorption refrigerator has a boiler that maintains a constant temperature during normal operation. Therefore, it is very easy to protect the refrigeration process and, in turn, the overall safety of the refrigerator. As a point of interest, the typical RV refrigerator boiler should never get much over 400 degrees Fahrenheit. When the boiler overheats, the cooling unit quits producing refrigerant, as described in the first article in this series. If the boiler overheats, irreversible damage occurs to the internal components of the refrigerator cooling unit. The ARP control limits the boiler temperature; thus it is a proactive safety device that automatically responds to conditions that can cause cooling unit failure leading to safety issues. Better yet, the ARP automatically restarts your refrigerator when the boiler cools.

In conclusion, the refrigerator controls are responsible for keeping the temperature in the refrigerator constant. This task is performed by using a temperature sensor that is generally of the thermistor type. The refrigeration control is also responsible for safety by monitoring the LP gas flame. Recent advances in control technologies further improve absorption refrigeration protection (ARP) and safety by monitoring the cooling unit boiler temperature. 

Reader Comments
We have had excellent comments regarding our RV fridge article series; we are glad we have stimulated thought. The initial installment of the series discussed the absorption refrigeration process. I did over-simplify the process and want to respond to some feedback.

Two Other Ingredients:
Reader: “It should be noted that, in addition to ammonia and distilled water in the refrigerator tubes, there are two other ingredients. One is a powder that coats the inside of the tubes to reduce corrosion.

”The “powder” is called sodium chromate, which is a corrosion inhibitor. Sodium chromate is highly soluble so it dissolves into the distilled water. If the sodium chromate is destroyed, the fridge will fail due to internal corrosion. Boiling the water in the cooling unit concentrates the sodium chromate, and, in turn, the sodium chromate becomes ineffective to prevent corrosion. The good news, the ARP control keeps the sodium chromate from being destroyed in this manner by preventing the water in the cooling unit from boiling. Simple solutions are the best.

Plugged Tubes:
Reader: “This powder can flake off and block up the small orifice at the top of the refrigerator and cause refrigerator failure.”

I often hear that absorption refrigerators have a “small orifice.” In fact, there is not an orifice in the RV-type absorption refrigerator. The absorption-type refrigerator does have two tubes that can be plugged by the failure of the sodium chromate or the resulting rust that “flakes off” in the cooling unit. One is the pump tube in the boiler and the other is the weak-solution tube. As mentioned above, as long as the water is prevented from boiling in the refrigerator, these tubes will not plug.

Hydrogen Gas:
Reader: “The final ingredient in the refrigerator tubes is flammable hydrogen gas. The evaporated ammonia passes through a small orifice at the top of the refrigerator into the hydrogen environment.”

All absorption-type refrigerators have what is called an assistant gas to facilitate the evaporation of the refrigerant (ammonia) based on the principle of Dalton’s Law of Partial Pressures. Hydrogen makes the best assistant gas because of its molecular structure. Although hydrogen is flammable, so is ammonia. Dalton would tell you that hydrogen is not the main fire issue because there is not much of it in the system compared to the volume of ammonia. The flammability of ammonia is evidenced by fertilizer plant fires, so ammonia is flammable and is more of a fire problem than the hydrogen. Over the years, helium has been used as an assistant gas, but these refrigerators have the same issues with destruction of the sodium chromate while still containing flammable ammonia.

Absorption refrigerators have been used safely for 150 years.  It is the control of the boiler process that is central to keeping the system reliable and safe.

Paul and Mao Unmack are mechanical engineers. Paul ran an automotive repair business in Red Lodge, Montana, for 20 years before receiving his engineering degree. He has practiced nuclear, fire suppression and industrial process control systems design. Mao designed pressure vessels for ammonia plants in China for 12 years, then came to the U.S. to get a master’s of welding engineering. She designed biodiesel plants and worked for a government-funded research and development organization. Paul and Mao run the entire ARP control business while taking on engineering consulting gigs. www.arprv.com 


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