FIG. 1: An example of a microprocessor.
Service technicians in the HVACR field will sooner or later encounter the microprocessor in their daily service work. Equipment manufacturers are using microprocessors more and more to make their products more reliable, less complicated, easier to troubleshoot, and more self-diagnostic, as well as to cut down on the amount of hard wiring that goes into the control circuit.

In this article, we will discuss a microprocessor (Figure 1) as part of the control circuit (Figure 2) of a medium-size commercial ice machine. Notice that the microprocessor has taken the place of a lot of the hard wiring that used to be inside a machine of this nature.

This type of technology is often referred to as "clean technology." Now, most modem control circuits consist of a microprocessor, a few starting components like a start and run capacitor, potential relay, compressor contactor, and the main loads or power-consuming devices.

In this case, the main loads include the compressor, harvest motors, hot gas and purge solenoids, and water pump. Most technicians are not afraid of the main loads or power-consuming devices when troubleshooting. It is the microprocessor that is the most feared. However, the microprocessor can actually be the simplest component to troubleshoot with a little patience, understanding, and practice.

The microprocessor is actually a small computer that has a sequence of events stored into its memory. It has many sophisticated solid-state devices in its internal circuitry that are needed for its proper operation.

However, a technician does not need to know how to troubleshoot or understand how each solid-state device operates in order to tell if the microprocessor is good or bad.

What the technician does have to know is the microprocessor's sequence of events, its self-diagnostic functions, and how to input/output (I/O) troubleshoot the microprocessor using its external terminals. The microprocessor's external terminals are where most of the wires are coming in and out of the microprocessor. These wires are the "inputs" and "outputs" of the microprocessor.

FIG. 2: The microprocessor as part of a control circuit.


The sequence of events of a microprocessor is usually found in the service manual. Often, a service manual cannot be found on site with the piece of refrigeration equipment. In this case, the owner or manager of the establishment where the refrigeration machinery is located must be contacted to see if the service manual is filed in some safe location.

If the service and operations manual still cannot be located, the company who manufactured the equipment must be contacted. Usually a company employee needs only the model and serial number of the machine to pinpoint the manual.

All the technician needs is the pages of the service and operations manual that includes the sequence of events of the microprocessor and how to initiate the self-diagnostics of the machine if it has any. Simply ask the company employee to fax the pages from the manual that include the pertinent information for servicing and troubleshooting.

A service manual will include the step-by-step sequence of events and the self-diagnostics of the microprocessor, respectively. Now, at least the service technician has the knowledge of what the machine is supposed to do at a certain time or temperature.

FIG. 3: Thermistor.


Input/output troubleshooting a microprocessor is actually an easy method if some common sense is used. All that is needed is a voltmeter and knowledge of the sequence of events from the service manual.

An ohmmeter should not be used directly on a microprocessor because of the ohmmeter's battery voltage. Many times this voltage is too high and may damage the intricate solid-state components and/or the magnetic memory internal to the microprocessor. However, if certain components (inputs and/or outputs) to the microprocessor are detached from the microprocessor, an ohmmeter can then be safely used.

Microprocessors are fed with information from their input devices to their input terminals. Input devices can be analog as with a thermistor or variable resistor (Figure 3), or digital (on/off) as with a switch. After processing the data from the inputs, an output signal is sent. The output signal can be read with a meter from the output terminals of the microprocessor.

Both the input and output terminals of the microprocessor are labeled for the technician to troubleshoot. It is from these input and output terminals that most troubleshooting can be accomplished using a meter. Most of the time the input signals are low voltage (AC or DC) or resistance signals and the output signals are of higher voltage (usually AC) going to the power-consuming devices - but not always. Always refer to the service manual for specifics.

Many times, input devices are for nothing but the digital read-out for a condensing temperature, evaporating temperature, or a sequence mode.

FIG. 4: Tape can be used to mount voltmeter probes as a way to prevent electrical shorts when measuring inputs or outputs from a microprocessor.


Let's say a technician is servicing an icemaker and has determined that the machine will not come out of a hot gas defrost mode as quick as it should. Too long of a defrost will cook off the metallic coating on the evaporator. Instead of the harvest motor's probe pushing the ice off the coil that will terminate defrost with a curtain switch, the evaporator gets very hot from the prolonged hot gas defrost.

In other words, a cookout occurs. The ice slowly falls off the evaporator in chunks by gravity. Sometimes, one of these chunks will trip the curtain switch and bring the machine out of defrost, and other times it will not.

The technician then gets out a voltmeter and measures the output terminals of the microprocessor labeled motor 1 and motor 2 while in defrost. The technician measures 115 VAC at both terminals and determines that this is the correct voltage. This process told the technician that the microprocessor was doing its job and sending an output signal to the harvest motor.

The microprocessor is not to blame. The problem must then lie in the harvest motor or the wires leading to the harvest motor. The technician then checks voltage at the harvest motor itself and gets 115 VAC. This check eliminated the problem being in the wiring between the microprocessor and the harvest motor.

The problem then must lie in the harvest motor itself. The technician knows that if the correct voltage is going to a power-consuming device, and the power-consuming device is not operating, the problem lies within the power-consuming device. The technician then retrieves a new motor from the service van and installs it.

The new motor works perfect and the ice machine is watched for two complete cycles. Everything operates as it should.

It is important that the technician use as short of a measuring probe as possible to avoid shorts to other terminals of the microprocessor. A direct short could ruin any microprocessor. It is suggested to you to tape your voltmeter probes halfway up as a way to prevent electrical shorts when measuring inputs and/or outputs from a microprocessor (Figure 4).


Now let's say a technician servicing an icemaker that has a microprocessor observes that the ice machine will not come out of "ICE 1" mode. The technician then consults the service manual and finds out that the evaporator must reach 14°F before an internal timer will start. The technician wonders if it is the microprocessor's fault, or is it in one of the inputs to the microprocessor.

The technician then reads the manual further and discovers that the microprocessor is fed an input signal from a thermistor connected to the evaporator. A thermistor is nothing but a resistor that varies its resistance as the temperature changes. This thermistor is what signals the digital Light Emitting Diode (LED) display what mode the icemaker is in. It is this thermistor that also will tell the microprocessor when the evaporator has reached 14°.

The technician reads further and finds out that the manufacturer has said to bathe the thermistor in a solution of ice and water at 32°. Its resistance should be 10.0 Kohms at 32°. This is accomplished by placing some crushed ice in a small amount of water and stirring.

The thermistor lug is then taken off of the evaporator and is also electrically disconnected from the microprocessor's evaporator input terminal. The thermistor lug is given a chance to stabilize its temperature at 32° in the ice water bath.

A resistance reading is then taken with an ohmmeter. The reading is 5.50 thousand ohms or 5.50 Kohms. This indicates a defective thermistor and explains why the microprocessor is not going out of ICE 1 mode. A new thermistor is ordered from the manufacturer.


Many microprocessors come with a self-diagnostic or component test mode built in. These tests usually allow the technician to distinguish between a defective microprocessor, a defective power consuming device, or circuitry between the two. Again, these tests involve troubleshooting the microprocessor's output terminals.

Many times error codes can be read directly off the LED display. This is a valuable troubleshooting and timesaving tool. However, usually regularly occurring problems - but not all problems - can be diagnosed with error codes. A word of advice: Always check to see if the fuse on the microprocessor is good before condemning the processor. Most microprocessors have these fuses.

As one can see, the microprocessor is not as complicated as it may seem. All the technician really has to know is input/output troubleshooting, understanding the sequence of events, and how to find and follow an instruction manual.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached at

Publication date: 03/06/2006