Service technicians must be able to troubleshoot microprocessor-controlled heating and cooling systems.
Electronic controls are packaged in closed modules or on printed circuit boards where the components are visible.
Microprocessor-based controls are like small computers. When provided with the correct input, the control produces a predictable output.
A typical input signal might be a call for cooling from the room thermostat. The output from the control would be a 24-volt signal to energize the compressor contactor.
The presence of the correct input and output signals can be confirmed with a voltmeter.
Microprocessor controls are more reliable, more efficient, and easier to troubleshoot than older electromechanical controls.
Conventional control circuits show all the components and are easy to trace. In an electronic control, much of the circuitry remains invisible and unknown.
The unknown and invisible circuits in the electronic control are often shown on the schematic as a black box.
Understanding the operating sequence of the system containing the electronic control is essential before attempting to troubleshoot.
The microprocessor-controlled equipment will often contain operating sequences and/or features not seen on equipment with conventional controls.
It is safe to assume that a voltage signal will be applied to the control as an input. The output is usually a voltage signal to energize a component such as a gas valve or relay.
Electronic controls are very reliable, with most failures coming as the result of outside influences.
The control will often contain diagnostic capabilities that can identify problem areas by displaying fault messages. The control may even signal if it has failed.
Control circuit voltage and other input signals can be checked at test points specified on the equipment’s wiring diagram.
When a microprocessor control fails, it should be determined if some external factor contributed to or caused the failure.
Exposing an electronic control to a harsh environment such as excessive dust, extreme dampness, or corrosive fumes can damage or destroy the control.
The National Electrical Code (NEC) defines various damaging environments and outlines procedures to be followed when installing equipment in those places.
Improperly installed equipment can be subjected to vibrations that will cause electronic controls to fail.
Problems with the equipment power supply such as low voltage, voltage spikes and surges, and harmonics can cause an electronic control to fail.
Electrostatic discharge such as that experienced when walking across a carpeted floor and touching a doorknob, can damage or destroy electronic controls.
A simple way to prevent electrostatic discharge damage is to always ground yourself to a grounded metal surface before touching any electronic control or component.
Electrostatic discharge damage to spare parts and other components can be prevented by storing them in protective isolation containers.
A thermistor is a device used with electronic controls that provides temperature input signals. It does this by varying its resistance according to temperature.
Some thermistors increase their resistance with an increase in temperature while other thermistors decrease their resistance with an increase in temperature.
The accuracy of the thermistor can be checked by measuring the resistance and comparing it to the resistance on a table or graph provided by the equipment manufacturer.
Microprocessor controls are commonly used in packaged gas heating/electric cooling units.
The electronic control board is prominently featured in the wiring schematic.
Packaged electronic controls are very effective for controlling heat pumps because the logic in the defrost circuits is more complex than the circuits needed to control straight cooling units.
Economizers use microprocessor controls to determine if the temperature and humidity condition of the outside air make it suitable for cooling the structure.
Depending on the condition of the outdoor air, the control will open dampers in the duct system and allow free cooling or close the dampers and turn on the compressor.
The monitoring of system conditions and the initiation and termination of defrost is easily accomplished by microprocessor-based heat pump controls.
No special test instruments are needed when working on units with microprocessor controls. A VOM can be used to check input and output voltages.
The ohmmeter function of the VOM can be used to check the resistance or continuity of other system components.
Many manufacturers provide equipment-specific diagnostic devices that can be plugged into the system to help identify and isolate problems.
To cut costs, manufacturers are moving toward standardization of control boards. For example, all induced-draft furnaces might use a common control.
While the control board might be common, the manufacturer might program different operating sequences for different versions of the same basic product.