Needham’s Troubleshooting Controls

Troubleshooting Basic Controls

Matthew Needham explains how to diagnose common control-system problems by reading schematic diagrams, checking voltage, and eliminating components that are working. The examples cover a simple switch-and-light circuit and a residential-style heating and cooling system.

Core Troubleshooting Principles

• Troubleshooting means finding the cause of a problem, not simply replacing parts. Use the system’s symptoms and electrical measurements to narrow down likely faults.
• Voltage is a difference in electrical potential between two points. An open switch does not automatically mean there will be no voltage reading; the reading depends on whether there is a complete reference path between the meter’s test points.
• In the simple circuit, an open switch or thermostat can leave voltage present at some points while other point-to-point measurements read zero. A disconnected or burned-out light bulb can still have voltage across its terminals.
• In a 115/230-volt example, each line measures about 115 volts to ground, while the voltage across the two lines is about 230 volts. The lecturer uses a two-car collision analogy to illustrate how the two line-to-ground potentials add across the circuit.
• Electrical safety matters: A person can become a path to ground and receive a shock. Voltage readings should not be confused with whether a point is safe to touch.

Reading the Heating and Cooling Schematic

The larger schematic uses colors to distinguish functions:

• Blue: Cooling controls and components
• Red: Heating controls and components
• Green: Indoor-fan controls and components

The lecturer identifies motors, heater elements, relays, contactor components, switches, a transformer, and a thermostat. In the diagram, the transformer steps 230 volts down to 24 volts for the control circuit. Loads are generally drawn on the right side of the schematic and switches on the left.

A Systematic Diagnostic Approach

1. Test the indoor fan first. Set the thermostat to off and switch the fan to on.

   • If the fan runs, that helps rule out several shared components in this example, including the breaker, disconnect, main fuses, transformer, indoor-fan relay, fan motor, and its capacitor.
   • This is an initial elimination test, not proof that every part of the system is working.

2. Follow the control sequence. Check whether the thermostat is sending 24 volts to the relevant relay or contactor coil, and whether that coil closes its contacts to supply power to the load.

3. Use the symptom to focus the search. If one part of the system works while another does not, look for components specific to the failed function. If nothing works, start with components shared by the whole system.

4. Confirm a suspected fault before replacing parts. For example, if a relay coil is energized but its contacts remain open, the relay may be faulty. The lecturer recommends checking the coil rather than replacing the part without verification.

Fan Operation and Motor Basics

• The indoor-fan relay closes when the fan is set to on, allowing power to reach the indoor-fan motor.
• The motors shown are PSC (permanent split capacitor) motors. Each uses a run capacitor that remains in the circuit while the motor operates.
• If the fan does not run despite power reaching the relay coil, a stuck-open relay is one possible cause. The motor, capacitor, and wiring should also be considered and checked.

Cooling Operation and Likely Faults

• When the thermostat calls for cooling, it energizes the contactor coil. The contactor closes and supplies power to the compressor and condenser fan.
• The indoor-fan relay is also energized during cooling when the thermostat’s fan setting is on auto.
• If the compressor and condenser fan operate but the indoor fan does not, the indoor-fan relay, motor, or capacitor may be at fault. Poor indoor airflow can lead to evaporator icing and potentially allow liquid refrigerant to reach the compressor.
• If the indoor fan runs but neither the compressor nor condenser fan runs, the contactor is a likely shared suspect. The coil or contacts may have failed.
• The lecturer gives a typical air-conditioning temperature drop of roughly 20°F, with an approximate range of 18–24°F depending on humidity and conditions.

Heating Operation and Likely Faults

• When the thermostat calls for heat, it energizes the heating relay. The relay switches power to the electric heater and also brings on the indoor fan.
• A limit switch is positioned to interrupt heating if the supply-air area becomes too hot. The lecturer says the opening temperature is commonly well below the maximum figure allowed by code.
• If there is no heat but the indoor fan works, possible causes include a failed heater element, an open limit switch, a blown fuse, or a problem with the heating-relay contacts.
• If the entire system is inactive, check shared power and control components first: a tripped breaker, an open disconnect, a failed main fuse, a bad transformer, or a faulty thermostat.

Causes of Overheating and Icing

A heater can overheat and trip its limit switch if insufficient air is moving across it. Possible causes include:

• A dirty or blocked air filter
• Closed supply registers or a blocked return
• A failing indoor-fan motor, capacitor, relay, or contact
• Other restrictions in the airflow path

An evaporator coil can ice up because of electrical problems that stop the indoor fan while cooling continues, or because of non-electrical airflow and operating issues. Examples include a dirty filter or evaporator and a thermostat set excessively low for an extended period.

Speakers and Sources Featured

• Speaker: Matthew Needham, lecturer
• Source referenced: Refrigeration and Air Conditioning Technology, Unit 15, mentioned as a textbook diagram that the lecturer adapted
• Other references mentioned: The Uniform Mechanical Code, in connection with limit-switch temperature, and California regulations concerning high-efficiency gas furnaces, mentioned as background context