Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.

50K thermistor resistance vs temperature graph, blue curve dropping as temperature rises

Bubba Moore

Technical Services Manager Southeast District

March 10, 2022

Isolating the Problem in The Unit

There are many ways to diagnose problems with a unit. You can listen to the customer describe the operations of the unit, and you can observe the operations for yourself. Then comes the time when you have to get your tools and meters to start checking why some parts of the unit are working, or why nothing is working.


One way of checking out why a motor or compressor is not working is the Hopscotch Troubleshooting Method. Hopscotching is a way of testing for voltage readings across components in the circuit that is not working. You would place one meter lead on one side of the unit’s line voltage or control voltage, depending on what was not working. For example, let’s say the motor is not running: place one meter lead on one side L2 going to the motor, then you would take the other meter lead and put it on L1 going to the motor before any components that this wire would connect to. Let’s say your voltage is 230v at this point, as you move your meter lead that is on L1 to the next place (the other side of a contactor or relay switch) leaving the other meter lead on the L2 and not moving it. This is how you hopscotch through to the controls to the part that is not working. When the meter lead that is testing the L1 side does not read 230v, that is where the problem is.

Diagram of hopscotch troubleshooting for a relay circuit, showing voltmeter readings across switches and load.

There may be burnt out points on a contactor, and/or the relay switch didn’t close due to being stuck open. If you had 230v at the motor, you probably have a motor that failed, but the process of hopscotching confirmed that there were no problems with the controls before the motor. The same process can be used to check the control voltage that turns on the components.


This is just one way to isolate problems with components within the equipment. You can use the hopscotch method in troubleshooting resistive loads like crankcase heaters or heater elements, as well as inductive loads such as relay coils, solenoid coils, and transformers. It can also be used to check switch contacts as in low/high pressure switches, contactors or relays, etc. This method allows you to check the complete circuit from start to the end.

Circuit diagram showing open contactor troubleshooting with voltmeter readings and labeled wires/components.
Circuit diagram of a motor not running, showing 230V hopsotch troubleshooting with voltmeter readings.

Sources:


HVAC SERVICING PROCEDURES

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