It is of utmost importance for service technicians to understand voltage troubleshooting when servicing HVACR equipment. The majority of service problems are electrical problems, which usually cause mechanical problems.

This article will probe a bit deeper into voltage troubleshooting using a voltmeter.

The diagram (pictured at right) is of a 230-volt, single-phase electrical schematic of a typical commercial refrigeration system. The diagram includes a timer assembly with a defrost termination solenoid (DTS), evaporator fans, defrost heaters, temperature activated defrost termination/fan delay (DTFD) switch, low-pressure control (LPC), high-pressure control (HPC), compressor contactor assembly, and a compressor/potential relay assembly.

The system is drawn in the refrigeration mode. This simply shows what voltages would be measured across certain points of the schematic if a voltmeter were used in troubleshooting. The diagram also shows where Line No. 1 (LI) is in relation to Line No. 2 (L2) for ease of understanding the measured voltages.

Notice that anytime the voltmeter probes see both Line No. 1 and Line No. 2, 230 volts will be read on the voltmeter.

Anytime the voltmeter probes see the same line (L1 to L1 or L2 to L2), 0 volts will be read on the voltmeter because there is a voltage difference between the measured points. So, if the service technician can determine where L1 and L2 is when voltage troubleshooting, the rest comes easy.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at

Publication date: 12/05/2005