Electrical Troubleshooting Using A Voltmeter

December 1, 2005
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