May 22, 2025
In this episode, Bryan and Bert dive into the often misunderstood world of compressor testing, focusing on common mistakes technicians make when using ohmmeters, compressor testers, and megohm meters. The conversation highlights how diagnostic errors can lead to unnecessarily condemned compressors and wasted time and resources.
The hosts begin by discussing fundamental concepts like the difference between "open" and "short" conditions, emphasizing that a path doesn't necessarily indicate a short circuit—it simply means there's a low-resistance path. They explore how many technicians get confused when testing terminals, mistakenly thinking that normal winding-to-winding measurements indicate a problem. Through practical examples from their field experience, they illustrate why understanding normal readings is crucial before taking any measurement.
A significant portion of the discussion centers on proper testing techniques for scroll compressors, which often show different measurements to ground compared to other compressor types due to their design, with windings submerged in refrigerant and oil. Bryan and Bert explain why standard motor testers can lead to false diagnoses when they indicate "bad" at threshold levels like 20 megohms—a reading that would be normal for many functional scroll compressors. They recommend proper megohm meters calibrated to appropriate voltage levels for accurate testing.
Key Topics Covered:
- The difference between "open" circuits and "shorts" and how to properly interpret ohm readings
- Common mistakes when testing compressor terminals, especially misinterpreting normal winding-to-winding measurements
- How to properly check for shorts to ground and the importance of clean connections
- Why scroll compressors naturally show different resistance-to-ground readings due to their design with submerged windings
- Problems with inexpensive compressor testers that give false "bad" readings on functional equipment
- The "redneck test" for isolating a compressor to determine if it's truly the source of breaker trips
- The importance of understanding meter auto-ranging and correctly interpreting readings in ohms, kilohms, and megohms
- Why a reading below 0.5 megohms to ground is the general threshold for concern with scroll compressors
- Differences between megohm meters, high pot testers, and standard multimeters
- The misconception that winding-to-winding shorts are common without corresponding shorts to ground
Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.
Purchase your tickets or learn more about the 6th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.
Subscribe to our podcast on your iPhone or Android.
Subscribe to our YouTube channel.
Check out our handy calculators here or on the HVAC School Mobile App for Apple and Android