Bryan Orr talks about how HVAC technicians can isolate problems to diagnose faulty systems. He particularly focuses on using isolation diagnosis on a grounded compressor and in the low-voltage circuit. This presentation was given at the HVAC Conference in Birmingham, AL, presented by BTrained, LLC.
Isolation diagnosis requires us to make a hypothesis about the system and isolate our testing to the problematic part of the system. Instead of going in and testing every single wire, you have to develop a process of elimination to get to the bottom of the issue efficiently.
At Kalos, when our techs find grounded compressors, we make our techs pull the wires off, isolate the wires, put the top back on, and see if everything else runs when power is applied once again. (We call that the "redneck compressor test," and it's an alternative to bringing out your ohmmeter.) If everything else runs, then we can be confident that the compressor is grounded and proceed with further diagnosis. However, if nothing else runs, then we will know that we have a problem elsewhere.
In cases with grounded compressors, we would bring out the megohmmeter to check resistance at the compressor terminals after we confirm that the problem is indeed isolated to the compressor. The lacquer in compressor motors can wear down and cause issues with the motor windings.
In cases with time-delay issues on the low-voltage circuit, we do isolation diagnosis by checking for shorts in the Y or G circuits. We can rule out the red circuit and reversing valve immediately when we experience time delay issues.
Once you have all your tools and an idea as to how you'll tackle a problem, you can use isolation diagnosis to troubleshoot the system. In cases where we have voltage, we need to make sure that the voltage doesn't drop. We can use components to test the voltage drop; for example, we can see if 24v power will energize the contactor at various points (though things get tricky when the common gets involved).
Isolation diagnosis can help you solve all sorts of weird problems that your meters alone can't help with.
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