Basic Electrical Troubleshooting or “How To Use Meters” (part 2)

Before we can start to use meters, we need to understand the basic electrical circuits in a piece of equipment.  How I was taught about electricity is pretty simplistic but it made me really understand circuit diagnostics and that was — in every circuit you have a power source , you have a switch(es),  and you have light bulbs.  It does not matter what you are working on, each circuit has at least these 3 items.


In today’s heating and air conditioning equipment, there can be multiple power sources, switches, and light bulbs. Some power sources can be light bulbs — a transformer is a power source for one part of the circuit and a light bulb for another part of the circuit.  Power terminates at one side of the transformer making it the light bulb.  But, now this light bulb becomes a power source for another part of the circuit.


Every electric circuit has a power source–24  volts, 110 volts, 220 volts, 46o volts.  Most control system have multiple power sources — 110 volts to 24 volts, 220 to 110 or 24 volts, and so on.

Light bulbs are basically anything that “consume” power — Motors, Gas Valves, Contactor coils, Transformers, etc. They are an end point.

When we look at switches,  switches are anything in a circuit that can open or close, either making or breaking the power source  or varying the power to the light bulb.  They can be a simple temperature Klixon switch. They can be a contact closure in a relay. They can be part of a circuit board.  They are there to act as safeties, on/off controls, sequencers, etc. Even a temperature sensor can be considered a “switch” (think of it as a “dimmer switch” since it can vary voltage to the light bulb).  To put it simply:  Switches control the power source.  Control Boards are nothing more than multiple light bulbs and switches — the on-board relays = light bulbs and the relays = on/off switches.  All they are there for is to control what happens between the power source and the light bulb. We do not need to worry about the “electronics” in the board since we are not rebuilding boards.  We need to see if the switches are working when we have the power source present. If the power source is present and the switches are working but the “light bulb” is not activating, then we need to diagnose the light bulb.

There are different types of circuits, but the 3 most common on today’s HVAC equipment are;

Series circuits in which the switches  are connected one to the next. This is common in a safety string

Parallel circuits in which the power can have more than one path:

  • •Parallel Circuits – More Than One Path
    • .Each Path Has Same Voltage As Applied Voltage

And then there is the combination of both the above, the Series-parallel circuit

Understanding how circuits are constructed will help in making proper diagnosis of the system.  This can then be applied to reading the wiring diagram, which is the road map for the system you may be working on.

Where some technicians get lost when doing electrical diagnostics is the start in the middle of a circuit at a “switch”. WRONG! When performing electrical checks, you  need to start at either the power source and work forward or at the light bulb and work back.  When you start in the middle, I can guarantee you are going to miss something.

No matter how complicated a circuit may be, no matter how many controls you may see in that circuit, if you just remember that everything you are looking at is either a power source, a switch, or a light bulb, and track it through with that in mind, starting at one END or the OTHER, electrical diagnostics will become easy.

Now that you have an understanding of how a control circuit is constructed, we need to understand how electricity “flows” through these circuits and what you meter is “looking for”.  In my next posting, I will try to explain electricity since this is necessary to understand so we use meters correctly.

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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3 Responses to Basic Electrical Troubleshooting or “How To Use Meters” (part 2)

  1. joe d martinez says:

    Thank for teaching fisherman how fish.god bless you.

  2. wayneshirley says:

    Off the wall question…didn’t York, at some point in history, have a heat pump low voltage design scheme, where “R” was grounded and “C” was fused? If I’m remembering correctly, do you have a schematic for that?


    • Yes. Both a heat pump and Rtu. Heat pump was an E1CP and Rtu was D1SS. Reason for that was it also met Canadian standards of the time.
      You can contact your local distributor possibly for wiring diagrams.
      Those units were all built before 1992 — about 23 years or more ago.

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