I see techs in the field get the most frustrated when they have a low-voltage short they can’t seem to locate. The challenge techs face with low voltage diagnosis is a combination of poor process and some root misunderstanding about what they are looking for.

First, let’s work to get a handle on what sort of issues exist and what they’re called.



The word “short” is an overused term that often means “something’s wrong electrically that I can’t explain.” Your cousin Randy might diagnose his blacked out TV as having a short, any range of mysterious electrical problems in vehicles are called a short, and the list goes on and on ...

The funny part is, even among those of us in the trade, we will often call a range of conditions a short, and defining it can be tricky. Here is a pretty good dictionary definition:

Short circuit — In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.

In the definition above, we would correctly diagnose something as a short when a path connects between two points of differential voltage, such as common and hot on a 24V transformer, without first traveling through a load that provides proper resistance to the movement of electrons. In this “no load” or low-resistance path, there is too little resistance. Therefore, the amperage gets too high, resulting in a blown fuse, tripped breaker, or something overheating and melting.

In the field, we also refer to cases where two energized conductors (wires) connect in an undesigned way as a short, even when it doesn’t result in a low-resistance path, a blown fuse, or tripped breaker. A common example of this is a case when two conductors in a cable have damaged insulation and make contact. If the “W” heating conductor makes contact with the “G” blower conductor, the heat will always run when the blower is turned on, and the blower will always be activated with the heat. We will often call this a short between W and G, meaning an undesired path or connection. In other words, something is connecting that shouldn’t be.

So, when we are seeking a short, we are looking for conductors or components that are connecting to something they shouldn’t be connected to, which often leads to decreased resistance and increased amperage, causing a fuse to blow. However, a short isn’t the only thing that can cause a blown fuse.



When you put too much load in the bed of your truck, it bottoms out. When you place too much load on an electrical circuit or device, it either has a protection device to shut it off, or it fails. We certainly see overload conditions in the case of motors, but occasionally, we can also see overload conditions in low-voltage cases. We see overload in low-voltage circuits when techs or installers simply put too many devices on the transformer. This includes add-on items, such as damper motors, UV lights, and photocatalytic oxidation (PCO) air purifiers. It’s easy to tell if a transformer is overloaded; simply divide the VA rating of the transformer by the secondary voltage. So a 40VA, 24V transformer can only carry 1.67 amps of load. In many cases, you will need to add another 24V transformer to carry the load of accessory components.

In 24V controls, electromagnetic coils are often used to open and close contacts and relays as well as activate valves. These controls generally have a relatively low amperage unless they become stuck in the deactivated position. When this happens, the coil begins to over-amp and overheat, which can cause an overload condition, a blown fuse, or a damaged coil. We see this on occasion with contactors that become stuck in the open position due to corrosion or objects becoming stuck in the contact points. We also see this when solenoid coils are not properly mounted or when a solenoid coil is energized, and the valve itself is stuck.



If you set a battery on the table, that is an open circuit. The battery contains stored energy that we call potential or voltage, but it doesn’t do anything because there is no path from the (+) to the (-) sides of the battery. When there is no path (or no path of sufficiently low resistance), we call it an “open” or an open circuit.

In HVAC controls, I often see techs looking for a short when the issue is actually an open. If you have the thermostat set to cooling, the time delay elapses, the compressor contactor doesn’t pull in, and no fuses blow, you should be looking for an open, not a short.

Open circuits are usually found by using a voltmeter, keeping one meter lead on a common point and “walking” the other lead through the circuit until voltage is lost. Alternatively, a tech may choose to de-energize the circuit and use an ohmmeter to check for a path between the two points as is often done to test a fuse.



The first step in finding low-voltage issues and repairing them quickly is to do a visual inspection. Notice the type of equipment, understand the sequence of operation, look for any aftermarket parts or recent repairs, and visually inspect for signs of trouble.

Low-voltage issues can often be found and repaired by simply looking for the obvious: poor connections, damaged cables from animals or lawn equipment, and rub-outs inside the equipment or where wires run into the cabinet. It is also a good idea to remove the thermostat or control from the wall and make sure it is wired properly with no exposed portions of the conductor.

If you find anything that is clearly an issue, it’s best to stop, quote, and repair. Let the customer know you will need to test for any other issues after the repair is made. Once all the obvious short circuits, overload situations, and open circuits are eliminated, the real diagnosis starts.

For the sake of keeping it short (pun intended), we’ll keep to typical 24V residential and light commercial controls.

If the fuse is blown or the low-voltage breaker is tripped, as will generally be the case in a short, first replace the fuse, or, better yet, use a test breaker, like the Supco Pro Fuse (in place of the fuse while testing) to eliminate wasting fuses. Turn off the service disconnect, go to the thermostat and turn all modes to off, place an ammeter on the 24V power line coming out of the transformer secondary, and then turn on the high-voltage power to the system. Next, observe at what point in the sequence of operation the amperage spikes and/or the fuse blows. If it blows right away, you will focus your attention on the R circuit (constant 24V). If it doesn’t, turn on the blower only by switching the fan to “on.” If it still doesn’t spike or blow, you move to heat, then cool, etc.

By watching the meter and observing the point at which the fuse blows or the low-voltage breaker trips, you can be certain which circuits are the culprit for the short. Keep in mind that some circuits may be connected to multiple components and safeties, so simply finding the circuit isn’t enough. You must now isolate the exact location.

This is the way I find short circuits and have done it this way for years out in the field. It is a practical process, but you need to adapt it for the specifics of the application. It would be unsafe to do it this way with controls over 24V, and it would be ineffective for communicating on 4-20 milliamp controls.

Disconnect the suspected circuits from one another at the air handler or furnace in a split system or from the main board on a package unit at the field connections, leaving the common wiring connected.

Next, take each suspected wire and tap it to constant 24V coming from the transformer at the wire-nut connection or the R terminal on the board. You will often see a large arc when you find the shorted or overamping circuit, and you can locate the source of the short without blowing a fuse at all.

Once you find the circuit (wire) with the amperage spike, you will know if it is a conductor shorted going back to the thermostat or if it is between the air handler/furnace and the outdoor components.

I then continue isolating components in that circuit by disconnecting them until the short circuit disappears. For example, let’s say we have a heat pump system, and the fuse didn’t blow with the thermostat set to “off,” and the blower runs fine in the “on” position. I then disconnect the orange wires in the air handler and find the arc is coming from the condenser side rather than the air handler side. Afterwards, I go outside and disconnect orange at the condenser, and the short disappears. I now know it is either the defrost board, wiring to the reversing valve, or the valve solenoid.

After doing a thorough inspection for visible signs, I would isolate each, starting by disconnecting the solenoid and working my way back. After disconnecting the solenoid and the wire between the board and the solenoid, if the short still exists in the orange circuit with the defrost board connected, then we have isolated the defrost board as the cause of the short (very rare, by the way).

Some will point out they prefer to do these tests with an ohmmeter instead of using the system voltage by checking from isolated conductors and components to ground and common. I agree that this method often works, but it won’t find overload issues and intermittent shorts as readily as when using a combination of the actual 24V and an ammeter.

Before condemning an expensive part or rerunning a cable, use an ohmmeter to confirm a short across the device to ground and to common. Whenever possible, find visual evidence to confirm the diagnosis, document it, and tag and save the part should a question arise in the future.

In summary, my preferred process for low-voltage diagnosis is:

1. Visual inspection;

2. Test using sequence of operation;

3. Use system voltage and ammeter to locate problems when possible; and

4. Double check your diagnosis visually and with an ohmmeter when possible.

A good tech understands all of the diagnostic tools at their disposal and will reconfirm before making a repair.

Publication date: 3/19/2018

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