Locating a Low Voltage Short in Residential A/C Systems

Newer technicians often get hung up and frustrated when searching for low voltage shorts. This is understandable due to the broad spectrum of possibilities for the location of the short. However, this doesn’t mean that the process needs to be complex. The time it takes to find a low voltage short may vary greatly depending on where the short is located, which components are failed, and how tedious the equipment is to access. Regardless of these variables, there are a few common processes that can make a technician’s life a bit easier when diagnosing a low voltage short. [Quick note: this is a guide for diagnosing a dead short in the low voltage circuit. In other words, the fuse immediately blows upon return of power to the appliance.]

The first step is ALWAYS a visual inspection. You can save a lot of time and frustration by simply using good observation skills. Look for rubouts, loose connections at switches and coils, discoloration, wire splices, splits in wire insulation, etc. These can all give a technician a great starting point to searching for a short of any kind. I’ve done many visual inspections and found other issues unrelated to the short that may have gone unnoticed without thorough observation. This is why good observation skills and a thorough visual inspection are great tools to use, no matter what you’re diagnosing.

The second recommended step would be to power down the appliance and install a resettable fuse. You can find this valuable tool for cheap at any supply house, or you could even make your own from an old transformer that utilized a resettable fuse. Using a resettable fuse prevents a technician from blowing through 20 fuses before the source of the problem is found. Be careful when selecting a resettable fuse product; some of them don't trip as quickly as the factory, and we have seen transformers and boards fail due to this. We suggest going to a 3A version rather than 5A when possible for additional protection. 

Step three: Rule out the transformer and thermostat. These components are rarely ever the issue, but the thermostat is also one of the first things newer techs will replace when panicked and trying to solve a low voltage problem. The first quick tests will help rule them out entirely. With your meter, check primary and secondary voltage against the rated voltage on the transformer. If the transformer secondary voltage is 24v, it is typical to see a range between 22-28v. If you measure higher or lower than the normal voltage from the transformer, it may be a good idea to disconnect the transformer from the circuit, ohm out the windings, and check for low resistance, which would result in higher amperage. 

Remove the thermostat from the wall and unwire all the wires except Common. Then, using either a pair of jumpers or a wire nut, connect R, G, Y, O, and W wires together. Now, re-energize the system. If the fuse pops, the thermostat is NOT the problem because it isn’t even in the circuit. If the fuse holds, that indicates that the equipment is running perfectly fine without the thermostat in place. THEN, you may start to suspect the thermostat. 

Next, remove the jumpers or wire nut and isolate R, G, Y, O, and W wires from each other. Reset the fuse and jump G, Y, O, and W to R one by one. Eventually, one of those combinations will pop the fuse, and it will be in that circuit the short is located. For example, let’s say the Y wire circuit pops the fuse when jumped to R.

At this point, you’ve isolated the problem circuit. You can begin testing everything related to that circuit. On a split system, the Y wire circuit will have the wire run from the thermostat to the indoor unit, from the indoor unit to the outdoor unit, from the outdoor unit to any defrost boards and switches, and from those components to the compressor contactor. The best way to determine what is in the circuit is to read a wiring diagram. Then, follow the wire to verify the schematic. At this step, a technician will repeat the visual inspection; this time, the tech will be more focused on a specific circuit.

Now, it’s time to test all circuit components (i.e., switches, relays, contactors, circuit boards, wire splices, etc.). Look for loose connections, burn markings, bare wire, rubout locations (like wire bending over sharp edges of the chassis), etc.

If your testing leads you to suspect the wiring itself, you may isolate the wire by completely disconnecting the low voltage wire from the outdoor unit. If the fuse still trips without any appliance connected to it (except the transformer power), then you can be certain the short is in the wire harness.

The final step in the process is to make all necessary repairs. Don’t forget to remove your resettable fuse and install a new appropriately sized fused for the appliance!

This process is one of MANY processes senior technicians have developed, and you may find yourself using your mentor’s methods instead, and that’s perfectly fine. As always, remember to diagnose the WHOLE system! You never know what else might be happening once the short is repaired and you can operate the system again.

For another take on a low voltage short diagnostic that comes with a little entertainment, here’s #BERTLIFE Ep. 4:




Mike Henderson
Mike Henderson
3/31/21 at 08:01 PM

This is great stuff, just wanted to add break out your meter before you install a pop it. Test circuits and isolate issue.
Used this today with a tech, hopefully tomorrow he shows me this page.


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