We all hate the grounded low-voltage wire that seems so hard to find.
To make this challenging diagnosis and repair easier, I take a step-by-step approach when dealing with a low-voltage issue in a residential unit.
1. The first step is to remove all low-voltage wires at the outdoor unit, thermostat, and indoor unit. Make sure that you label the wires that you remove to make it simple to reinstall them back in the correct place.
2. Next, separate the stat wires going to the thermostat and to the outdoor unit. Using a megaohm setting on your multimeter, determine if it’s the stat wire that’s causing the problem or if the problem is in the unit.
Start at the indoor section of the unit, touching each low-voltage wire with one lead of your meter and the other to an indoor section with good grounding. Be careful not to let your fingers touch the leads. Follow the wire that shows any grounding to determine where it’s grounded.
If the stat wire is long, you may want to cut it close to the middle. Cut it at a junction where it can be spliced back together to cut down the amount of wire necessary to trace and replace.
3. The next step is to test each wire in each bundle for possible grounding beneath the wire shielding. Then, jumper the low-voltage to bring on the indoor unit with test leads to see if the problem is in the indoor low-voltage circuits.
4. Finally, test all of the low-voltage circuits in the outdoor section, eliminating one low-voltage load at a time.
I always add a 3A or 5A automobile-style fuse to the low-voltage circuit if it doesn’t already have one. This will help prevent blown transformers and wasting low-voltage fuses during testing.
Try this approach and I think you’ll be amazed how much faster the job goes and how much more profitable you’ll be on the job.
Rob Mullis is a service technician at Love Heating & Air Conditioning, Indianapolis, IN. He can be reached at 317/353-2141, or by e-mail at firstname.lastname@example.org