AC Compressor Won’t Start: A Technician's Troubleshooting Guide

An air conditioner's compressor is the heart of the system, responsible for circulating refrigerant and enabling the cooling process. When a compressor fails to start, the entire system comes to a halt, leaving your customers with a warm and uncomfortable home. As an HVAC technician, being able to quickly and accurately diagnose a compressor that won’t start is a critical skill.

This guide will walk you through the most common reasons why an AC compressor may not start and provide you with a systematic approach to troubleshooting the issue.

Common Causes of Compressor Starting Issues

There are several potential culprits when a compressor fails to start. These can range from simple electrical issues to more serious mechanical failures. Here are some of the most common causes:

• Faulty Capacitor: A weak or failed run capacitor is one of the most common reasons for a compressor to not start. The capacitor provides the initial boost of energy needed to get the compressor motor spinning. [1]
• Bad Relay: The compressor relay is an electrical switch that controls the flow of power to the compressor. If the relay is bad, the compressor will not receive the power it needs to start.
• Overload Protection: The compressor has a built-in overload protector that will shut it down if it gets too hot. This can be caused by a variety of issues, including a dirty condenser coil, a low refrigerant charge, or a failing condenser fan motor.
• Low Voltage: The compressor needs a certain amount of voltage to start. If the voltage is too low, the compressor will not be able to overcome the initial resistance and will fail to start.
• Mechanical Failure: In some cases, the compressor itself may have a mechanical failure, such as seized bearings or a broken internal component. In these situations, the compressor will need to be replaced.

A Step-by-Step Troubleshooting Process

When faced with a compressor that won’t start, it’s important to follow a systematic troubleshooting process to avoid misdiagnosis and unnecessary repairs. Here are the steps to follow:

1. Check for Power: The first step is to verify that the condensing unit is receiving power. Check the circuit breaker and the disconnect switch to ensure that they are both in the “on” position.
2. Inspect the Capacitor: Use a multimeter to test the capacitor. If the capacitor is weak or failed, it will need to be replaced.
3. Test the Relay: With the power off, you can test the compressor relay for continuity. If the relay is open, it will need to be replaced.
4. Check the Voltage: Use a voltmeter to check the voltage at the compressor terminals. If the voltage is low, you will need to investigate the cause of the voltage drop.
5. Check for Overload: If the compressor is hot to the touch, it may be in an overload state. Allow the compressor to cool down and then try to start it again. If it starts, you will need to investigate the cause of the overload.
6. Ohm out the Compressor: If you have ruled out all of the above issues, the final step is to ohm out the compressor windings to check for an open or shorted winding. If the windings are open or shorted, the compressor will need to be replaced.

Recommended Parts and Tools

Having the right parts and tools on hand is essential for quickly and efficiently resolving compressor starting issues. At HVACProSales.com, we carry a wide selection of capacitors, relays, and other HVAC parts to help you get the job done right. We also offer a variety of diagnostic tools to help you accurately diagnose the problem.

References

[1] Compressor Doesn't Start Reliably