My AC Guy Says My Compressor Is Locked Up
Your AC guy says:
“Your compressor is locked up.”
Maybe.
A compressor absolutely can become mechanically locked and fail to start.
But before we condemn one of the most expensive parts of your AC:
Let’s make sure it isn’t simply failing to start for another reason.
What Does “Locked Up” Mean?
Inside the compressor is an electric motor driving the mechanical compression components.
When a compressor is truly locked, the motor is energized but the mechanical assembly cannot start rotating.
The compressor may draw very high current and then trip its internal overload or a breaker.
That’s a legitimate failure.
But:
Compressor isn’t running
doesn’t automatically equal:
Compressor is mechanically locked.
What Else Can Keep a Compressor From Starting?
Before condemning it, we’d want to check things like:
- capacitor
- contactor
- supply voltage
- voltage while attempting to start
- wiring and connections
- compressor windings
- overload condition
- starting components, when applicable
A failed capacitor can leave a compressor sitting there humming and unable to start.
That can look pretty dramatic.
It doesn’t automatically mean the compressor itself is dead.
What Does LRA Have to Do With It?
You’ll often see LRA — Locked Rotor Amps on the equipment data plate.
If the compressor attempts to start but can’t rotate, amperage may climb toward locked-rotor current.
Then the overload or another protective device may open.
That’s important evidence.
But we still want to determine why it isn’t starting.
If proper voltage is reaching the compressor, the capacitor and starting circuit are good, the windings test appropriately, and the compressor repeatedly pulls locked-rotor current without starting?
Now:
“Locked compressor”
is looking considerably more convincing.
What About a Hard-Start Kit?
Sometimes additional starting torque can help a compressor that’s struggling to start in an appropriate application.
But a hard-start kit isn’t a resurrection device.
If the compressor is mechanically seized, we’re not going to keep adding increasingly enthusiastic capacitors until it changes its mind.
At some point:
It’s locked.
Can It Be Repaired?
Not internally in your driveway.
A residential hermetic compressor is sealed.
If it’s truly mechanically locked, we’re generally looking at:
compressor replacement
or
system replacement.
Which makes sense depends on the equipment’s age, warranty, condition and repair cost.
What Should You Ask?
Ask:
“How did you determine the compressor itself is locked?”
A good answer should involve more than:
“It won’t start.”
We want to know that the electrical components, voltage and starting circuit were checked before the compressor got sentenced.
The Klein Answer
Yes, compressors really do lock up.
And when one is genuinely mechanically seized, that’s a major failure.
But:
Won’t start ≠ locked compressor.
First make sure the compressor is actually being given everything it needs to start.
If it is…
and it sits there pulling locked-rotor current anyway…
Yep. Your AC guy may be completely right.
Sometimes the compressor isn’t waiting for another capacitor.
Sometimes it’s just done.