Checking Compressor Valves and Mechanical Compressor Diagnostics
A compressor can run electrically and still fail to pump refrigerant correctly. Mechanical compressor diagnostics determine whether the compressor can create and maintain the pressure difference required by the refrigeration system.
This lesson focuses specifically on the mechanical side of compressor diagnosis: refrigerant pumping ability, suction and discharge valve sealing, service-valve operation, and pressure response.
Mechanical Diagnostics Only
This lesson does not cover electrical compressor testing.
Motor windings, resistance measurements, grounded windings, capacitors, contactors, overloads, voltage, current draw, and other electrical tests are covered in the separate electrical course sequence.
What You Will Learn
By the end of this lesson you should be able to:
Recognize that the test determines whether the compressor can create and maintain a pressure difference.
Connect gauges, position service valves, and prepare the compressor according to the source procedure.
Determine whether the compressor can pull the low side into the vacuum specified in the lesson.
Determine whether low-side pressure remains in vacuum after the compressor stops.
Explain why the discharge service valve must not be front-seated while the compressor is operating.
Recognize when a mechanical pumping test is appropriate and when electrical testing belongs in the separate electrical course.
What Are We Checking?
A reciprocating compressor depends on internal suction and discharge valves to control refrigerant flow through the cylinders.
During normal operation, low-pressure vapor must enter through the suction valves, be compressed by the pistons, and leave through the discharge valves as high-pressure vapor.
Suction Side
The compressor must be able to reduce low-side pressure sufficiently as it removes refrigerant vapor from the suction side.
Discharge Side
The discharge valves must prevent high-pressure refrigerant from leaking backward through the compressor after compression.
Mechanical Question
Can the compressor pump, and can its internal valves hold the pressure difference it creates?
Mechanical Failure Is Not the Same as Electrical Failure
A compressor may have a mechanical problem even when the motor starts and continues to run.
Likewise, a compressor that does not run cannot be evaluated mechanically until the reason it does not run has been determined.
This Lesson Covers
- Refrigerant pumping ability
- Suction valve condition
- Discharge valve condition
- Pressure response
- Vacuum produced by the compressor
- Mechanical service-valve procedure
Electrical Course Covers
- Motor winding condition
- Grounded or open windings
- Supply voltage
- Current draw
- Capacitors
- Contactors and relays
- Overloads and electrical controls
Service Valve Positions Matter
The source procedure requires the technician to change the position of the compressor service valves during the test.
Understanding the terms back-seated and front-seated is therefore important before performing the procedure.
Back-Seated
The valve is positioned for normal system operation while isolating or limiting access through the service port as designed.
Front-Seated
The valve is turned toward its front seat to close the refrigerant path through that service valve.
Critical Safety Rule
Never front-seat the discharge service valve while the compressor is running.
This warning is specifically included in the source compressor-valve procedure. :contentReference[oaicite:1]{index=1}
Before Beginning the Mechanical Test
The source procedure begins with connecting refrigeration gauges to the unit and preparing the service valves.
Install the Gauge Set
Connect the refrigeration gauges to the appropriate service connections.
Purge Air as Required
The source instructs the technician to bleed air from the gauge connections at the service valves before beginning the test.
Prepare the Low-Pressure Control
The source procedure instructs the technician to jump the low-pressure control so the compressor can continue running during the pumping test.
Start the Compressor
Operate the compressor so its pumping ability can be observed through the gauge readings.
These initial steps are taken directly from the source procedure. :contentReference[oaicite:2]{index=2}
Establish the Initial Pressure Readings
After the compressor is running, the source instructs the technician to adjust the low-side service valve and read low-side pressure.
The high-side service valve is then adjusted and the high-side pressure is also observed. :contentReference[oaicite:3]{index=3}
Low-Side Reading
Shows the pressure at the compressor suction side before the pumping test is performed.
High-Side Reading
Shows the pressure being produced on the compressor discharge side.
Checking Suction-Side Pumping Ability
The first mechanical test determines whether the compressor can pull the low side into a sufficiently deep vacuum.
Compressor Running
Operate the compressor with the gauges connected and service valves prepared according to the procedure.
Front-Seat the Low-Side Service Valve
The source procedure closes the low-side path so the compressor pumps the suction side down.
Observe Low-Side Pressure
Watch how rapidly the compressor pulls the suction pressure into a vacuum.
The source specifies that if the compressor reaches approximately 20 inches Hg vacuum within two minutes, the low-side valves are considered good. :contentReference[oaicite:4]{index=4}
Source Test Result: Suction Valves Pass
Approximately 20 in. Hg vacuum is reached within two minutes.
Source Test Result: Suction Valves Fail
The compressor cannot reach approximately 20 in. Hg vacuum within two minutes.
Source Procedure Reference
The 20 in. Hg / two-minute criterion is the test standard given in this particular compressor lesson. It should not automatically be applied to every compressor without considering the equipment and manufacturer’s service information.
What Happens When Suction Valves Leak?
If suction valves do not seal correctly, compressed vapor can leak backward during the piston cycle.
This reduces the compressor’s ability to lower suction pressure and reduces effective pumping capacity.
Good Suction Valves
The compressor can remove refrigerant vapor from the low side and pull the suction side into a deep vacuum during the source test.
Leaking Suction Valves
The compressor has difficulty producing the required vacuum because refrigerant leaks backward through the valve area.
Checking Discharge-Valve Sealing
After evaluating the compressor’s ability to pull a vacuum, the source procedure next checks whether the discharge valves can prevent pressure from leaking backward through the compressor.
Low Side Remains Front-Seated
The low-side service valve remains in the test position after the compressor has produced the vacuum.
Stop the Compressor
The source instructs the technician to stop the compressor while the low-side valve remains front-seated.
Watch the Low-Side Gauge
Observe whether the vacuum remains or begins to rise.
The source states that if the low side remains below approximately 20 inches Hg vacuum, the discharge valves are considered good. If the pressure rises above that vacuum level, the source identifies the discharge valves as bad. :contentReference[oaicite:5]{index=5}
Source Test Result: Discharge Valves Pass
The vacuum remains below approximately 20 in. Hg after the compressor stops.
Source Test Result: Discharge Valves Fail
Low-side pressure rises above approximately 20 in. Hg vacuum after shutdown.
Why a Leaking Discharge Valve Reduces Capacity
During normal compression, the discharge valve should allow high-pressure vapor to leave the cylinder and prevent that vapor from flowing backward.
If the valve leaks, part of the compressed refrigerant can return toward the cylinder or low-pressure side.
Mechanical Effect
A leaking discharge valve reduces the compressor’s ability to maintain the pressure difference between the high and low sides.
The Complete Mechanical Valve-Test Sequence
The source procedure can be summarized as the following sequence:
Install refrigeration gauges.
Bleed air at the service-valve gauge connections as directed by the procedure.
Jump the low-pressure control.
Start the compressor.
Adjust the low-side service valve and observe low-side pressure.
Adjust the high-side service valve and observe high-side pressure.
Front-seat the low-side service valve and observe the vacuum produced.
Determine whether approximately 20 in. Hg vacuum is reached within two minutes.
Stop the compressor while the low-side service valve remains front-seated.
Observe whether the low-side vacuum holds or rises.
Back-seat both service valves after completing the test.
Remove the refrigeration gauges using the appropriate service procedure.
The source ends the procedure by instructing the technician to back-seat both valves and remove the gauges. :contentReference[oaicite:6]{index=6}
Interpreting the Mechanical Test
Mechanical Interpretation
Suction-side pumping ability is acceptable according to the lesson procedure.
The source procedure identifies the low-side compressor valves as defective.
Discharge valves are sealing according to the source procedure.
The source procedure identifies discharge-valve leakage.
What This Test Does Not Tell You
A mechanical valve test evaluates pumping ability and valve sealing. It does not provide a complete diagnosis of every compressor problem.
It Does Not Test Motor Windings
The mechanical test does not determine whether compressor motor windings are open, shorted, or grounded.
It Does Not Evaluate Starting Components
Capacitors, relays, contactors, overloads, and other electrical starting components require electrical testing.
It Does Not Replace System Diagnosis
Low capacity can also result from refrigerant charge, airflow, metering-device, condenser, evaporator, or other system problems.
Separate the Two Diagnostic Paths
Mechanical diagnosis asks whether the compressor pumps correctly.
Electrical diagnosis asks whether the compressor motor and electrical circuit operate correctly.
Service-Valve Procedure Must Be Followed Carefully
The mechanical test deliberately changes refrigerant flow through the compressor service valves.
Incorrect valve positioning can create abnormal compressor pressures or isolate the compressor incorrectly.
Never front-seat the discharge service valve while the compressor is running.
This is the critical procedural warning contained in the source valve-checking instructions. :contentReference[oaicite:7]{index=7}
Put the Concepts Together
Mechanical compressor diagnostics evaluate the compressor’s ability to pump refrigerant vapor and maintain a pressure difference.
The suction-side test determines whether the compressor can pull the low side into the vacuum specified by the source procedure.
The source uses approximately 20 in. Hg vacuum within two minutes as the suction-valve test criterion.
The discharge-valve test observes whether the low-side vacuum remains after the compressor stops.
A rising low-side pressure after shutdown indicates discharge-valve leakage according to the source procedure.
The discharge service valve must never be front-seated while the compressor is running.
Mechanical compressor testing does not replace electrical compressor diagnosis.
Electrical testing of the compressor motor and electrical circuit belongs in the separate electrical course sequence.
Can You Explain the Mechanical Compressor Test?
You should be able to answer these questions before continuing.
1. What part of compressor operation is evaluated by this lesson?
2. What compressor tests are intentionally left to the electrical course sequence?
3. What is the purpose of the suction-valve pumping test?
4. What vacuum and time criterion are given in the source procedure?
5. What does failure to reach the specified vacuum indicate according to the source?
6. How is discharge-valve sealing checked?
7. What does a rising low-side pressure after shutdown indicate?
8. What service-valve position is used on the low side during the pumping test?
9. What critical restriction applies to the discharge service valve while the compressor is running?
10. What should be done with the service valves after the mechanical test is complete?
What You Should Have Learned
A compressor can operate electrically and still have a mechanical pumping problem.
Internal suction and discharge valves must seal correctly for a reciprocating compressor to maintain the required pressure difference.
The source procedure evaluates suction-side pumping ability by observing how deeply and how quickly the compressor can pull a vacuum.
The source uses approximately 20 in. Hg vacuum within two minutes as the low-side valve criterion.
Discharge-valve sealing is evaluated by observing whether the vacuum holds after the compressor is stopped.
A pressure rise after shutdown indicates discharge-valve leakage according to the source test.
The discharge service valve must never be front-seated while the compressor is operating.
Electrical compressor diagnosis is a separate subject and is covered in the electrical training sequence.
Mechanical Compressor Diagnosis
The purpose of this test is simple:
Can the compressor create the pressure difference, and can its internal valves hold that pressure difference?
This is the mechanical side of compressor diagnosis.
Electrical testing of the compressor motor and electrical circuit is covered separately in the electrical course sequence.