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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Appliance Repair › Refrigeration Principles and System Diagnosis › Lesson 4
BUS-APPL73-03 · Lesson 4 · 12 clock hours · demo

Compressor and Start Component Diagnosis

Objective. Students will use a multimeter to test compressor winding resistance and continuity, test PTC starters and overload protectors, and determine whether a compressor is electrically serviceable.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Compressor Diagnosis Matters

The hermetic compressor is the heart of any refrigeration system. When a refrigerator, freezer, or window air conditioner stops cooling, the compressor is one of the first components suspected—but it is also one of the most expensive to replace. Accurate electrical testing lets you confirm whether the compressor itself is faulty or whether a cheaper start component is the real problem. Replacing a good compressor because of a bad PTC starter wastes the customer's money and your credibility.

Safety Before You Touch Anything

Compressors operate on 120 V AC or 240 V AC. Before performing any resistance test, disconnect the appliance from the power source and apply lockout/tagout (LOTO) procedures. Verify the circuit is de-energized with your meter set to AC voltage before touching any terminals. Wear safety glasses and insulated gloves rated for the voltage involved. Capacitors associated with compressors can hold a charge even after power is removed; discharge them through a resistor before handling. Never probe live terminals with a resistance-range setting—the meter's internal current source is not designed for live circuits and the result can injure you or destroy the meter.

The Three-Terminal Resistance Test

A hermetic compressor has three external terminals labeled C (common), S (start), and R (run). Internally, two windings share the common terminal:

Set your digital multimeter to the ohms (Ω) function. Touch the probes to each pair of terminals in turn and record all three readings:

| Measurement | Notation |

|---|---|

| Common to Run | C–R |

| Common to Start | C–S |

| Run to Start | R–S |

The golden rule: R–S should equal C–R plus C–S. If your three values satisfy this relationship (within normal meter tolerance), the windings are electrically intact. If any reading is zero or near zero, that pair of windings is shorted. If any reading is OL (overload/infinite), the winding is open. Either fault means the compressor must be replaced.

Also touch one probe to a compressor terminal and the other to the bare metal shell. The meter should read OL (no continuity). Any measurable resistance here means the winding is grounded to the shell—the compressor is condemned.

Identifying Terminals Without Labels

Terminals are sometimes unmarked. Measure all three pairs. The highest reading is always R–S. The middle reading is C–S, and the lowest reading is C–R. Label accordingly.

Overload Protectors

An external overload protector is a thermal/current device wired in series with the common terminal. Its job is to open the circuit if the compressor overheats or draws excessive current, preventing winding damage. To test it:

1. Disconnect it from the circuit (LOTO in place).

2. Set the meter to continuity or ohms.

3. At room temperature, a good overload should read near zero ohms (closed).

4. An open reading (OL) at room temperature means the overload has failed open and must be replaced.

Always allow a suspected tripped overload to cool for 30 minutes before testing; some will reset and show continuity once cool, indicating the real fault is elsewhere (restricted airflow, dirty condenser, low voltage).

PTC Start Devices

A PTC (positive temperature coefficient) thermistor is used as a start device on many household compressors. At room temperature its resistance is low, allowing current through the start winding. As it heats up from current flow, its resistance rises sharply, effectively removing the start winding from the circuit. There are no moving parts.

To test a PTC device:

1. Allow it to cool to room temperature for at least five minutes if recently operated.

2. Measure resistance across its two terminals with the meter on ohms.

3. A good PTC will show a low resistance (commonly in the range of a few to a few tens of ohms—consult manufacturer data for the specific part). An OL reading on a cool PTC means it is open and must be replaced.

4. Visually inspect for cracks, burn marks, or melted housing; any of these is cause for replacement regardless of the resistance reading.

Current-Sensing Relays

A current-sensing (amperage) relay uses a low-resistance coil wired in series with the run winding. High start current energizes a magnetic plunger that closes the start winding contact. Once the motor accelerates and run current drops, the relay drops out and opens the start circuit. To test:

1. Shake the relay gently near your ear—a broken or corroded plunger will rattle abnormally.

2. Measure coil resistance; an open coil (OL) means replace.

3. Measure the normally-open start contacts; at rest they should read OL (open). If they read continuity at rest, the relay is welded closed and must be replaced.

Determining Serviceability

Combine your findings into a repair decision:

Always document your measured values on the diagnosis form. Good records protect you professionally and help the next technician who opens that unit.

Key terms10

Hermetic compressorA refrigeration compressor permanently sealed inside a welded steel shell, with motor windings and the compressor mechanism sharing the same refrigerant atmosphere.
Common terminal (C)The compressor terminal shared by both the run and start windings; always one end of both internal windings.
Run windingThe main compressor motor winding, made of heavier wire with lower resistance, which carries current throughout normal operation.
Start windingThe auxiliary compressor motor winding, made of lighter wire with higher resistance, used only during motor startup to create the phase shift needed for torque.
PTC starterA positive temperature coefficient thermistor used as a solid-state start device; its resistance rises sharply when heated, removing the start winding from the circuit after startup.
Current-sensing relayAn electromagnetic relay wired in series with the run winding that uses high starting current to close the start winding circuit, then drops out as current falls once the motor is running.
Overload protectorA thermal or current-sensitive device in series with the compressor common terminal that opens the circuit to prevent motor damage from overheating or excessive current draw.
Grounded windingA fault condition where winding insulation has broken down, creating an electrical path between the winding conductors and the compressor shell; measured as continuity from any terminal to the shell.
Lockout/tagout (LOTO)An OSHA-required safety procedure in which energy sources are isolated and physically locked off, with a warning tag attached, before any service or testing work begins.
Open windingA fault condition where a break in the winding wire results in no continuity (infinite resistance) across the affected pair of terminals.

Worked examples4

1. A technician measures the three terminal resistances on an unmarked hermetic compressor and records: pair 1 = 4 Ω, pair 2 = 6 Ω, pair 3 = 10 Ω. Identify which pair corresponds to C–R, C–S, and R–S, and verify the readings are consistent with a good compressor.
  1. Recall the rule: the highest reading is R–S, the middle reading is C–S, and the lowest reading is C–R.
  2. Assign: R–S = 10 Ω, C–S = 6 Ω, C–R = 4 Ω.
  3. Verify: C–R + C–S should equal R–S. Calculate 4 + 6 = 10.
  4. Compare: 10 Ω = 10 Ω. The relationship holds.
Answer: C–R = 4 Ω, C–S = 6 Ω, R–S = 10 Ω. Because 4 + 6 = 10, the windings are consistent with a good compressor—no open or shorted windings indicated.
2. A technician tests a compressor and records C–R = 3 Ω, C–S = OL, R–S = OL. What is the diagnosis and why?
  1. An OL reading means infinite resistance, indicating an open (broken) circuit in that winding path.
  2. C–S = OL means there is a break somewhere in the start winding path (from C through the start winding to S).
  3. R–S = OL is consistent with the start winding being open: because S is isolated, any measurement that must pass through the start winding will also read OL.
  4. C–R = 3 Ω is a normal low reading, confirming the run winding itself is intact.
  5. Conclusion: the start winding is open.
Answer: The start winding is open. Because both readings involving the S terminal are OL while C–R is normal, the fault is isolated to the start winding. The compressor must be replaced.
3. A technician records C–R = 5 Ω, C–S = 8 Ω, R–S = 13 Ω, and then touches one probe to the C terminal and the other to the bare compressor shell, reading 7 Ω. What is the diagnosis?
  1. Check the R–S relationship: 5 + 8 = 13. This matches R–S = 13 Ω, so neither winding is open or shorted to each other.
  2. Evaluate the shell test: a reading of 7 Ω (not OL) from the C terminal to the shell indicates the winding insulation has broken down and current can flow to the metal shell.
  3. This is a grounded winding—a serious fault that creates a shock hazard and will cause the breaker to trip or fuse to blow during operation.
Answer: The compressor has a grounded winding. Despite normal winding-to-winding resistance values, continuity to the shell condemns the compressor; it must be replaced.
4. A PTC start device is tested at room temperature and reads 180 Ω. The manufacturer specification for that part number states the room-temperature resistance should be between 3 Ω and 30 Ω. What is the correct action?
  1. Compare the measured value (180 Ω) to the specification range (3–30 Ω).
  2. 180 Ω is far above the upper limit of 30 Ω.
  3. A PTC that has excessively high resistance at room temperature will not pass enough current through the start winding during startup, meaning the compressor may fail to start or will draw locked-rotor current and trip the overload.
  4. The device is outside specification and is faulty.
Answer: Replace the PTC starter. Its room-temperature resistance of 180 Ω far exceeds the acceptable maximum of 30 Ω, meaning it will not properly energize the start winding.
In class: Watch instructor demonstrate the three-terminal resistance test on a hermetic compressor (common-to-run, common-to-start, run-to-start windings) using a digital multimeter; then each student repeats the test on two separate trainer compressors and records results on a diagnosis form. · Test a PTC (positive temperature coefficient) start device and a current-sensing relay for correct operation using resistance and visual inspection methods taught by the instructor; log findings and compare to the manufacturer's specification or general acceptance criteria reviewed in class. · Interpret a set of five pre-filled diagnosis forms representing different compressor and start-component faults; for each, select the correct repair action (replace compressor, replace start device, replace overload, or no fault found) and justify the decision in writing.
Materials: Digital multimeter (auto-ranging), assorted hermetic compressors (working and known-faulty), PTC starters, current-sensing relays, overload protectors, diagnosis forms, manufacturer service data or appliance service manuals (matching trainer units), safety glasses, insulated gloves.
Check: Each student must perform the winding resistance test independently and record all three resistance values; instructor verifies technique and recorded values are within 10% of actual; five-fault interpretation exercise graded by instructor—minimum 4 of 5 correct diagnoses to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Work Practices
1. Before measuring winding resistance on a hermetic compressor, a technician must first:
Electrical Diagnosis
2. A technician measures compressor terminal resistances and finds C–R = 8 Ω, C–S = 12 Ω, and R–S = 20 Ω. Which statement best describes this compressor?
Electrical Diagnosis
3. When testing an unmarked hermetic compressor, which terminal pair always produces the highest resistance reading?
Electrical Diagnosis
4. A compressor shows normal winding-to-winding resistance values, but the technician measures a low resistance between the common terminal and the compressor shell. The correct diagnosis is:
Component Diagnosis
5. A current-sensing relay is shaken gently next to the technician's ear and produces a heavy rattling sound. The most appropriate next step is to:
Component Diagnosis
6. An external overload protector reads OL (open) on the ohmmeter immediately after the technician has removed it from a compressor that had been running for hours. The best action is to:
Component Diagnosis
7. A PTC start device tests at a very high resistance (open) when cool. The likely result if it is reinstalled and the appliance is restarted is:
Electrical Diagnosis
8. A diagnosis form shows these compressor readings: C–R = 6 Ω, C–S = 0 Ω, R–S = 6 Ω. Terminal-to-shell reads OL. Which fault does this indicate?
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