How to Test a Compressor Start Relay, Overload and Windings

Why this matters

"The fridge clicks every few minutes and is getting warm" is one of the most common refrigerator calls, and it has two very different answers. Most of the time the start relay or the overload has failed: an inexpensive part on the outside of the compressor, replaced in twenty minutes. Some of the time the compressor itself is open, shorted, grounded or locked, which is a compressor replacement, a sealed-system job and often a conversation about replacing the refrigerator. Three resistance readings and two part tests tell those apart before anything is ordered. This card is that procedure for a conventional single-speed compressor, with the readings from one real unit carried through each step. It stops at the compressor shell: anything inside the refrigerant loop routes to 'What Is Inside the Sealed System Line', and variable-speed compressors are a different test, covered in 'Inverter Compressors and DC Fans: What a Meter Cannot Tell You'.

The terminal layout

COMPRESSOR TERMINALS (one common layout,
pins identified by reading, not position)

            (C) common
           /   \
         (S)---(R)
        start   run

 START RELAY (PTC or current type) plugs
 onto S and R: feeds R directly, feeds S
 through its PTC pill or relay contacts
 OVERLOAD plugs onto C

 readings, power off, relay and overload off:
   C to R   lowest
   C to S   middle
   S to R   highest, about C-R plus C-S

The run winding is thicker wire and fewer turns, so it reads lowest. The start winding is finer and reads higher. S to R measures both in series through the common point, so it should come out at about their sum. That last relationship is what lets you name the pins on a compressor whose label is gone, and it is the first check that the windings are intact.

Start state

The unit and what you need. A conventional single-speed refrigerator compressor, identified as non-inverter by the tech sheet or by a plug-on relay and overload on three pins. A meter on its lowest ohms range, zeroed by touching the leads together, a clean bare spot to read ground against (a scraped section of the suction or discharge line, or the copper process stub, never paint), and the tech sheet's winding resistance values and relay test.

Before you begin. Confirm the complaint is a start fault rather than a cooling fault with the compressor running: listen for the hum and click, and feel whether the compressor shell is very hot. If it is, the overload may simply be open because it is hot, and its test waits until it has cooled.

Steps, with one unit's readings

The unit: a top-freezer refrigerator. Fresh food at 49 F, freezer at 24 F. The compressor hums for a few seconds, clicks off, and repeats every few minutes. Condenser fan runs when the compressor tries.

1. Unplug, verify dead, and give the parts time. Pull the plug and verify absence of voltage at the relay's connector with a meter proven on a known live source before and after, per 'Appliance Electrical Safety Check Before Service'. Let a hot compressor cool before touching the overload or reading it. Why: the relay and overload carry line voltage, the compressor shell can be hot enough to burn, and an overload read hot reads open whether or not it is good. On this unit: verified dead; shell warm, so the tech starts on the windings and leaves the overload for last.

2. Remove the relay and overload straight off the pins. Note or photograph which wire goes where, then pull each part straight back without rocking it. Why: the pins are the weakest point of a hermetic compressor. A pin bent or loosened in its glass seal can leak refrigerant or, later, fail violently when energized; that is why live readings are never taken at bare pins and the terminal cover always goes back on before power does. On this unit: relay and overload came off cleanly; pins straight, no oil or scorching at the base of any pin.

3. Read C to R, C to S, and S to R. Probe the pins directly, firmly, on clean metal. Why: these three readings prove the windings continuous and not shorted to each other, and the sum relationship confirms them. On this unit:

C to R      4.2 ohms
C to S     11.8 ohms
S to R     16.0 ohms
check: 4.2 + 11.8 = 16.0, matches S to R
tech sheet: run and start values within
its stated ranges

What would fail this step: any reading OL (an open winding), C to S and C to R nearly equal or both near zero (a short), or a sum that misses S to R by a wide margin. Any of those condemns the compressor and the procedure ends here, routed as a sealed-system job.

4. Read each pin to ground. One lead on a pin, the other on a scraped spot on the copper tubing, meter on its highest ohms range. Why: a winding that has broken down to the shell can pass step 3 perfectly and still trip the breaker or the overload, and a grounded compressor is a shock hazard on the cabinet. On this unit: C, S and R each read OL to the suction line. Any reading at all on a basic meter, even in the megohm range, is treated as a grounded winding: the compressor is not energized again. A megohm tester at the voltage the compressor maker specifies, read against the maker's minimum, confirms it before the compressor is condemned.

5. Test the start relay. Follow the tech sheet's test for the relay type fitted. Why: the relay is the most common failure on this complaint, and it has two common forms that test differently.

• PTC start device (a solid-state pill in a plastic housing): across the two terminals the sheet names (the ones that feed R and S), it should read a low resistance at room temperature, in the range the sheet gives. OL means the pill has cracked or burned open. A rattle when shaken often means the same thing.
• Current (magnetic) relay: its contacts are position sensitive. Held in its mounted orientation, the S contacts read OL; turned upside down so the armature drops, they read near zero. The coil reads near zero across its terminals.

On this unit: a PTC device, room temperature, OL across the R and S terminals where the sheet expects a low reading, and it rattles. The relay has failed.

6. Test the overload once it is at room temperature. Across its two terminals it should read near zero. Why: an overload that is open cold has failed open; one that is open hot may be doing its job, and the question is why it got hot. On this unit, after twenty minutes off the compressor: 0.2 ohms. Good.

7. Replace what failed with the exact match. Use the relay or relay and overload combination the parts list gives for that compressor, not a universal part, unless the compressor maker's cross-reference approves it. Why: a relay's PTC rating and an overload's trip curve are matched to the compressor's windings; a mismatch either fails to start it or fails to protect it. On this unit: the listed PTC relay and the original overload refitted, wires on the terminals they came off, terminal cover clipped on.

8. Restore power and prove the start. With the terminal cover on and your hands clear of the compressor compartment, plug the refrigerator in. Clamp the meter on the compressor's supply lead. Why: this is the step where line voltage goes back onto the compressor, and the only proof the repair worked is a clean start. A good start shows a brief high current that drops within a second or two to a steady running current at or below the rating plate's figure, and the compressor keeps running. On this unit: start within the board's delay, a brief spike, then a steady draw below the rating plate value; no click-off over the next ten minutes, the condenser coil warming and the freezer floor getting colder.

Where the call forks

• Windings pass, ground passes, relay and overload pass, and the compressor still hums and trips on overload. Check the supply first: read line voltage at the relay connector during the start attempt (hands clear, cover on, at the connector, not at the pins). A line that sags well below nominal, more than about 10 percent below 115 V during the start, points at the outlet or an extension cord, not the compressor. With the supply proven, the compressor is mechanically locked, which is a compressor replacement and a sealed-system job.
• The relay keeps failing. A replaced relay that fails again in weeks points at a compressor drawing high current, a weak winding, or a supply problem. Read running current against the rating plate and the voltage under load before fitting a third one.
• A board-controlled conventional compressor may run its relay from a board output. If the relay and windings pass and the compressor never gets a start attempt, the question becomes whether the board is calling, which is a back-probe reading at the board, per 'How to Back-Probe a Control Board Connector Without Shorting Pins'.

The sealed-system boundary

Everything above is outside the line. The relay, overload, terminal cover and harness are ordinary parts any tech replaces. A compressor that fails steps 3 or 4, or proves locked, means opening the refrigerant loop to replace it, which under EPA Section 608 (40 CFR Part 82, Subpart F) requires Type I or Universal certification, plus recovery equipment. Write the readings on the ticket, quote it or route it, and do not leave a compressor energized that read to ground.

References

• Manufacturer tech sheet: winding resistance values, relay type and test, and rating plate running current for the specific compressor
• EPA Section 608, 40 CFR Part 82 Subpart F, for any compressor replacement
• See related: What Is Inside the Sealed System Line
• See related: Inverter Compressors and DC Fans: What a Meter Cannot Tell You
• See related: How to Back-Probe a Control Board Connector Without Shorting Pins