Troubleshooting Guide

What Is a Visual Sign of a Start Capacitor Failure?

Motor won't start? Compressor humming but not running? A failed start capacitor is one of the most common — and cheapest to fix — causes of motor problems in air conditioners, heat pumps, pool pumps, and compressors. Here's how to diagnose it.

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SAFETY FIRST

Start capacitors can store a dangerous charge even after the equipment is turned off. Always discharge the capacitor before touching the terminals. Use an insulated 20kΩ resistor held across the terminals for 5 seconds, or an insulated screwdriver (expect a spark). Work with one hand when possible to prevent current from flowing across your chest.

Behavioral Symptoms: What You'll Notice

Before you open anything up, pay attention to what the equipment is doing. A failed start capacitor produces very specific behavior:

Motor hums but won't start

The motor receives power (you can hear the electromagnetic hum) but can't generate starting torque. This is the #1 symptom of a dead start capacitor.

Critical — turn off immediately

Motor starts slowly or struggles

The capacitor has lost some capacitance. It provides some starting boost but not enough for a clean, fast start. The motor may eventually reach speed but takes much longer.

Degraded — replace soon

Breaker trips or overload pops

A stalled or slow-starting motor draws excessive current (locked rotor amps). This trips the overcurrent protection. Resetting the breaker without fixing the capacitor will repeat the cycle.

Critical — diagnose before resetting

Motor runs but overheats

If the start capacitor is weak (partially failed), the motor may eventually start but the start winding stays energized too long, generating excessive heat.

Urgent — check capacitor and relay

AC blows warm air (compressor not running)

The indoor fan runs but the outdoor compressor can't start. You hear a click (contactor engaging) then a hum, then a click (overload tripping). Classic failed start cap.

Common HVAC symptom

Burning smell near the motor

If the motor has been trying to start repeatedly, the start winding insulation may be overheating. Stop immediately to prevent permanent motor damage.

Critical — stop and inspect

Visual Signs on the Capacitor Itself

Bulging or Swollen Top

The flat top of the capacitor is domed or puffed out. Internal gas pressure from electrolyte decomposition. Definite failure.

Leaking Oil/Fluid

Brown or clear oily substance around the terminals or base. The sealed case has ruptured. The capacitor is dead.

Burn Marks or Melted Plastic

Black marks, melted terminals, or charred housing. Indicates a catastrophic short circuit. Check wiring before replacing.

Cracked or Split Casing

The plastic case has split or separated. Usually from overheating or overvoltage. The capacitor has vented.

Important: A start capacitor can fail with no visible signs at all. If the motor shows behavioral symptoms but the capacitor looks fine, you must test it electrically. See our How to Test a Capacitor guide.

How to Test a Start Capacitor

Turn Off Power

Disconnect at the breaker and verify with a non-contact voltage tester. Wait 5 minutes.

Locate the Capacitor

Usually in a covered compartment near the motor. Start caps are cylindrical and black; run caps are oval and metal.

Discharge It

Bridge the two terminals with an insulated resistor (20kΩ 5W) for 5 seconds. This drains any stored energy safely.

Read the Label

Note the µF value (e.g., 88–108 µF) and voltage rating (e.g., 250VAC). You need this for comparison.

Set Multimeter to Capacitance

Turn the dial to the -||- symbol. Some meters auto-range; others need manual range selection.

Measure

Touch probes to the terminals. Wait 3–5 seconds for the reading to stabilize.

Good

Reading is within the labeled range (e.g., 88–108µF reads 95µF). Capacitor is healthy.

Weak

Reading is below the labeled range by more than 10% (e.g., 88–108µF reads 70µF). Replace.

Dead

Reading is 0, "OL", or wildly off. Capacitor is shorted or open. Replace immediately.

How to Match a Replacement Start Capacitor

Parameter	Rule	Example
Capacitance (µF)	Match exactly or within the labeled range	Original: 88-108µF → Use 88-108µF
Voltage (VAC)	Equal to or HIGHER than original	Original: 250VAC → Use 250V or 330V
Physical size	Must fit in the housing	Measure diameter and height
Terminal type	Match the connector (push-on, wire lead)	Most use 1/4" push-on terminals
Start vs Run	Never swap start for run (or vice versa)	Start = intermittent duty; Run = continuous

Start Capacitor FAQ

What is a visual sign of a start capacitor failure?

The most common visual signs are a bulging top, oil leakage near terminals, a cracked case, or scorch marks. Any of these indicate the capacitor should be replaced before re-energizing the motor.

What are the symptoms of a bad start capacitor?

The most common symptoms are: 1) Motor hums but won't start, 2) Motor starts slowly or struggles to reach full speed, 3) Motor trips the overload protector or breaker, 4) A loud buzzing or humming from the motor, 5) The motor runs in the wrong direction (single-phase motors), 6) Visible signs on the capacitor: bulging top, leaking fluid, burn marks, or cracked case.

What is the difference between a start capacitor and a run capacitor?

A start capacitor provides a large burst of energy to get the motor spinning (high capacitance, 70–400µF, used briefly). A run capacitor stays in the circuit continuously to improve efficiency and power factor (lower capacitance, 5–60µF, rated for continuous duty). Start capacitors are typically black and cylindrical; run capacitors are usually oval and metal-cased.

Can I test a start capacitor with a regular multimeter?

Yes, if your multimeter has a capacitance mode. Set it to capacitance (symbol: -||-), discharge the capacitor safely, and measure. The reading should be within ±10-20% of the marked value. If it reads significantly low or "OL", the capacitor is bad. A resistance test can also indicate shorts (reads 0Ω) or opens (stays at OL without climbing).

How long does a start capacitor last?

Start capacitors typically last 10-20 years in normal residential use. However, frequent starts (short cycling), high ambient temperature, voltage issues, and moisture can shorten life significantly. If your HVAC system short-cycles (turns on/off frequently), the start capacitor will wear out faster because it is stressed during every start event.

Can I replace a start capacitor with a different µF value?

You should match the µF value as closely as possible. Using too low a value means insufficient starting torque — the motor may not start at all. Using too high a value can overdrive the start winding and cause overheating. The voltage rating can be equal to or higher than the original, but never lower.

Why does my AC unit hum but not start?

A humming AC compressor that won't start is the classic symptom of a failed start capacitor. The motor is receiving power (hence the hum) but can't generate enough starting torque without the capacitor's phase-shifted current. Turn off the unit immediately — running a motor in a stalled condition overheats the windings and can destroy the compressor.

Can a bad capacitor damage the motor?

Yes. A weak or failed start capacitor forces the motor to draw excessive current while trying to start. This overheats the start winding. If the overload protector doesn't trip quickly enough, the winding insulation can break down, permanently damaging the motor. Replacing a $15 capacitor prevents a $500+ motor replacement.

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