Voltage Drop Testing

A continuity test — meter across two points with no current flowing — will tell you whether there’s a complete circuit. It will not tell you whether that circuit can actually carry current. A connection with 5 Ω of corrosion contamination passes continuity (“hey, it beeped”) but drops 2 volts at 1 amp of load — enough to make a furnace lock out or a contactor chatter or a motor stall.

Voltage drop testing — measuring voltage across a component while it’s carrying load — is how you find the faults that continuity misses. It’s one of the most underused techniques in HVAC, and it’s the one that differentiates techs who solve “intermittent” problems from techs who chase them forever.

Why unloaded testing misses things

Picture a relay with slightly pitted contacts. Clean silver at some points; at others, the contact is oxidized and rough.

Unloaded continuity test: the meter’s test current (1 mA or so) finds the path of least resistance through the cleanest pit, and reads 0.2 Ω or so. “Beeps, continuity is good.”

Loaded operation: the motor needs 5 A. Current flows through the same pitted contact. At 5 A through even 0.3 Ω of contact resistance, Ohm’s law gives V = IR = 5 × 0.3 = 1.5 volts dropped across the contact. That’s 1.5V that the motor isn’t getting — the motor sees 118.5V instead of 120V. Barely noticeable on a healthy motor, often catastrophic on a motor that was already marginal.

Multiply this across multiple aging contacts, corroded lug connections, and wire terminations, and you can easily lose 5-10 volts through the wiring path. A fresh motor reads fine at 120V; a motor seeing 110V is stressed and overheats.

Unloaded = low current = low voltage drop = missed diagnosis. Loaded = high current = significant voltage drop = visible problem.

The procedure

Voltage drop testing is a live-circuit test. The equipment must be running.

1. Identify the suspect connection or component. Relay contact, contactor, lug crimp, wire run, plug-pin connection.
2. Set meter to AC voltage (or DC, matching the circuit).
3. Put one probe on each side of the suspect. Black probe on the upstream side, red on the downstream side. You’re measuring the voltage across the component, not to ground.
4. With the circuit loaded (motor running, relay closed, etc.), read the meter.
5. Interpret:
   • 0 V: perfect contact, no drop. Healthy.
   • 0.1–0.3 V: normal healthy contact on a modern device. Acceptable.
   • 0.5–1 V: some resistance developing. Worth watching.
   • 1–3 V: significant problem. Arcing, pitting, corrosion, or loose connection. Will likely get worse.
   • >3 V: major problem. Component likely failing under load. Address immediately.

Canonical examples

Contactor contacts on a residential AC. Compressor should see full line voltage. Put probes on the line side and load side of one contactor pole with the compressor running. Should read a few tenths of a volt or less. If you read 1-3 V, contacts are pitted and need replacement. If you read 5+V, the contactor is near failure and the compressor is running on reduced voltage.

Transformer primary. If a furnace is behaving like it has low 24V (marginal relay pickup, missed calls), one possibility is low primary voltage due to voltage drop in the supply. Probe across the transformer primary terminals with the furnace running. Should read full line voltage. If you read 108V where you expect 120V, something upstream (a loose lug, a corroded ground, a high-resistance connection) is dropping voltage under load.

Pressure switch contacts. On a furnace locking out with “pressure switch open” errors but the switch is mechanically closing: probe across the switch contacts during a call, with draft established. Reading should be 0V (contacts closed, no drop). A few tenths to 1V indicates failing internal contacts — the switch is closing but can’t pass control current reliably. Replace.

Ground path. Probe from the furnace cabinet ground to the neutral conductor with the furnace running at full load. Should be very small (under 1V). Higher readings indicate a compromised ground return path, which affects flame sensing, ECM operation, and overall reliability.

When voltage drop isn’t the right tool

Voltage drop testing measures the voltage drop across a component under load. If there’s no load, there’s no drop, and you can’t test this way. Similarly, for entirely digital or solid-state components (circuit boards, microprocessors), voltage drop across a physical connection doesn’t tell you about logic-level faults.

The right tool for:

• Continuity or open circuit: ohmmeter, de-energized.
• Component ratings (capacitor µF, winding resistance): dedicated meter functions, de-energized.
• Logic signals on a control board: DMM reading actual control voltages, at specified test points.
• Voltage drop: a live, loaded circuit with a suspected resistive fault in the power path.

From the field

Callback on a compressor that “starts fine when cool, stalls out on hot days.” Homeowner had been living with it for weeks. Other techs had measured the cap, measured the compressor windings, measured line voltage at the disconnect — all good. Pattern didn’t fit common failures.

I did a voltage-drop test across the contactor’s load-side contacts with the compressor trying to start. The reading on a healthy contactor is a few tenths of a volt. This one read 14 volts. Under starting inrush (45+ amps), 14V of drop meant the compressor was seeing 226V instead of 240V — plenty of voltage difference to prevent a hot-day start where pressure differential is highest.

Contact erosion was not visible by eye — contacts looked adequate. But the voltage drop test showed the arc-welded layer of oxide that was creating high resistance. Replaced the contactor, voltage drop came down to 0.1V, compressor started reliably even on 95°F days.

No other test would have caught this — the contactor looked fine, beeped on continuity, and wasn’t visibly chattering. Voltage drop is what revealed it.

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Check your understanding

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01An ohmmeter reads 0.3 Ω across a contactor's closed contacts when the unit is de-energized. Why might this contactor still have a significant voltage drop problem when the compressor is running?

AOhmmeters are inaccurateBUnloaded, the meter's small test current finds the path of least resistance. Under load (e.g., 30A), even a 0.3 Ω contact drops 9V (V=IR). A few tenths of an ohm is invisible at test current but catastrophic at operating current.CContact resistance changes with temperatureDThe contactor is undersized

02A 240V compressor circuit has 7V drop across the contactor's closed contacts while the compressor runs. What's the correct response?

ANo action needed — under 10V drop is normalBReplace the contactor — 7V drop on a 30-40A load means nearly 300W of heat being dissipated at the contacts and the compressor is running on significantly reduced voltage; both are accelerating damageCTighten the contactor mounting boltsDReplace the compressor

03When would voltage drop testing NOT be the right diagnostic tool?

AWhen you need to test whether a circuit is continuous (use ohmmeter on de-energized circuit instead)BVoltage drop testing is always the right toolCWhen you're in a hurryDWhen the circuit is 240V

Voltage drop testing is the technique that separates techs who find the hard faults from techs who chase them. A DMM, a pair of probes, five minutes of live-circuit work, and you’ve found what every other test missed. Use it on every call where “everything tests fine but something’s clearly wrong.”