Which Meter First Voltage Vs Amp Vs Ohm No Start Sequencing Decision Tree

Why this matters

On a no-start electrical call, voltage, amperage, and resistance each answer a different question, and using them in the wrong order wastes time and can be dangerous. Voltage tells you whether power is present and arriving where it should. Amperage tells you what a running circuit is actually drawing. Resistance tells you the condition of a de-energized component. The hard rule is that resistance and continuity checks must be done with power removed and capacitors discharged, while voltage checks are done live, so mixing them carelessly damages meters and injures techs. Sequencing voltage first protects you and prunes the tree fastest, because most no-starts are a missing or interrupted supply, not a failed winding. This tree fixes the meter order so you find the open in the supply chain before you tear into a component.

Symptom presentation

"It will not start" covers a dead condenser, a humming compressor that will not turn, a blower that will not spin, and a board that will not call. The electrical fault sits somewhere in a chain: line power, disconnect, contactor, capacitor, control voltage, and the motor windings. Testing that chain out of order, ohming a winding before confirming the contactor even pulls in, sends you down a component path when the real fault is an open upstream switch.

Quick checks

1. Lockout/tagout and PPE: confirm rated gloves and a meter rated for the circuit.
2. Visible damage: burnt contactor, swollen capacitor, melted wire, tripped breaker, pulled disconnect.
3. Call present: confirm the thermostat is actually calling before chasing power.

Isolation tree

Step 1: Voltage first, energized, top of the chain down

Measure live, working from supply toward the load:

• Line voltage at the disconnect/contactor line side: expect nameplate (240 VAC class). Absent means the fault is upstream (breaker, disconnect, service). Present means proceed.
• Across the contactor load side with a call active: if the contactor is closed, you should read line voltage on the load side. No load-side voltage with a closed contactor means burnt or pitted contacts.
• 24 VAC at the contactor coil during a call: absent means the control circuit is open (stat, safety switch, float, transformer, or blown low-voltage fuse). Present means the contactor should pull in.

This single energized sweep locates most no-starts: no line power, open control circuit, or failed contactor.

Step 2: Amperage on what is energized

For a component that is getting voltage but not running correctly:

• Compressor or fan locked rotor: a unit that hums and draws locked-rotor amps but will not turn points to a weak run capacitor, a seized bearing, or a failed motor.
• No draw with voltage present at the terminals: an open internal winding or open overload.
• Compare running amps to nameplate full-load amps to judge a motor that runs but struggles.

Step 3: Resistance and continuity, power removed and capacitor discharged

Only after voltage and amperage point to a specific de-energized component:

• Capacitor: discharge through a resistor, then measure microfarads against nameplate; replace if outside roughly +/- 6 percent or shorted/open.
• Motor windings: check winding resistance start-to-run-to-common against spec, and each winding to ground (a reading to ground means a grounded, failed motor).
• Contactor coil: ohm the coil for open or shorted.
• Safety switches: continuity across each series safety to find the open one.

Why each meter mode has its place in the order

Voltage answers "is power present and arriving," and it is the only one of the three you can take fully energized with minimal risk, so it leads. Amperage answers "what is the running circuit actually doing," which only has meaning once voltage confirms the component is energized; a clamp on a dead conductor tells you nothing. Resistance answers "what is the condition of this de-energized part," and it requires the circuit isolated and capacitors discharged, so it comes last and only on the specific component the prior two steps implicated. Running ohms first wastes the time spent isolating a component that the voltage sweep would have exonerated in 30 seconds, and it tempts a tech to ohm a live circuit, which destroys the meter and endangers the operator.

Confirming diagnosis

The fault is proven when one component fails its proper test in sequence: no upstream voltage (supply fault), 24 VAC absent at a coil (control-circuit open), voltage present but no load-side output (failed contactor), voltage present with locked-rotor amps and a bad capacitor (capacitor fault), or a winding open or grounded on a de-energized ohm test (failed motor). Never condemn a motor on resistance alone if you skipped the voltage and capacitor checks.

Remediation

• Supply fault: reset/replace the breaker, repair the disconnect, restore service-side power.
• Control-circuit open: replace the blown low-voltage fuse and find the short, reset a tripped safety after fixing its cause, replace a failed transformer.
• Contactor: replace pitted contacts or a failed coil.
• Capacitor: replace with the correct microfarad and voltage rating.
• Motor: replace a grounded or open-winding compressor or fan motor.

References

• NFPA 70 National Electrical Code, Article 440 Air-Conditioning and Refrigerating Equipment
• NFPA 70E Standard for Electrical Safety in the Workplace
• AHRI Standard 210/240-2023 Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
• UL 60730-1 Automatic Electrical Controls for Household and Similar Use