A motor starter is not running, and the operator says it stopped on its own. The contactor is dropped out. The overload flag is not showing tripped. Pressing START does nothing. The control circuit is dead, and you need to find out where the break is. A multimeter set to AC volts and a systematic approach to probing terminals will locate the open in the circuit without guessing, without swapping parts, and without pulling wires. Every terminal in the control rung has a voltage relative to the return conductor, and that voltage tells you whether the circuit path is complete to that point or broken upstream of it.
Setting Up the Meter
Set the multimeter to AC volts. The control circuit operates at the control voltage of the installation: 120VAC or 24VAC if the starter uses a control transformer, or full line voltage (208V, 480V) if the control circuit runs at line potential.
Connect the black COM lead to L2 or to X2 on the control transformer secondary. This is the return side of the control circuit. On a transformer-fed circuit, X2 is typically the grounded conductor per NEC 250.20. On a line-voltage control circuit, L2 is the return conductor but is not a grounded conductor in a 208V or 480V system. Leave it there for every measurement that follows. The black lead is the fixed reference point. Every reading taken with the red lead is now the potential difference between that probe point and the circuit's return path.
This reference-lead technique turns the multimeter into a one-handed diagnostic tool. Instead of repositioning both leads for each measurement, one lead stays anchored and the other traces the circuit. The readings form a sequence: voltage present, voltage present, voltage present, voltage absent. The transition point is the break.
Live voltage measurements inside an energized panel require appropriate PPE and authorization per the facility's electrical safety program.
Measuring Line Voltage
Probe L1 to L2 at the starter's line-side terminals with the disconnect closed (or H1 to H2 on a control transformer primary). Expected reading: system voltage. On a 208V installation, the meter should read approximately 208VAC. On a 480V installation, approximately 480VAC.
If the reading is zero, power is not reaching the starter. The disconnect is open, a fuse has blown upstream, the feeder breaker has tripped, or the utility supply is interrupted. There is no point troubleshooting the control circuit until this measurement confirms that the starter has a source.
If the reading is present but low, the supply voltage is sagging under load or the feeder conductors are undersized for the run. Voltage problems at the source propagate through every circuit downstream.
This is always the first measurement. It takes three seconds and eliminates the most common false start in troubleshooting: spending twenty minutes tracing a control circuit that has no supply.
Measuring Control Voltage
With line voltage confirmed, probe the control circuit source. On a transformer-fed installation, probe X1 to X2 on the secondary. Expected reading: the transformer's rated secondary voltage, typically 120VAC or 24VAC. On a line-voltage control circuit, this measurement is the same as the line voltage measurement above.
If the reading is zero with line voltage present on the primary side, the transformer has failed or the secondary-side fuses have blown. Control transformers typically have fuse protection on the secondary. These fuses are small, often tucked inside the transformer's terminal cover, and easy to overlook. A blown secondary fuse kills the entire control circuit while leaving the power circuit unaffected.
If the reading is present and correct, the control circuit has a source and you can begin tracing downstream through the control rung.
Tracing Voltage Through the Control Rung
This is the diagnostic technique. Start at L1 (or X1 on a transformer-fed circuit) and probe each terminal in the series path toward the coil, keeping the black lead on L2 (or X2).
In a standard motor starter control rung, the series path from L1 to L2 runs through the STOP contact (NC), through a junction where the START contact (NO) parallels the seal-in auxiliary (NO), through the contactor coil, and through the OL auxiliary contact (NC) to L2.
At each NC contact that is closed, you should read control voltage on both the line side and the load side of the contact. The closed contact is a conductor. Current can flow through it, so the potential on both sides is the same. The reading confirms the contact is closed and the path is intact through that device.
At the first open device in the series path, the reading changes. The line side of the open device reads control voltage (the path from the source to that point is complete). The load side reads zero (no path continues past the open device to the source). The voltage has dropped across that single device.
Probe terminal by terminal:
L1 side of STOP. Should read control voltage. If zero, the source connection to the rung is broken.
L2 side of STOP. Should read control voltage if the STOP contact is closed. If zero here but voltage is present on the L1 side, the STOP contact is open. Either the button is pressed, the contact has failed, or the wire from the STOP terminal is broken.
L1 side of the coil. Should read control voltage if the path through the STOP contact and the START/seal-in junction is complete. If the motor is not running and no one is pressing START, the seal-in auxiliary is open and the START button is open. Both are NO contacts. In this state, zero voltage at the coil's L1 terminal is expected because no closed path connects the junction to the coil. This is the normal resting state of the circuit, not a fault.
L2 side of the coil. Should read zero with the coil de-energized because the closed OL contact connects this terminal to L2, placing it at L2 potential. When the coil is energized and the motor is running, this terminal still reads close to zero because the OL contact has negligible resistance and drops negligible voltage.
Driving voltage past the junction. In the resting state, both NO contacts (START and seal-in) are open, so no voltage from L1 reaches the coil or the OL contact. To diagnose a fault downstream of the junction, have someone hold START pressed while you probe. This drives voltage from L1 through the STOP contact, through the now-closed START contact, and into the coil and OL circuit.
OL contact (95-96). With START held and the motor running normally, both OL terminals should read close to zero (both are near L2 potential). If the OL contact is open (relay tripped), holding START drives voltage through the coil to the OL contact. The coil side of 95-96 reads control voltage and the L2 side reads zero. Full control voltage across the OL contact means it is the break.
The pattern repeats regardless of the circuit's complexity. Every terminal either reads control voltage (path from source is complete to that point) or zero (path from source is broken somewhere upstream). Walk the rung, find the transition, and you have found the open.
What the Readings Tell You
The underlying principle is a voltage divider applied to troubleshooting. In a series circuit, closed contacts and intact wires have near-zero resistance. They drop near-zero voltage. An open contact or a broken wire has effectively infinite resistance. It drops all of the source voltage.
Voltage present at a terminal: every conductor, contact, and connection between the source and that terminal is intact. The path is complete.
Voltage absent at a terminal: something between the source and that terminal is open. The break is somewhere upstream of the probe point.
Full control voltage across a single device (voltage on one side, zero on the other): that device is the open element. It is dropping all of the source voltage because it is the only discontinuity in the series path. This reading is conclusive. The open is not "somewhere in the circuit." It is across the device you just measured.
This technique works on any series control circuit, regardless of the number of devices in the rung, the control voltage level, or the complexity of the circuit. Add more contacts, more interlocks, more pilot devices, and the method is the same: anchor the reference lead, walk the rung, find the transition from voltage-present to voltage-absent.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Practice voltage measurements with the in-simulator multimeter →Frequently asked questions
How do you measure voltage on a control circuit?
Set the multimeter to AC volts, connect the COM lead to L2 or X2 as a reference, and probe each terminal in the control rung with the red lead. Each reading tells you whether the circuit path is complete from the source to that point. Voltage present means the path is intact. Voltage absent means the path is broken upstream of the probe.
What voltage should you read across a closed contact?
Zero volts, or very close to zero. A closed contact is a conductor. It has negligible resistance and drops no significant voltage. If you read full control voltage across a contact that should be closed, the contact is open or has failed.
What voltage should you read across an open contact?
Full control voltage. The open contact is the only break in the series path, so it drops all the source voltage. This reading confirms that the rest of the circuit is intact and this specific contact is the break point.
Why use L2 or X2 as the reference point?
L2 or X2 is the return conductor of the control circuit. On a transformer-fed circuit, X2 is typically the grounded conductor. On a line-voltage circuit, L2 is the return conductor but not a grounded conductor. Using the return side as a fixed reference means every measurement with the red lead reads the potential difference from the source through the circuit to that point. You only need to reposition one probe to trace the entire rung.
What does zero volts at L1 mean?
No power is reaching the starter. The disconnect is open, an upstream fuse or breaker has tripped, or the feeder is de-energized. Check the disconnect position and the upstream protective devices before troubleshooting the control circuit.
How does the simulator multimeter work?
The Electrimancer multimeter models the same point-to-point voltage measurement technique used in the field. You select two probe points on the workbench, and the meter reads the voltage between them based on the circuit's energized state. It responds to contact positions, wire connections, and power supply state, so you can practice the systematic tracing technique described in this article.