Runs When Jumpered But Not on Control: A Decision Tree
Why this matters
A load runs fine when you jumper across its control terminals but will not run on its own control wiring. This is one of the most useful diagnostic splits in the trade because it cleanly separates the power side from the control side: the motor, contactor, and power conductors are proven good the instant it runs jumpered, so the fault has to be in the control circuit, the sensing that closes it, or the supply to it. Misreading this leads techs to condemn good contactors and motors. Understanding the split turns a vague no-run complaint into a short list of control-circuit suspects.
Symptom presentation
- The load starts and runs when the control terminals are jumpered or the contactor is manually pushed in.
- On its own controls (a switch, thermostat, pressure switch, float, timer, or PLC output), it does nothing.
- No fuse blows and no breaker trips when it fails to start on control.
- The contactor coil does not pull in, or it chatters, when called normally.
What the jumper test proves
Jumpering across the contactor or control terminals bypasses the entire control logic and forces the coil to energize (or forces the power contacts closed). When the load then runs, you have proven the line-side power, the contactor power contacts, the overloads (if not the cause), and the motor are all healthy. The fault is therefore upstream of the jumper in the control path: the control transformer or control voltage source, the control conductors, the controlling device (switch, sensor, relay), or the coil itself. The jumper test is a deliberate divide-and-conquer that isolates power from control in one move.
Quick checks
- Identify the control voltage and source. Many controls run on a low-voltage control transformer (commonly 24 V or 120 V control on a 240/480 V power circuit). Measure the control voltage at its source with the system calling.
- Determine what the jumper bypassed. Jumpering the coil terminals tests the coil and downstream; jumpering across the controlling contact tests only that contact. Know which you did.
- Verify the call signal. With the controlling device asking for run, measure voltage at the coil terminals. Control voltage present at the coil but no pull-in means a bad coil; no voltage at the coil means an open in the control path.
Isolation tree
Control voltage absent at its source. The control transformer is dead (failed transformer, blown control fuse, open primary). Measure the transformer primary and secondary. Replace the fuse or transformer and find why it failed.
Control voltage present at source, absent at coil on a call. An open in the control path. Walk the control circuit from the source through each series device (disconnect interlock, overload auxiliary contact, the controlling switch or sensor, safety contacts) measuring across each. Voltage drops to zero across the open device or open conductor. That is the fault.
Control voltage present at the coil on a call, coil does not pull in. The coil is open or burned, or the contactor is mechanically jammed. Read coil resistance with power off; an open coil reads infinite, a shorted coil reads abnormally low. Replace the coil or contactor.
Coil pulls in on a call but the load still will not run on control, runs on power-contact jumper. The pull-in is marginal or the contacts are not transferring. Suspect low control voltage under load (an undersized transformer sagging when the coil draws inrush) or worn power contacts that the manual jumper bridged. Measure control voltage at the instant of the call, not at rest.
Controlling sensor or switch never closes. The logic device is the fault: a stuck float, a failed pressure switch, a thermostat not calling, a PLC output not energizing, or a tripped safety that is correctly preventing operation. Verify the sensor is actually supposed to be calling before forcing it.
Confirming diagnosis
Confirm by measuring control voltage at three points while the system is called: the control source, the controlling device's load side, and the coil terminals. The point where voltage disappears is the open, and the device immediately downstream of the last good reading is the fault. If voltage reaches the coil and the coil still does not actuate, an off-power coil-resistance reading confirms an open or shorted coil. If a safety device is open (an overload auxiliary, a high-limit, an interlock), confirm whether it is faulty or correctly protecting the equipment before defeating it; jumpering a safety to make the load run is not a repair.
Jumpering controls energizes power circuits and starting motors. Never jumper out a safety interlock, an overload, a high-limit, or a pressure/temperature safety to keep a machine running; those devices prevent injury and equipment destruction, and a jumpered safety can start machinery unexpectedly or run it past a dangerous limit. Use the jumper strictly as a momentary diagnostic, then remove it. Treat all control and power conductors as live, use category-rated instruments, and follow NFPA 70E for energized work and motor-control hazards including unexpected startup.
Remediation
- Replace a failed control transformer or control fuse and correct what caused it to fail.
- Re-terminate or replace the open control conductor or failed controlling device.
- Replace an open or shorted contactor coil, or the contactor if power contacts are worn.
- Replace a failed sensor or switch; restore, do not defeat, any safety device that was correctly open.
- Re-verify operation on the normal control path under load and confirm the coil pulls in cleanly.
References
- NFPA 70, National Electrical Code, Article 430 (motors, motor circuits, and controllers).
- NFPA 70, National Electrical Code, Article 725 (control and signaling circuits).
- NFPA 70E, Standard for Electrical Safety in the Workplace (energized work, unexpected startup, motor-control hazards).
- UL 508A, Industrial Control Panels (control-circuit and contactor application).