Won't Start But Tries vs Totally Dead: A Decision Tree

Why this matters

"It won't turn on" covers two completely different failures, and confusing them wastes hours. A unit that tries to start - hums, clicks, dims the lights, trips a breaker - is getting power and attempting to run. A unit that is totally dead - no sound, no light, no reaction - is not getting power at all, or its control circuit never closed. Sorting these two apart in the first thirty seconds tells you whether to chase the power supply or the load itself.

Start here: does it react when you call for start?

Stand at the equipment, call for it to start, and watch and listen. Pick a branch:

• Tries to start: any hum, buzz, click, momentary movement, light dimming, or a breaker trip. Power is arriving; the load or its start components are failing.
• Totally dead: nothing at all. No sound, no indicator, no breaker reaction. The problem is upstream - power, control circuit, or a safety lockout.

Do this safely. If a unit trips a breaker repeatedly, stop resetting it. A breaker that trips on a dead short or ground fault is protecting you; repeatedly resetting into a fault is dangerous. Leave it off and find the fault.

Branch A: it tries but won't run

The control circuit closed and power reached the load. The load cannot complete the start.

• If it hums and trips the overload or breaker, the load is drawing locked-rotor current and cannot turn. Suspect a seized bearing, a jammed mechanism, a shorted winding, or a failed start component.
• If a motor hums but does not spin, then spins freely by hand, suspect the start circuit: a weak or dead capacitor, an open start winding, or a failed start relay. Many single-phase motors will not start without a healthy start capacitor even though they run fine once nudged.
• If it clicks repeatedly (a relay or contactor chattering), the control voltage is marginal or the coil is failing. Measure the control voltage at the coil during the attempt.
• If lights dim hard and it strains, supply voltage may be sagging too low to start under load. Measure voltage at the moment of attempted start.

Branch B: it is totally dead

Nothing reacts, so trace power and control from the source toward the load.

• Confirm power is present first. Verify voltage at the disconnect or supply terminals with a meter. Do not assume the breaker being on means power is reaching the unit. A tripped breaker, blown fuse, or open disconnect kills everything downstream.
• If supply power is good, check the control circuit. Most equipment will not energize the load until a chain of switches, safeties, and a control signal all close. One open safety - a tripped high limit, an open float switch, a lockout, an interlock, or an off setting - leaves the unit fully dead.
• If there is a control transformer or low-voltage control, confirm it has output. A blown control fuse or dead transformer makes a healthy unit look completely dead.
• If power and control both reach the load and still nothing happens, suspect an open in the load itself or a failed switching device feeding it.

The trap: a safety lockout looks like a dead unit

A unit locked out on a safety can be totally silent and look like a power failure. Before condemning power, check for a tripped limit, a reset button that needs pressing, a lockout indicator, or an interlock that is not made (a panel removed, a door open, a switch off). Many "totally dead" calls are a single open safety in the control string.

Reading the difference

• Tries to start equals power present, load or start-components failing - work from the load back.
• Totally dead equals power or control circuit open - work from the source forward.
• Trips a breaker every time equals a hard fault (short or ground) - stop resetting and find it before powering up.

Decide the branch before you reach for a part. The reaction at start is the cheapest diagnostic you have.

References

• OSHA lockout/tagout and electrical safety practices
• NFPA 70B, recommended practice for electrical equipment maintenance
• Manufacturer wiring diagrams for the control and safety string
• See related: "Ground Fault vs Overload vs Short" and "Multimeter: Which Setting for This Test"