The Wiggle Test for Intermittents

Why this matters

Intermittent faults are the ones that ruin your day and your reputation. The system fails, you arrive, it works perfectly, you leave, it fails again. Nothing tests bad because the fault is not present while you are watching. Most intermittents come down to a connection that is making and breaking: a loose terminal, a corroded contact, a cracked solder joint, a marginal crimp, a wire flexing where it enters a connector. The wiggle test is how you force that fault to show itself on demand instead of waiting for it to happen on its own. It is a staple across every trade that deals with wiring, plumbing connections, and mechanical linkages.

The core idea: make the fault happen on purpose

An intermittent that comes and goes on its own can usually be provoked. If you can put a meter or an indicator on the circuit so you can see the fault the instant it occurs, then physically disturb the suspect areas one at a time, you can catch the exact spot that triggers it. The principle is to convert "it fails sometimes" into "it fails when I touch this," which is a finding you can repair.

This works because most intermittents are mechanical at heart. A connection that is barely making contact will break contact when flexed, tapped, heated, or cooled. Disturb it deliberately and the fault appears under your hand.

Set up to see the fault

You cannot wiggle-test what you cannot observe, so instrument first.

• Put a meter on the circuit in a mode that shows the fault. Continuity or resistance across a suspect connection, or voltage across a junction, set so a momentary open or a jump in resistance is visible. A meter that captures a brief min/max excursion helps, because the fault may last only a fraction of a second.
• Or watch the system's own behavior. If the symptom is a device cutting out, run it and watch the device while you disturb the wiring. The device itself becomes your indicator.
• For non-electrical intermittents, watch the relevant gauge or output: a pressure reading, a flow, a level, while you disturb fittings and linkages.

Safety first for energized work

If the circuit must be live to show the fault, set up to do it safely. Where you can find the fault with the circuit dead (continuity across a connection, for example), de-energize and verify dead first and test it cold. When the fault only appears energized, plan the measurement, keep one hand clear, use rated tools and insulated probes, and never let the wiggling put you into contact with a live conductor. A live wiggle test on a high-energy circuit is a real hazard; treat it like one.

Step by step

1. Reproduce or get close to the fault condition if you can, so the marginal connection is near its breaking point. Some faults only show warm, or only under load, or only at a certain position.
2. Start at the most likely spots. Connectors, terminals, splices, anywhere a wire flexes or enters a fitting, recently serviced joints, and anything visibly corroded, loose, or discolored.
3. Disturb one area at a time, gently and deliberately. Wiggle the wire, flex the harness, tap the connector, push and pull the terminal, work the linkage. Move one thing, watch the indicator, then move the next. Change one spot at a time so the reading points at exactly that spot.
4. Watch the indicator the instant you disturb each point. A flicker on the meter, a momentary open, a jump in resistance, the device cutting out, the gauge twitching. That reaction marks the fault.
5. Add other stressors if wiggling alone will not provoke it. Gentle heat or cold on a suspect joint (within safe limits) can expose a connection that only fails at temperature. Load can expose one that only fails under current or pressure.
6. Confirm by repeating. A real find reacts every time you disturb that one spot and stays quiet when you disturb its neighbors. One lucky flicker is not proof; repeatable provocation is.

Reading and fixing the result

When one specific point reliably triggers the fault and the others do not, you have found it. Repair the connection properly: re-terminate, re-crimp, clean and tighten, replace the connector, re-solder, or replace the flexing conductor, depending on what you found. Do not just snug it and hope; a connection that went marginal once will do it again if you only nudge it.

After the repair, run the wiggle test again on the fixed spot to confirm it no longer reacts, and disturb the surrounding areas to make sure there is not a second marginal connection hiding behind the first. Document where the fault was and what you did, because the next intermittent on that system will be tempting to blame on the same spot whether or not it deserves it.

References

• See related: The Power Supply vs the Load: A Decision Tree
• See related: Proving a Circuit Dead the Right Way
• NFPA 70E (electrical safety for any energized testing)
• Trade-standard troubleshooting practice (provoking intermittent faults at the connection)