Intermittent Electrical Faults: The Wiggle Test & Beyond – Original Text

The Fault That Disappears When You Look At It

You’ve been there. Customer complains the heated seats cut out randomly. You test them—perfect. The dashboard goes dark occasionally. You check every fuse and relay—spotless. The parking sensors beep when they shouldn’t. You run diagnostics—no codes, everything nominal.

They drive away. Next week, they’re back. Same fault. Still intermittent.

Intermittent electrical faults are the ghosts of the automotive world. They appear just long enough to frustrate your customer, then vanish the moment the vehicle reaches your workshop. But here’s the truth: they’re not mysterious at all. 90% are connector or wiring issues, and there’s a systematic way to find them.

The Answer: The Wiggle Test (But Done Properly)

Wiggle every connector in the affected circuit while monitoring live data, voltage, or the component itself. When the fault appears or disappears during wiggling, you’ve found your problem.

No expensive diagnostic equipment required. No hours of wiring diagram tracing. Just methodical stress testing that replicates exactly what’s happening on the road.

Why This Works: The Physics of Failure

When we considerwhat happens to electrical connections in a vehicle:

Thermal Cycling:

• Engine bay temperatures swing from -10°C to 120°C (UK winter to hot summer engine bay)
• Connectors expand when hot, contract when cold
• Marginal connections break at temperature extremes
• Your workshop is not the real world, so the fault often doesn’t show up

Vibration Damage:

• Every pothole, speed bump, and rough road stresses connections and wiring looms
• Every where the wiring loom is secured to the chassis or drivetrain is a stress point
• Locking tabs wear, terminals lose tension
• Connections that look fine are actually intermittent
• Static testing in the workshop won’t reveal this

Corrosion:

• UK weather = moisture ingress
• Copper oxidises, creates high resistance
• Intermittent contact as oxide layer breaks and reforms
• Voltmeter shows voltage present, but connections can’t carry current

The wiggle test replicates the failure modes whilst you’re monitoring the system.

The Systematic Wiggle Test Procedure

Step 1: Set Up Monitoring

Before you touch anything, you need eyes on the fault:

Option A: Component-Level Monitoring

• Turn on the affected system (heated seat, radio, dashboard lights)
• Leave it running or cycling
• Watch for the fault to appear during wiggling
• Best for obvious failures (light goes out, motor stops)

Option B: Live Data Monitoring

• Connect scan tool to affected system
• Monitor sensor readings or module communication
• Watch for dropouts or erratic values
• Best for sensor circuits or CAN bus issues

Option C: Voltage/Current Monitoring

• Multimeter on voltage and or current at component
• Watch for drops or spikes during wiggling
• Best for power feed or ground issues
• Pro tip: Set multimeter to record min/max values
• If the component has failed, circuit testing can continue using an appropriate load (test light/load)

Step 2: Map the Circuit

Don’t just wiggle randomly. Work systematically:

1. Start at the component (the heated seat, the sensor, the module)
2. Work backwards towards the power source and ECU
3. Include every connection point:
   • Component connector
   • Intermediate connectors in the harness
   • Relay/fuse contacts
   • ECU connector
   • Ground points

Step 3: The Wiggle Technique

This isn’t just shaking things randomly:

For each circuit:

• Wiggle side-to-side (tests terminal tension)
• Push in and pull out slightly (tests locking tab)
• Flex the wiring either side (tests wire breakage near connector)
• Twist the connector (tests for worn housing)
• Use a heat gun to heat-load connectors and components
• Use a plumbers freeze spray to lower the temperatures of connectors and components
• Do this for 10-15 seconds per connector

When the fault appears:

• Stop immediately
• Mark the connector with tape
• Continue testing (there might be multiple faults)
• Come back to marked connectors after full circuit test

Step 4: Don’t Forget the Grounds

Here’s what catches out most technicians: intermittent grounds are more common than intermittent power feeds.

Every ground point in the affected circuit needs testing:

• Engine block grounds (corroded mounting points)
• Chassis grounds (painted or rusted contact surfaces)
• Shared grounds (multiple systems on one ground point)
• ECU grounds (high-resistance causing strange faults)

The killer tip: If multiple unrelated systems fail together, it’s almost always a shared ground. Check wiring diagrams for common ground points.

Advanced Diagnostic: When Wiggling Isn’t Enough

Technique 1: Thermal Stress Testing

Some faults only appear at temperature extremes:

Heat Testing:

1. Use a heat gun on suspect connectors/modules
2. Don’t exceed 80°C (plastic melts, you damage things)
3. Watch for fault to appear as temperature rises
4. Common culprits: soldered joints, ECU components, relays

Cold Testing:

1. Freeze spray or ice pack on suspect areas
2. Watch for fault to appear as components contract
3. Common culprits: connectors with worn terminals, cracked solder joints

Technique 2: Voltage Drop Testing

Wiggling found the suspect connector. Now prove it:

1. Connect multimeter across the connector (parallel to circuit)
2. Set to mV scale (you’re looking for millivolts)
3. Load the circuit (turn on the component)
4. Any reading above 0.2V = high resistance = your problem

Why this matters:

• A corroded connector might show 12V with a voltmeter
• But it can’t carry current (high resistance)
• Voltage drop testing reveals the problem
• This is how you prove the fault to the customer

Technique 3: The Road Test Wiggle

Some faults only appear under specific conditions:

1. Take a passenger (safety first)
2. Drive the route where the fault occurs
3. Passenger monitors scan tool or component
4. Driver recreates conditions (speed bumps, cornering, braking)
5. Note when fault appears
6. Back in workshop, focus on that specific stress condition

Real example: Rear parking sensors cutting out on left turns. Wiggle test in workshop—nothing. Road test revealed harness rubbing on rear subframe during body roll. 15 seconds to find once you knew what to look for.

Technique 4: Thermal Imaging

Some faults only appear after an overnight sit or during specific conditions. Thermal imaging can reveal hot and cold spots on relays, in fuse boxes on wiring looms, connector blocks and with components:

1. Overview of the affected area can be carried out quickly
2. Zoom in on fuses and relays. Cold signature = lower current. Hot spots = higher current
3. Control units and components give tell tales. Units drawing higher current give a hot spot signature.
4. Green stuff in a wire acts as a resistance. Resistances generate heat via electrical ‘friction’. Hot spots in looms show warm right before the problem area.

The “Impossible” Electrical Faults

Some faults seem to defy logic. Here’s what to check:

Fault Pattern 1: Multiple Unrelated Systems Failing

Most likely cause: Shared ground or power feed

Diagnostic approach:

• Wiring diagram time
• Find common connections between affected systems
• Test ground resistance (should be under 0.1Ω to chassis)
• Check battery earth strap (the most overlooked component)

War story: Van with intermittent failure of lights, radio, and dashboard. Wiggle test found nothing. Ground test revealed battery earth strap with internal corrosion—perfect continuity when cold, intermittent when hot. 30 minutes of testing, £15 part, problem solved.

Fault Pattern 2: Fault Appears Only When Wet

Most likely cause: Moisture ingress into connector

Diagnostic approach:

• Water spray test on suspect connectors
• Look for white/green corrosion inside connector
• Check for damaged connector seals
• Examine harness routing (is water pooling somewhere?)

The fix:

• Don’t just clean and refit
• Replace the connector (cheap insurance)
• Improve harness routing if needed
• Dielectric grease on terminals

Fault Pattern 3: Fault Appears Only At Specific RPM/Speed

Most likely cause: Vibration frequency resonance

Diagnostic approach:

• Note exact RPM/speed where fault occurs
• Suspect engine-mounted or transmission-mounted components
• Look for harnesses without proper support
• Check for wiring touching exhaust or moving parts

The fix:

• Secure harnesses with proper clips
• Add rubber grommets where harness passes through metal
• Replace work-hardened wiring (UK road salt accelerates this)

Fault Pattern 4: No Fault Codes, But Module Functions Intermittently

Most likely cause: CAN bus communication issue

Diagnostic approach:

• Check CAN bus termination (120Ω at diagnostic connector)
• Scope CAN-H and CAN-L signals (should be clean differential pair)
• Wiggle test every module connector in the CAN network
• Check for aftermarket accessories causing interference

The killer clue: If multiple modules lose communication together, it’s the CAN bus trunk wiring or a shared ground, not individual modules.

The Tools You Actually Need

Essential (you probably own these):

• Digital multimeter with min/max recording
• Thermal imaging camera
• Basic scan tool with live data
• Test light / test load (yes, really—sometimes the simplest tool is best)
• Your hands (the mechanics of the wiggle test )

Highly Recommended:

• Wiring diagram access (Autodata, Haynes Pro, manufacturer subscription)
• Heat gun (£25 from any DIY shop)
• Freeze spray (£10 from any DIY shop)
• Dielectric grease and contact cleaner
• Terminal files
• Decent headtorch (free both hands for wiggling and monitoring)

Advanced (if you specialise in electrical work):

• Oscilloscope (for CAN bus analysis and ignition testing)
• Current clamp (for parasitic draw and circuit loading tests)
• Signal generator (for testing sensors and modules)
• Thermal camera (instant identification of high-resistance connections)

Common Mistakes That Waste Hours

Mistake 1: Wiggling Without Monitoring

Wiggling blind is pointless. You must have eyes on:

• The component itself, or
• Live scan data, or
• A voltmeter reading

Otherwise you’ll wiggle right past the fault and never know it.

Mistake 2: Only Testing In Workshop Conditions

Workshop = 18°C, dry, smooth floor, stationary vehicle.

Customer driving = -5°C to 30°C, wet, speed bumps, vibration, engine heat.

If you can’t recreate their conditions, you might not find their fault.

Mistake 3: Fixing The Symptom, Not The Cause

You found a dodgy connector on the heated seat module. Don’t just clean it and move on.

Ask why:

• Why did it corrode? (water ingress)
• Why is there water ingress? (failed seal, poor harness routing)
• If you don’t fix the root cause, it’ll be back
• The same goes for wire failures. Don’t just repair the single wire, look for potential issues in adjacent wires. You found the biggest problem – look for the future problems too.

Mistake 4: Replacing Modules Without Testing Connectors First

ECUs and modules are expensive. Connectors are cheap.

Always test:

• Power feed voltage under load
• Ground resistance
• Connector terminal condition
• Module connector pinout (check for corrosion or backed-out pins)

War story: Technician replaced £800 ABS module for intermittent fault. Customer back two weeks later, same fault. Wiggle test found corroded connector. £800 module was fine all along.

Mistake 5: Not Documenting What You Find

Intermittent faults are hard to replicate. When you find one:

• Photograph the faulty connector
• Note the conditions when it failed
• Document the fix (helps with comebacks)
• Update your own fault database (future you will thank you)

Pro Tips From 30 Years of Electrical Diagnosis

Tip 1: The “Newspaper Test” for Vibration

Can’t recreate the vibration that causes the fault?

1. Roll up a newspaper
2. Gently tap suspect areas while monitoring
3. Replicates road vibration without damage
4. Surprisingly effective for finding loose connections

Tip 2: The “Hairdryer Method” for Thermal Testing

Don’t have a heat gun? Use a hairdryer.

• Safer (lower temperature)
• More controlled airflow
• You can borrow one from the office
• Works perfectly for connector testing

Tip 3: The “Multimeter Alarm” Trick

Most multimeters have a continuity beeper:

1. Set it to continuity mode
2. Connect across the circuit
3. Walk away and do other jobs
4. When the beep stops = you’ve found the intermittent break

Free hands for other work while you wait for the fault.

Tip 4: Check The Obvious First

Before you spend an hour wiggling:

• Battery terminals tight and clean?
• Main earth strap good?
• Alternator charging properly?
• No aftermarket accessories poorly installed?

20% of “mysterious electrical faults” are just loose battery terminals.

Tip 5: Learn Your Common Faults

Every vehicle has known weak points:

• BMW door lock modules (water ingress)
• VAG coil pack connectors (heat damage)
• Ford rear lamp clusters (corrosion)
• Land Rover body grounds (corrosion, obviously)

Know your common faults. Start there. Save hours.

When To Give Up (And What To Do Instead)

Some intermittent faults are genuinely difficult to replicate. If you’ve spent 2 hours and found nothing:

Option 1: Fit A Data Logger

• Modern scan tools can record intermittent fault codes
• Customer drives with logger fitted
• Fault records with conditions (RPM, speed, temperature)
• Come back with actual data to work from

Option 2: Repair The Most Likely Suspect

• You’ve narrowed it to 2-3 possible connectors
• Customer needs the vehicle back
• Repair the most likely culprit preventatively
• Document what you’ve done (for comeback management)
• Charge accordingly (you’ve done diagnostic work)

Option 3: Be Honest With The Customer

• Some faults need to get worse before they’re findable
• Explain this to the customer
• Give them a plan (“if it happens again, try to note exact conditions”)
• Don’t guess and replace expensive parts

Your reputation is built on honesty, not miracle fixes.

The Next Step: CAN Bus Diagnostics

If your wiggle test revealed communication faults or multiple modules misbehaving, you’re likely dealing with CAN bus issues. This requires more advanced techniques:

• Termination resistance testing
• Differential voltage measurement
• Signal waveform analysis
• Bus topology understanding

Read our CAN Bus Fault Finding Guide → (coming soon)

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Key Takeaways: Electrical Faults

✅ 90% of intermittent electrical faults are connectors or wiring – not modules

✅ Wiggle test while monitoring – eyes on the fault, hands on the circuit

✅ Check grounds first – especially if multiple systems affected

✅ Thermal and vibration stress testing – replicate real-world conditions

✅ Voltage drop testing proves the fault – shows high resistance connections

✅ Document everything – intermittent faults are hard to replicate