How humidity failures show up
Electrical equipment on board lives with humidity, salt mist, oil and vibration. Failures rarely happen suddenly. Typical early signs are weak cranking, a solenoid that clicks repeatedly, low charging current, voltage fluctuation, insulation alarms and discoloured terminals. Most of these come from reduced insulation resistance and oxidised contacts, not from burnt windings.
Four failure points on starters
- Solenoid contacts — oxidation or burning raises contact resistance; the motor clicks but does not turn.
- Brushes and commutator — worn brushes and carbon build-up cause weak cranking with sparking.
- Bearings and seals — grease washed out by seawater leaves the motor stiff.
- Terminals — oxidised copper and loose bolts heat up and degrade insulation over time.
Four failure points on alternators
- Insulation resistance — cold readings drop after humid spells; dry out and re-test.
- Rectifier diodes and AVR — excitation faults cause unstable or absent output.
- Bearings — noise and temperature rise; neglect leads to rotor damage.
- Terminal box seals — ageing seals let moisture reach the windings directly.
Troubleshooting order
- Measure insulation resistance cold and hot; keep a trend record.
- Check terminals and contacts for heat, oxidation and looseness.
- Inspect brushes, commutator and bearings.
- Only then suspect windings and electronic modules, avoiding unnecessary replacement of major parts.
Reducing humidity failures
- Use anti-condensation heaters or periodic energisation while alongside.
- Replace terminal box seals annually rather than waiting for water ingress.
- Run idle motors briefly once a month.
- Store spares dry and check rubber part dates.
For starter motors, alternators, brushes, AVRs and rectifier modules, see Generator & Starter Motor and Marine Electrical Equipment, or contact us with nameplate details and symptoms.