Short answer
Measure voltage across each connection while the circuit is under load, not at rest. Put one probe either side of a joint, crimp or switch: a healthy connection shows a very small drop, while a corroded one shows a significant fraction of system voltage. Work along the circuit until you find the joint that is eating it.
You will need: Multimeter, torch, wire brush, contact cleaner
Marine DC faults are usually resistance, not breakage. A corroded joint passes enough current to light a panel lamp and nowhere near enough to turn a starter — which is why the panel looks fine while nothing happens.
The method
The test only works under load. At rest, a badly corroded joint reads perfectly.
- Put the circuit under its working load — cranking for a starter circuit, running for a charging one.
- Place probes on either side of a single joint, crimp, switch or length of cable.
- Read the voltage difference. Small is healthy; a meaningful fraction of system voltage is your fault.
- Work along the circuit joint by joint, including the earth return, which is where a surprising share of marine faults live.
Where to look first
Battery posts and clamps, the isolator switch, crimps that were made without the right tool, the engine earth strap, and anything that has been under water or spray. Corrosion often hides under insulation and heat-shrink, so a joint that looks clean can still be the culprit.
Fixing it properly
Clean and remake the connection with marine-grade terminals and the correct crimping tool, protect it, and re-test under load. A smeared-on fix in a damp bilge is a fault you will find again next season.