REFEVIA Marine Brief · Issue 01Marine directory ↗
Marine electrical · Field diagnostics · September 25, 2026

Find the voltage drop before replacing the electronics.

A repeatable 12 V DC test for service technicians: compare voltage at the battery posts and at the equipment terminals while the circuit carries its normal load.

Technical brief for marine electricians, yard technicians, and service teams · 5 minute read

The fault path

A chartplotter that resets when another load starts may have adequate battery voltage but insufficient voltage at its own terminals. Resistance in the supply or return path can produce a drop only when current flows. A no-load reading alone cannot rule out a poor connection.

12 volt DC supply and return pathBattery positive passes through a fuse, switch, and load, then returns to battery negative. Both legs and each connection can contribute to voltage drop.Battery+ / − postsFuseSwitchElectronicsLoad terminalsReturn conductor and connectionsPositive supply conductor and connectionsDiagram is conceptual; actual distribution layouts vary.
Illustration 1. Follow the complete circuit, including the return. Fuse holders, switches, terminals, and crimps each add possible resistance.

The loaded measurement

  1. Identify the circuit and its rated operating range. Confirm the equipment manual, fuse, and system voltage. This procedure concerns low-voltage DC circuits; isolate shore power and AC work from the test.
  2. Run the affected equipment under a stable, representative load. Measure across the actual battery posts, then across the equipment’s positive and negative input terminals with the same load operating.
  3. Subtract the readings. The difference is the combined supply and return loss at that load. Record load state, both voltages, and the measured current if available.
  4. Locate the loss. A qualified technician can compare voltage across individual conductors or connections under load to find the segment causing the largest drop. De-energize before cleaning, tightening, or replacing parts; follow the component maker’s torque instructions.
Worked voltage drop comparisonBattery-post reading 12.6 volts, equipment-terminal reading 11.9 volts under the same load. Difference 0.7 volts, about 5.6 percent of 12.6 volts.At battery posts12.6 VAt equipment terminals11.9 V0.7 V lost · 5.6% of 12.6 V
Illustration 2. Example readings only. The measurements must be made under the same operating conditions; a 0.7 V difference points to a path worth investigating, not a battery diagnosis by itself.

Interpret and correct

Blue Sea Systems uses 3% allowable voltage drop for critical circuits such as electronics, navigation lights, bilge blowers, and panel feeders when sizing conductors; it lists 10% for selected noncritical loads. In the example above, 0.7 ÷ 12.6 is about 5.6%, above that 3% design target. This comparison is a screening clue, not a substitute for the equipment’s own voltage specification or a full circuit assessment.

Inspect the segment with the measured loss for corrosion, loose connections, poor crimps, undersized conductors, and excessive run length. Size a replacement using the round-trip length, current, installation environment, ampacity, and appropriate circuit protection. Do not increase a fuse rating to mask the fault.

Field note

If the battery-post voltage itself collapses under load, investigate the battery, charging system, and load separately. If battery voltage remains comparatively steady while voltage at the equipment falls, focus on the distribution path.

Technical references

Blue Sea Systems: voltage drop and wire sizing · Choosing DC wire size · Victron Energy: wiring and voltage drop. The diagrams and worked numbers above are REFEVIA illustrations, not copied manufacturer figures.

02
Bilge systems · Hose routing

A running pump with no discharge

A centrifugal bilge pump can run normally while an air pocket and trapped water prevent useful flow. Confirm the actual outlet flow, not just motor sound.

What to inspect

Follow the discharge hose from pump to outlet. Xylem’s Rule demonstration shows how a sag forms a water trap: after the pump stops, retained water can leave air at the pump, and the next cycle may fail to prime. Route the line continuously upward where the pump maker and vessel layout permit. Verify the float can move freely and inspect the strainer and hose for obstruction, kinks, or collapse.

Service sequence

  1. Check the pump’s automatic and manual controls and observe water leaving the outlet during a controlled water test.
  2. Trace every low spot in the hose; correct routing and secure the hose without creating another sag. Check the actual installation instructions for any vented loop or anti-siphon requirement.
  3. Repeat the fill, start, discharge, stop, and restart cycle. Record whether it primes reliably on the second cycle.

Source: Xylem Rule airlock demonstration and Rule float switch instructions.

Bilge hose route comparisonTop line rises continuously from pump to outlet. Lower line sags and traps water, potentially causing an airlock.PumpOutletContinuous risePumpWater trapSag can retain water
Illustration 3. Conceptual hose profiles; verify required routing and outlet geometry for the specific vessel and pump.
03
Diesel service · Fuel supply

When a filter will not hold prime

After filter service, repeated air in the fuel supply calls for a seal and routing check before another element is installed.

Trace the suction side

For a typical Racor suction-side installation, fuel moves from tank through the separator to the transfer pump and engine. Parker’s troubleshooting sequence starts at the vent plug, drain valve, fittings, filter head, and bowl. It then checks fuel lines for pinches or unnecessary bends and the tank pickup for restriction. A new element alone will not correct an unsealed drain or a loose lid.

Service sequence

  1. Identify the exact filter model and engine priming instructions. Shut down and cool the engine; ventilate and keep ignition sources away.
  2. Inspect the bowl, lid, drain, and vent for correct seating and seal condition. Drain collected water into an appropriate container using the model-specific procedure.
  3. Prime as the filter and engine manuals direct. Run the engine, inspect for leaks, and correct leaks with the engine off. Log the model, element, symptom, and post-service result.

Source: Parker Racor 400 Series installation and troubleshooting. Procedures vary by filter model.

Typical fuel supply flowFuel tank flows to separator, then transfer pump and engine. Drain and vent seals are highlighted for prime-loss inspection.TankFilter /separatorPumpEngineVent / lid sealDrain / bowl sealTypical suction-side layout; confirm actual plumbing and filter manual.
Illustration 4. Map the fuel path before diagnosing the loss of prime. Seals at the separator are key inspection points.
04
Sonar · Installation geometry

Depth readings lost at speed

When bottom tracking is stable at idle but fails as the vessel accelerates, document the operating condition and inspect transducer water flow before replacing hardware.

Look upstream of the sensor

Garmin’s placement guide notes that strakes, steps, rivets, fittings, intakes, discharges, and propeller wash can introduce bubbles and turbulence. A transom transducer needs smooth water at the speed where the customer expects a reading. The correct position depends on hull material, propulsion, deadrise, and the specific transducer instructions.

Controlled comparison

  1. Record depth behavior at idle, transition, and operating speed, along with vessel loading and trim. Photograph the transducer and hull features ahead of it.
  2. Inspect bracket condition, orientation, immersion, cable routing, and any upstream turbulence source. Do not assume a menu setting fixes a flow problem.
  3. Use the model-specific placement and installation guide for adjustment; water test again under comparable loading and speed. Record the before-and-after result.

Source: Garmin Transducer Placement Guide. Its dimensional guidance applies to the installation types described there; follow the actual device manual.

Transducer and turbulent waterWater flows aft under the hull. A strake upstream generates bubbles near one candidate sensor location; another candidate position has smoother flow.Hull bottomWater flow aft →StrakeBubble-affectedSmooth flowConceptual view only; actual location requires model-specific guidance.
Illustration 5. Air from an upstream hull feature can degrade a downstream sensor. The drawing is conceptual, not a mounting template.
Beyond the workbench

Events, weather, and marine work

Three useful starting points for planning the next job, finding the industry, and keeping current with conditions. Check the original source before acting on dates, forecasts, or openings.

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Calendar

Events

Browse boat shows and marine industry gatherings. Confirm schedules, venue details, and registration with the organizer.

Explore marine events →
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Conditions

Weather

Start with marine forecasts and official condition resources before a service visit, delivery, sea trial, or day underway.

Check marine weather →
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Careers

Jobs

Explore marine employment and hiring resources, from technical service to operations. Confirm each opportunity with the employer.

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Images are illustrative and do not depict a specific forecast, event, or job opening.

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