Troubleshooting Tools

The marine environment can be especially harsh on the components of your boat’s electrical system. When trouble occurs you will want the capability to make accurate and reliable measurements quickly. The basic tool for this job is a Fluke digital multimeter. Digital because of the better resolution… Fluke because it is more accurate, rugged and reliable.

This application note provides guidelines for testing some basic electrical components commonly found on inboard marine engines including batteries, starters, alternators, and ignition systems.

But the applications don’t end here. Once you own a Fluke digital multimeter, you will be able to check the wiring on your boat
trailer, perform corrosion potential testing on your zinc/bonding system, and even check the wiring in your house and car. When you think of the various electrical items that you want to add to your boat, you quickly realize that a good quality digital multimeter is an essential part of your boat’s tool kit. A list of recommended Fluke multimeters is included at the end of this application note.

Troubleshooting Tools

Work Safety

The voltages and currents present in electrical power systems can cause serious injury or death by electrocution. Consequently, when testing or troubleshooting, carefully adhere to all industry standard safety rules that apply to the situation. Read and follow directions and safety warnings provided by the equipment manufacturer.

Fluke cannot anticipate all possible precautions that you must take when performing the test described in this application note. At a minimum, however, you should:

  • Be sure that all power has been turned off, locked out, and tagged in any situation where you will be in direct physical contact with live circuit components – and be certain that the power can not be turned on by anyone but you.
  •  Use only well maintained test equipment. Inspect all test leads and probes and fuses before use. Repair or replace any test leads or probes with damaged insulation.
  •  Be very cautious when working on electrical systems when fuel vapors are present. Remember that vapor from gasoline and propane are heavier than air and will collect in the bottom of bilges and other closed compartments. Sparks generated by making connections with live electrical components can start a fire or cause an explosion when fuel vapors are present.
  •  Be aware that charging of unsealed, lead-acid batteries generates hydrogen gas. This hydrogen can explode if exposed to a spark generated at the battery terminals when connecting or disconnecting a battery charger. Always verify that the battery charger power is off before connecting or disconnecting the charger leads at the battery terminals.


Often the first sign of a battery problem will occur when the starter won’t turn the engine over. Use your multimeter to get a rough idea of the battery’s state of charge. To perform a no-load test, set the digital multimeter switch function to Volts DC (V—) and measure across the terminals.

The voltage test tells only the state of charge, not the battery condition. To gain additional information about the battery’s condition, test the specific gravity of the electrolyte in each cell using a hydrometer. If the specific gravity is low but relatively
the same across all cells, recharging may be able to bring the battery back to good health, unless the plates are sulfated. If one cell shows a specific gravity much lower than the rest, the cell is probably dead and recharging will not help.

In a lead-acid battery, each cell produces about 2.1 volts at full charge. Therefore, a 12V battery has 6 cells in series. If the no-load test reads 10V instead of 12V, a dead cell is likely and the battery should be replaced.Battery


A Digital Multimeter’s accuracy and display make regulator/alternator diagnosing and adjusting easy. First determine if the system has an integral (internal) regulator, then whether it’s type A or B*. Type-A has one brush connected to battery + and the other brush grounded through the regulator. Type-B has  one brush directly grounded and the other connected to the regulator.

Next, isolate the problem to alternator or regulator by bypassing the regulator (full-fielding). Ground Type-A field terminal. Connect Type-B field terminal to Battery +. If the system now charges, the regulator is faulty. Use a rheostat in series with the field connection if possible. Otherwise, just idle the engine (lights on) so the voltage doesn’t exceed 15V.

Good alternator

Verifying a Good Alternator

The battery must be fully charged (see Figure 1). Run the engine and verify that no-load voltage is 13.8 to 15.3V (check as in Figure 1). Next load the alternator by turning on dc loads such as lights, radio, etc. Run the engine at 2000 RPM. Check the current with an i410 or i1010 current clamp.Alternator

Checking Field Current

Worn brushes limit field current, causing low alternator output. To test: load unit as in Figure 3 and measure field current with current clamp or use 10A jack on DMM.Readings range from 3 to 7 amps. On integral GM units: with alternator not turning, jump terminals #1 & #2 together and connect both to Batt + with DMM in series set to measure 10 amps. Field current should be between 2 & 5 amps, higher current with lower battery voltage. Control battery voltage by turning on lights, radio, etc.Current

Checking Ripple Voltage

Ripple voltage (AC voltage) can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the “BAT” terminal on the back of the alternator, (not at the battery). A good alternator should measure less than 0.5 VAC with the engine running. A higher reading indicates damaged alternator diodes.Rip Voltage

Alternator Leakage Current

To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the engine is not running. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps. Use care when disconnecting the alternator output wire; make sure the battery is disconnected first. A leaking diode can discharge the
battery when the engine is off.


Fluke analog/digital multimeters can also be used for checking automotive capacitors (condensers). The movement of the bar graph will show that the DMM is charging the condenser. You’ll see the resistance increase from 0 to infinity. Be sure to switch the leads and check both ways. Also make sure to check condensers both hot and cold.Condenser Leakage

Spark Plug Wires

Most modern gasoline engines have resistance wire for the high voltage connections between the distributor cap to the spark plugs. The resistance reduces radio interference and produces a cleaner spark.

Plug wires should be checked for open circuits if they are more than a couple years old. Not all wires indicate the date they were
manufactured. Due to the heat of the spark plug insulator, a spark plug boot may bond to the spark plug. Pulling a spark plug boot straight off the spark plug can damage the delicate conductor inside the insulated wire. Rotate the boot to free it before pulling it off.

If you suspect bad wires, test the resistance of the wire while gently twisting and bending it. Resistance values should be about 10 kW per foot (30 kW per meter), depending on the type of wire being tested; some may be considerably less. You should compare readings to other spark plug wires on the engine to provide a relative reference for a typical good reading.