Application Note

TroubleshootingMarine Engine

Electrical Systems

Troubleshooting toolsThe marine environment can beespecially harsh on the componentsof your boat’s electrical system.When trouble occurs you will wantthe capability to make accurate andreliable measurements quickly. Thebasic tool for this job is a Flukedigital multimeter. Digital becauseof the better resolution... Flukebecause it is more accurate, ruggedand reliable.

This application note providesguidelines for testing some basicelectrical components commonlyfound on inboard marine enginesincluding batteries, starters, alter-nators, and ignition systems.

But the applications don’t endhere. Once you own a Fluke digitalmultimeter, you will be able tocheck the wiring on your boattrailer, perform corrosion potentialtesting on your zinc/bonding sys-tem, and even check the wiring inyour house and car. When youthink of the various electrical itemsthat you want to add to your boat,you quickly realize that a goodquality digital multimeter is anessential part of your boat’s tool kit.A list of recommended Flukemultimeters is included at the endof this application note.

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Work safetyThe voltages and currents present inelectrical power systems can causeserious injury or death by electrocution.Consequently, when testing or trouble-shooting, carefully adhere to all industrystandard safety rules that apply to thesituation. Read and follow directions andsafety warnings provided by the equip-ment manufacturer.

Fluke cannot anticipate all possibleprecautions that you must take whenperforming the test described in this ap-plication note. At a minimum, however,you should:

• Be sure that all power has been turnedoff, locked out, and tagged in anysituation where you will be in directphysical contact with live circuitcomponents - and be certain thatthe power can not be turned on byanyone but you.

• Use only well maintained test equip-ment. Inspect all test leads and probesand fuses before use. Repair or replaceany test leads or probes with damagedinsulation.

• Be very cautious when working onelectrical systems when fuel vapors arepresent. Remember that vapor fromgasoline and propane are heavier thanair and will collect in the bottom ofbilges and other closed compartments.Sparks generated by making connec-tions with live electrical componentscan start a fire or cause an explosionwhen fuel vapors are present.

• Be aware that charging of unsealed,lead-acid batteries generates hydrogengas. This hydrogen can explode ifexposed to a spark generated at thebattery terminals when connecting ordisconnecting a battery charger.Always verify that the battery chargerpower is off before connecting ordisconnecting the charger leads at thebattery terminals.

BatteriesOften the first sign of a battery prob-lem will occur when the starterwon’t turn the engine over. Use yourmultimeter to get a rough idea of thebattery’s state of charge. To performa no-load test, set the digital multi-meter switch function to Volts DC(V---) and measure across the termi-nals. Compare your readings to thegraph in Figure 1.

The voltage test tells only thestate of charge, not the battery con-dition. To gain additional informationabout the battery’s condition, testthe specific gravity of the electrolytein each cell using a hydrometer. Ifthe specific gravity is low but rela-tively the same across all cells,recharging may be able to bring thebattery back to good health, unlessthe plates are sulfated. If one cellshows a specific gravity much lowerthan the rest, the cell is probablydead and recharging will not help.

In a lead-acid battery, each cellproduces about 2.1 volts at fullcharge. Therefore, a 12V battery has6 cells in series. If the no-load testreads 10V instead of 12V, a dead cellis likely and the battery should bereplaced.

Sulfated plates can be detectedby measuring the output voltageunder load (after the battery hasbeen charged). Typical results for agood battery are given in Figure 2below.

Load test @ 1/2 CCA Rating8.5V @ 0 °F (-18 °C)8.8V @ 30 °F (-1 °C)9.4V @ 50 °F (10 °C)9.6V @ 70 °F (21 °C)

Figure 2: Battery test voltages for a goodbattery at 1/2 cold cranking ampere rating.

AlternatorsA Digital Multimeter’s accuracy anddisplay make regulator/alternatordiagnosing and adjusting easy. Firstdetermine if the system has an inte-gral (internal) regulator, thenwhether it’s type A or B*. Type-Ahas one brush connected to battery+ and the other brush groundedthrough the regulator. Type-B hasone brush directly grounded and theother connected to the regulator.

Next, isolate the problem to alter-nator or regulator by bypassing theregulator (full-fielding). GroundType-A field terminal. ConnectType-B field terminal to Battery +.If the system now charges, the regu-lator is faulty. Use a rheostat in se-ries with the field connection ifpossible. Otherwise, just idle theengine (lights on) so the voltagedoesn’t exceed 15V.

Figure 1: Measuring System VoltageMeasure the voltage across the batteryterminals (see chart). Voltage tests only tell thestate of charge, not the battery condition.

*Measured after battery has been at rest for atleast 24 hours.

*Type A is sometime referred to as P-Type andType B is sometimes referred to as N-Type

Figure 5: Checking ripple voltageRipple voltage (AC voltage) can be measured byswitching your DMM to AC and connecting the blacklead to a good ground and the red lead to the “BAT”terminal on the back of the alternator, (not at thebattery). A good alternator should measure less than 0.5VAC with the engine running. A higher reading indicatesdamaged alternator diodes.

Figure 4: Checking field currentWorn brushes limit field current, causing low alternatoroutput. To test: load unit as in Figure 3 and measure fieldcurrent 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 & #2together and connect both to Batt + with DMM in seriesset to measure 10 amps. Field current should be between2 & 5 amps, higher current with lower battery voltage.Control battery voltage by turning on lights, radio, etc.

Figure 3: Verifying a good alternatorThe battery must be fully charged (see Figure 1). Run theengine and verify that no-load voltage is 13.8 to 15.3V(check as in Figure 1). Next load the alternator by turningon dc loads such as lights, radio, etc. Run the engineat 2000 RPM. Check the current with an i410 ori1010 current clamp.

2 Fluke Corporation Troubleshooting Marine Engine Electrical Systems

Figure 6: Alternator leakage currentTo check alternator diode leakage, connect themultimeter in series with the alternator output terminalwhen the engine is not running. Leakage current shouldbe a couple of milliamps at most; more often, it will beon the order of 0.5 milliamps. Use care when discon-necting the alternator output wire; make sure the batteryis disconnected first. A leaking diode can discharge thebattery when the engine is off.

Starting systemsStarting system troubles are oftenconfused with charging systemproblems. Many dead batteries havebeen replaced when the real causewas a faulty charging system. Besure that the charging system isfunctioning properly before youreplace the battery. Make sure thebattery is charged and passes a loadtest, then look for resistance in thestarter circuit if the engine stillcranks slowly.

Investigate excessive currentdraw; check for worn-through insu-lation, a seized or tight engine, afaulty starter, etc. If the starter turnsthe engine slowly, the current drawis not high, and the battery is ingood condition, check the resistancein the starter circuit.

Troubleshooting Marine Engine Electrical Systems Fluke Corporation 3

Figure 7: Measuring starter current drawDetermine how much current the starter isdrawing by using Fluke’s i1010 InductiveCurrent Clamp on the starter cable. Thisaccessory will allow the multimeter to measurestarter current up to 1000 amps. Checkmanufacturer’s specs for exact figures.

Starter circuit voltage dropOhm’s law (E=IxR) tells us that evenvery low resistance in the startercircuit will cause the starter to turnslowly, because of low voltage. Forexample: in a system drawing 200amps, 0.01 ohms resistance in thestarter cable will cause a 2 volt dropin voltage at the starter; 0.01 ohmsis too little for all but the mostexpensive and sophisticated ohm-meters to measure, but measure-ments of voltage drop will indicatewhere there is excessive resistance(Figure 8).

Ignition systemsFluke digital multimeters will mea-sure from tenths of an ohm to sev-eral million ohms, making ignitiontests easy to interpret. If your enginehas breaker points in the distributor,use your multimeter to measure theresistance across the contacts whenthe points are closed. The readingshould be very low, typically 0.1 to0.3 ohms. Subtract the test leadresistance for best accuracy.

The primary and secondarywindings in the ignition coil can alsobe measured as described in Figure9. Use the manual ranging featureon the multimeter to avoid any oscil-lations between ranges that cansometimes be caused by the induc-tance of the coil. Ballast resistors inseries with the coil primary can alsobe measured. Look for low values inthe range of 0.5 ohm.

Figure 8: Testing for excessive voltage dropDetermine if there is resistance in the circuit bymeasuring the voltage drop across each connection andcomponent in the starter circuit while cranking theengine. Measure the voltage drop between the batterypost and the connecting cable, the solenoid posts and thewires that attach to them, and across the solenoid itself.Also check the connection on the starter, alternator (feedand ground side) and the ground strap connection to theengine block. A logical test sequence would be to start byfirst measuring the battery voltage between + and -terminals when the starter is cranking. Then measure thevoltage between the starter terminal and engine blockwhen cranking. If the starter voltage is significantly less,use the above procedure to isolate the voltage drop.

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Figure 9: Measuring internal coil resistanceIf you suspect a malfunctioning ignition coil, check theresistance of primary and secondary windings. Do thiswhen the coil is hot, and again when it is cold. Alsomeasure from the case to each connector. The primarywindings should have a very low resistance, typicallyfrom a few tenths of an ohm to a few ohms. Thesecondary windings have a higher resistance, typically inthe 10 kΩ to 13 kΩ range. To get the actual figures for aspecific coil, check the manufacturer’s specs. But as a ruleof thumb, primary windings range from a few tenths ofan ohm to a few ohms, and secondary winding may be10 kΩ or more.

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Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:U.S.A. (800) 443-5853 orFax (425) 356-5116Europe/M-East (31 40) 2 678 200 orFax (31 40) 2 678 222Canada (905) 890-7600 orFax (905) 890-6866Other countries (425) 356-5500 orFax (425) 356-5116Web access: http://www.fluke.com

©1998 Fluke Corporation. All rights reserved.Printed in U.S.A. 11/98 B0270UEN Rev B

Printed on recycled paper.

Fluke Multimeters for Marine ApplicationsThe following Fluke Digital Multimeters arerecommended for use in marine applications.

Fluke 36ClampMeter

• True-rmsresponding

• AC current to 600A

• DC current to 1000A

• AC and DC voltageto 1000 Volts

• Max Hold

• Continuity beeper

Fluke 27

• Fully sealed,waterproof case

• Touch Hold®

captures stablereadings

• Volts, ohms, amps,continuity, diodetest

• Current with rangesfrom 320 µA to 10A

Fluke 26 Series IIISame features asFluke 73 Series III,plus:

• Rugged, overmoldedcase

• Tough, electrical testlead set withsilicone insulationand alligator clip

• True-rms ac voltage

• AC/DC current withranges from 4.0 mAto 10A

• Lo Ohms

Fluke 73 Series III

• Touch Hold®

captures stablereadings

• Auto and manualranging

• Holster with Flex-Stand™ included

• Volts ac and dc

• Resistance

• Diode test/continuity beeper

• AC/DC current withranges from 32 mAto 10A

Fluke 12BPut basic testson automatic

• VCheck™ auto-maticallyswitches frommeasuring ohms/continuity to acor dc volts

• Capacitance, to10k microfarads

• Continuitycapture locatesintermittentopens and shorts

• Min/Max record-ing with relativetime stamp

Other marine application notes available from Fluke:• Testing Corrosion Protection Systems• Troubleshooting Outboard Motor Magneto Ignitions

Fluke. Keeping your worldup and running.

CAT

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Figure 10: Checking condenser leakageCheck for leaking condensers with the Ohmsfunction. As the condenser charges up, theresistance should increase to infinity. Anyother reading indicates that you shouldreplace the condenser.

CondensersFluke analog/digital multimeterscan also be used for checking auto-motive capacitors (condensers). Themovement of the bar graph willshow that the DMM is charging thecondenser. You’ll see the resistanceincrease from 0 to infinity. Be sureto switch the leads and check bothways. Also make sure to checkcondensers both hot and cold.

Spark plug wiresMost modern gasoline engines haveresistance wire for the high voltageconnections between the distributorcap to the spark plugs. The resis-tance reduces radio interferenceand produces a cleaner spark.

Plug wires should be checkedfor open circuits if they are morethan a couple years old. Not allwires indicate the date they weremanufactured. Due to the heat ofthe spark plug insulator, a spark

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plug boot may bond to the sparkplug. Pulling a spark plug bootstraight off the spark plug can dam-age the delicate conductor insidethe insulated wire. Rotate the bootto free it before pulling it off.

If you suspect bad wires, test theresistance of the wire while gentlytwisting and bending it. Resistancevalues should be about 10 kΩ perfoot (30 kΩ per meter), dependingon the type of wire being tested;some may be considerably less. Youshould compare readings to otherspark plug wires on the engine toprovide a relative reference for atypical good reading.