Outboard-Engine Starter Circuits

So, if the amperage specifications aren’t available from the engine maker, how do we get them? Easy! Just do a current-draw test when you know your starter is working normally and record the amperage the starter motor uses in your manual. Then when a problem does crop up, you’ll have a known value to work with as a benchmark.

To do the current-draw test, first make sure your starting battery is in good, serviceable condition. If you have an inductive-clamp multimeter capable of measuring up to about 500 amps, clamp the inductive pickup over the main battery cable going to the starter motor and take a reading while a helper cranks the engine. Now you have the normal amperage you can expect your starter motor to draw. If you don’t have an inductive-pickup multimeter, one of the inexpensive Snap-On inductive meters shown in my tool collection in chapter 1 will do the job.

When you have a starting problem that you think might be caused by the starter motor, first doublecheck your battery to make sure it’s charged and in good condition, then repeat the current-draw test. If your new reading is lower than the previously established benchmark reading, the starting problem is probably due to loose or corroded terminals in the battery cable connected to the starter motor. If the cable has been replaced since you established your benchmark and the terminals are clean and tight, the new cable is probably undersized and needs to be upgraded.

If the reading you get is higher than the benchmark reading, make sure that the problem is not caused by a mechanical fault such as a partially seized engine or a frozen drive unit. You may need to call in a pro to help out at this point. Once you’re certain that the engine is not causing the problem, you can be sure that any excess current drawn by the starter motor is due to a fault within the motor. Remove it and send it out for overhaul.

Voltage-Drop Test

Another useful test for your starter motor and starter circuit is to trace the circuit while checking for voltage drop at various points. This test will be outlined in the following section on outboardengine starter circuits and will work just as well for inboard engines.

A system overview of a typical starter-motor circuit on an outboard engine with remote control is shown in figure 8-6a on page 129. On many engines the remote-control harness plug is located under the engine cowl, so this plug is not as shown in the diagram.

If your outboard engine doesn’t have a remote ignition switch, it will have a starter button located on the engine, and may have a neutral-safety switch integrated into the mechanical shift linkage under the cowl. A simplified wiring diagram of this circuit is shown in figure 8-6b on page 129. Your engine may have some of these connections in a wiring junction box. Also, starter-motor battery terminals are often used by manufacturers as handy places to attach additional circuits, so refer to your wiring diagram and narrow the number of wires down to what you see in this drawing; ignore the rest.

All outboard engines use inertia-type starter motors that engage the flywheel ring-gear when centrifugal force throws the drive gear upward. Medium-to-large outboard engines also use a remotely mounted solenoid just like those used on inboard engines. Problems with inertia systems can be as simple as a low battery, or corroded terminals causing a cranking speed that’s too slow to generate enough inertia to engage the drive gear. So, as with any system, the first thing to check if trouble develops is the battery and all its connections.

The open-circuit voltage test described in chapter 5 will show you if the battery is fully charged. If it isn’t, charge the battery to bring it up to snuff before proceeding with any of the following tests. Of all the electrical circuits on your boat, the starter circuit is probably the one that draws the most amperage; until the engine starts, the starter motor needs all the juice the battery can give it.

Afteryoumakesureyourbatteryisfullycharged, it’stimetotracecircuits.Totesttheintegratedsystem foundonsmallengineswithoutremotecontrol,first checkforvoltageatpointsthroughoutthecircuit.Figure8-6conpage129showsthepointstocheckandthe sequenceinwhichyoushouldcheckthem.Makesure yourenginegroundandthegroundboltorcable(it shouldbetheblackone)arefreeofcorrosionandtight.

12 volt Battery Battery, ground cable

Starter motor Solenoid Remote control with key switch and neutralsafetyswitch

Remote control harness plug

Battery, Positive cable

Electrical junction

Engine ground

Fig. 8-6a. A typical outboard-engine starter-motor circuit with remote control.

At point 1, check the power to the push-button switch. With your meter’s black probe attached to an engine ground (the bolt or cable grounding the starter motor to the engine is a good point), use the red probe to check the other points along the circuit. The voltage at point 1 should be very nearly the same as your direct reading across the battery. If it isn’t, there is a bad wire or broken connection between the battery and the terminal at point 1. This test illustrates how battery voltage gets to the hot side of the starter button, usually via a red wire.

If the voltage is good here, proceed to point 2. Disable the ignition to prevent the engine from starting as you do the next four tests. Press the starter button while holding the red probe to point 2, the output side of the starter button. You should find a reading of approximately 12 volts. If you don’t, your starter button is defective and will need to be replaced.

Neutralsafety switch

Starter pushbutton Starter motor, grounded to engine Neutral safety switch

Starter pushbutton

❸ ❷

❶ ❹ ❺ Starter motor, grounded to engine

Engine ground point

Fig. 8-6b. An outboard engine with integrated starter-motor circuit and no remote control. Engine ground point

Fig. 8-6c. Using a voltmeter to check the outboard integrated starter-motor circuit. Use the voltmeter to check voltage at each point indicated in the circuit.

If you do find 12 volts at point 2, proceed to point 3 and connect the red probe to the hot side of the neutral-safety switch, which disconnects the startermotor when the transmission is in gear. You should get another 12-volt reading. If not, the connection is bad or the wire between the starter button and the neutral-safety switch has a break in it. Repair or replace the wire as needed.

Next, be sure the transmission is in neutral and move your probe to point 4 (the output side of the safety switch). Push the button and take a reading; you should get 12 volts. If you don’t, the neutralsafety switch is defective or out of adjustment. To check for proper adjustment, unbolt the switch from its bracket, allowing the switch button to extend fully. If you still cannot get a 12-volt reading at point 4, the switch is bad and must be replaced. If you do get a 12-volt reading, adjust the neutral-safety switch by repositioning it in its mount so that the shift linkage extends the button as far as it will go.

If you do get a 12-volt reading at point 4 and the engine still won’t crank, move the red probe to point 5 at the battery terminal on the starter motor (the connection with the large red wire). With the engine in neutral, press the starter button and check for 12 volts. If you don’t find it, there is a poor connection or a broken wire between points 4 and 5. Repair or replace the wire as required. If you do find 12 volts here, the problem is in the starter motor, and it will have to be removed for rebuilding or replacement.

Voltage-Drop Test

Another simple test that can help you to locate any bad connections, undersized wires, or faulty parts that could cause excessive resistance and slow-crank condition in a starter circuit is called the voltage-drop test. This test requires a digital multimeter (you’ll be checking for readings of 0.3 volt or less) set to the low-volts scale if it isn’t self-scaling.

The meter connections and the sequence for the voltage-drop test are shown in figures 8-7a–d.It’s a good idea to get a set of the thread-on alligator-clip probes (many multimeters now come with interchangeable probes as standard equipment), available at Radio Shack and other supply houses that sell multimeters. The alligator clips let you keep your probes attached to a wire or terminal while you crank the engine and take a reading, eliminating the need for an extra set of hands.

First check the connections shown as A and B in figure 8-7a. The red wire attached to connection A is from the ignition switch or starter button and provides power to the solenoid when the ignition switch or starter button is engaged. The black wire from connection B is the ground wire to the coil within the solenoid. Make sure that this ground is good.

Figs. 8-7a–d. Sequence of a starter-motor circuit voltage-drop test being performed. Excessive voltage drop at any point in the circuit indicates a bad connection or possibly wire cabling that’s too small.

Starter solenoid

A B Starter motor, grounded to engine

Starter solenoid

A B Starter motor, grounded to engine

A

Engine ground point

B

Engine ground point

With the key off, use your multimeter set up to read resistance and check for continuity (a reading of very close to zero ohms) between the terminal at connection B and ground on the engine block. If you don’t find continuity, repair the connections or replace the wire.

Next, turn the key to start and check for 12 volts at the terminal labeled connection A. If 12 volts is not present, the problem is somewhere in the wire from your starter-motor switch or neutral-safety switch. Follow the steps described later to correct this problem. If all seems well here, proceed with the voltage-drop test.

For each of the four steps to this test, the engine must be cranking but not firing. In step one, connect the meter as shown in figure 8-7a. Your voltage reading with the engine cranking should not exceed 0.3 volt. If it does, then the connection at the positive battery post is bad, the connection at the solenoid is bad, or the battery cable is too small and must be upgraded to a larger one. An easy way to check for a too-small battery cable is to feel it as you crank the engine. If it gets warm to the touch, it’s too small.

Step two of the voltage-drop test, as shown in figure 8-7b, is to check the voltage drop through the solenoid. Connect the meter directly to the two largest terminals on the solenoid. A reading here in excess of 0.2 volt indicates a fault inside the solenoid, and it will have to be replaced.

Next connect the voltmeter as shown in figure 8-7c with one lead to the terminal on the output side of the solenoid and the other to the large positive terminal on the starter motor. While cranking the engine, your voltage reading should not exceed 0.2 volt. If it does, the connection at the solenoid is bad or, as with the battery cables, the wire connecting the solenoid to the starter motor is too small. (This wire should be the same size as the main battery cables.) A too-small cable would only be a problem if someone changed the cable before you. Factory wiring is carefully engineered for size and is never too small. Difficulties are caused when improper repairs are made.

Battery Cable Size

Here are a few guidelines for sizing battery cables

For engines 15 horsepower and under with a distance of less than 10 feet (3 meters) to the battery, 10 AWG cable is usually adequate. For runs of 10 to 15 feet (3 to 5 meters), use 8 AWG. For runs of 16 to 20 feet (5 to 7 meters), use 6 AWG. On engines in the 20-to-30-horsepower range, use 6 AWG, 4 AWG, or 3 AWG, respectively, for the same cable runs. On the larger engines (V4 and V6), use 4 AWG, 2 AWG, and 1 AWG, respectively.

Starter solenoid

A B

C

Engine ground point Starter motor, grounded to engine Starter solenoid

A B

D

Engine ground point Starter motor, grounded to engine