Yamaha Outboard Battery Voltage and Charging System Check

The starter cranks slowly, the fault memory shows battery voltage below spec, and next to it there are entries that seem to have nothing to do with the battery. That's a common picture, because with too little voltage, sensors, the throttle body and the shift system all start reporting nonsense. M-JET DIAGNOSTIC shows you the voltage that actually reaches the ECU, on every supported range from the F20 to the HPDI Z series.

Three readings that tell you a lot

• Key on, engine off: a fully charged 12 volt battery sits around 12.6 to 12.8 volts at rest.
• While cranking: if voltage drops way down, suspect the battery, terminals or ground cable.
• Idle and a bit above: now voltage should be above the resting value. If it stays flat or drops, the engine isn't charging.

What you can check without a toolbox

Start at the terminals. Salt and green corrosion between clamp and post are the most common cause, and you often only see it once the clamp is off. Then check the ground cable at the block and the connection at the battery switch. The steps for the voltage fault are short: look at voltage in live data, check the connection between cable and battery post, then measure the peak output voltage of the charging system. For the HPDI Z series the check procedure specifically calls for at least 12 volts at the battery. A practical tip: measure once right at the battery posts with a multimeter and compare that with the value in the app. A big difference means voltage is getting lost along the way at terminals, the battery switch or the cable.

When the engine won't charge

If voltage stays at battery level with the engine running, look at the charge coil and the rectifier regulator. Charge coil peak voltage is measured with a multimeter and a peak voltage adapter, and the app doesn't show it. On the big F275 to F450 engines some ECUs have a separate generator system no signal fault. In that case you check the wiring between ECU and rectifier regulator, the wiring to the crank position sensor and the sensor's supply voltage. On F175 to F450 the app can also show accessory battery voltage, depending on the ECU.

Don't forget fuses and relays

Some entries in the fault memory aren't about the battery itself but about the supply behind it. That includes the main relay power supply and the starter relay power supply, each with the instruction to check the matching fuse first and then the wiring. On F275 to F450 some ECUs add a communication error with the fuse box. When several faults show up at once, charge the battery, clean up the corrosion, clear the memory and see what comes back after the next run. Usually only the real fault is left.

Related topics

• Yamaha Outboard Won't Start: Checking Kill Switch, Shift, Pulser Coil and Battery
• Yamaha Outboard Live Engine Data on Your Phone
• Yamaha Outboard Electronic Throttle (ETV): Checking TPS and APS
• All Yamaha outboard diagnostics topics

Everything runs via app over Bluetooth, for Yamaha outboards with control unit generations from 2003. Learn more about the M-JET DIAGNOSTIC for Yamaha WaveRunner & Outboard.