An outboard tachometer reads pulses from the engine’s charging system or ignition and turns them into rpm, so it must be set to match the engine. If the reading is roughly half, double or otherwise off by a fixed amount, the tach’s pole or pulse setting is wrong for your engine; check the engine maker’s chart. If it reads zero, check the signal wire, the tach’s power and ground, and on some engines the charging components that produce the signal.
Why the setting matters
Analog outboard tachs typically count pulses from the alternator stator, and stators have different numbers of poles depending on the engine. The tach has a switch or setting on the back to match it. Engine makers publish the correct setting for each model; the tach maker’s application chart lists it too.
Tachometer not working: what to check
- No reading at all. Check the tach’s power and ground, then the signal wire connection at the engine harness.
- Reading wrong by a fixed ratio. The pole or pulse setting does not match the engine.
- Erratic or jumping needle. Loose or corroded connections, a poor ground or electrical noise.
- Tach and charging both dead. On some engines the tach signal comes through the rectifier or regulator, so a charging fault can also stop the tach.
Digital gauges on newer engines
Many newer EFI outboards send rpm and other data over a digital network to compatible gauges or displays. An older analog tach may not work with them without the maker’s interface, so check compatibility before buying a gauge.
Using the tach to check propping
Every outboard has a full-throttle rpm range in its manual. With a normal load, run at full throttle and read the tach: if rpm is below the range, the propeller pitch is probably too high; above it, too low. Running outside the range can hurt performance and strain the engine. See boat propeller pitch and sizing and outboard loses power at full throttle.
Hour meters
Many tachs include an hour meter, and some engines log hours in their computer. Hours drive the service schedule and matter when buying or selling; see how many hours is a lot for an outboard?
Installing a tach
Connect power to a switched ignition source, ground to the boat’s negative bus, and the signal wire to the engine’s tach lead as the engine wiring diagram shows. Set the pole or pulse switch before you power it up, and protect connections from moisture.
Related
See outboard motor maintenance and browse engines on outboard motors.
More guides: prop slip · outboard trim sender and gauge · outboard ignition switch.
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