1999 "EE" 90, 115 FFI, 150, 175 V4, V6 FFI Outboards Service Manual, P/N 787024, Page 143Get this manual
Perform the following tests if the tachometer does not operate INote The tachometer circuit is driven by the ECUImportant Verify that all engine and boat wiring and connectors are in good condition1Check voltage at the batteryUse this reading as your reference for battery voltage2Check for battery voltage between the tachometer purple lead and black lead at the dash with the engine NOT running and the key switch ON If the voltmeter shows battery voltage, go to Step If the voltmeter shows less than battery voltage, check the purple, purplered, and black circuits, Position fuse, key switch, and the battery connections3Disconnect instrument harness gray lead and black lead at tachometerSet Fluke 29 Series II meter, or equivalent, to Hz scaleConnect meter between gray lead and black lead With engine running at 1000 RPM, meter should indicate 90-105 Hz If reading 90-105 Hz, replace tachometer If other than listed above, check gray lead output at ECU- Reading OKcheck condition of gray circuit- Reading not OKreplace ECU