A tachometer measure the revolutions per minute, or rpm, of a motor, and is a common device on vehicles of all kinds. Mercruiser tachometers are used on watercraft, where tachometers are often even more useful than land-based versions. A properly working tachometer will tell you how well the engine is performing, if there are any sudden problems related to motor issues, and the overall efficiency of the engine system.
Sometimes tachometers are not set to the proper number of alternators. For a normal outboard motor, this is 12, so the tachometer should be set to the 12 position by moving a switch on the back of the device. Tachometer manuals generally recommend using a test light, or ohmmeter, to check for wiring damage. If there is not a 12-volt reading for the power or ground circuits, you can replace the wire and the tachometer should be repaired. If there is no power, it is possible the tachometer's inline fuse has blown.
Tachometers, especially those with advanced electrical circuits, depend on the accuracy of their connections to give proper readings. However, in some cases the tachometer can give mistaken readings due to interference, either from poorly insulated wire or a strong electromagnetic force that may be interfering with the readings. Even a powerful radio may affect readings.
The rectifier works with the regulator to adjust motor function for readings by the tachometer, and the stator is an immovable part of the motor that is usually wrapped with coiled wire and helps create the electromagnetic force the motor needs. If the stator wire looks burnt or melted, then it needs to be replaced to fix the tachometer. Typically, the rectifier is part of the entire tachometer circuit, which will need to be replaced if there is a complete power loss.
Tachometers can become slowly decalibrated and lose the accuracy of their readings over time. Most devices have an adjustment screw that can be used to calibrate the tachometer and make it as accurate as possible. Anywhere within 100rpm from a professional shop readout is acceptable, but tachometers can vary by as much as 500rpm.
To narrow down the problem, you may want to attach a quick shop tachometer that comes with alligator clips to test the tachometer connection. If this one works fine, you will probably need to replace the tachometer itself or one of its parts. If it does not work, then the problem may be in the stator or another part of the engine.
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