Taking a peek behind the dashboard and seeing that twisted mass of wires can be frightening, but it isn't nearly as bad as it looks. Just a few basic tools, and an understanding of the type of instruments on your dashboard, and you should have no problem diagnosing instrument failures.
The most basic tool is a 12-volt test light. This light looks something like an ice pick, with a light bulb inside the handle, and a wire extending from it. The wire will usually have an alligator clip on the end, which will allow you to attach it to either a ground or positive source of power. When the clip is attached to a negative or ground wire, and the ice pick is touched to a positive wire or terminal, the light will illuminate.
If you understand how each instrument works, you will be able to better diagnose any problems with that gauge. The tachometer requires a positive wire that is turned on and off with the key switch, a negative wire that is a constant ground connection, and a pulse signal from the engine that is used to represent the engine RPMs.
Most engines use the pulses of electricity from the alternator for this signal. To check out a tachometer, start by verifying that the positive and negative connections are operating properly, attach the test-light clip to the negative post of the tachometer, and with the key switch turned on, touch the positive post with the ice pick end of the light. If the light illuminates, then these connections are good.
Next turn the key switch on and off while watching the face of the tachometer. You should see the indicator needle move to the "0" position each time the key switch is energized. If this does not happen, then the tach is probably bad and will need to be replaced. If the wire connections are functioning and the tachometer is reacting whenever you turn the key switch on or off, then the problem may be in the engine itself. You will need to get a service manual for your model engine in order to test the alternator, rectifier and/or voltage regulator for possible problems.
The speedometer and water pressure gauges simply display pressure readings from a hose that is attached to the rear of each gauge. These gauges may have positive and negative wires attached to the rear of them, but those wires are only there to turn on the instrument light for nighttime navigation purposes. If you disconnect the pressure hose for each gauge at the engine, and also at the gauge, you can then blow through the hose with your air compressor.
If there are any restrictions in the hose, you will either have to clear them or possibly replace the hose. If there are no restrictions, then reattach the hose to the gauge and blow into the other end of the hose while watching the face of the gauge to see if the indicator needle moves. This will tell you if the gauge is good or bad.
Temperature gauges, fuel gauges and power trim indicators are all ground rheostat gauges. Just like the tachometer, these instruments have a positive and negative connection, which can be tested in the same way as the tachometer.
Once you have verified that the positive and negative connections are hooked up and functioning, then you will have to determine if the gauge or the sending unit is causing the problem. You will need a short jumper wire with alligator clips on each end to perform the next test. With the key switch turned on, attach your jumper wire from the ground terminal of the gauge to the sender terminal of the gauge. If the gauge is good, you will see a full reading on it. A fuel gauge will show full tank, a temperature gauge will show a hot reading and a trim indicator will show a full tilt position.
Once you have determined that the instrument is good, you will next need to test the wire from the gauge to the engine, and the sender on the engine, to determine which is causing the problem. Start by removing the wire from the sending unit. Again with the key switch turned on, attach your jumper wire from engine ground to the sender wire. The gauge should react as previously and show a full reading. If it does not, then the sender wire is defective and will need to be replaced. If the wire works and the gauge shows a full reading, then by a process of elimination the only thing left is the sender, and it will have to be replaced.
Two other common gauges found on boats are the volt meter and the hour meter. These gauges are just simply powered by the positive and negative wires that are attached to them. Test these wires the same as the tachometer power leads. If you are getting power to the gauge when the key switch is turned on, but the gauge does not work, then replace the defective gauge.
If you have any questions about your boat, motor or trailer, you can e-mail me at firstname.lastname@example.org.