Marine gauges are a window into the operations of your boat, allowing you to know what is going on in your systems and warning you of impending danger. They can only do this, however, if they are in good working order. Since marine conditions tend to be inhospitable to electronics, marine gauges often require more upkeep than their inland counterparts. Though repairing them may seem complicated, it is only a matter of understanding the basics of how they work and what to do. As with many boating skills, the most important step is to pay attention. Observe your gauges closely, so that you will catch their issues before it impacts the safety of your boating experience.
As a note, this is a general guide. You may wish to look at your manual for specific information relating to the type of gauge you have. On a boat, some of the most common types of gauges on the dashboard are the tachometer, fuel, speedometer, trim, voltmeter, oil pressure, and water temperature gauges. The trim, a gauge found only on boats, measures how high the engine is raised or lowered.
Fortunately, marine gauges are not complicated in their construction. When it comes to a faulty gauge, there are three components that could be at the root of the problem: the gauge itself, the wiring, or the sender.
Most gauges have at least three terminals on the back. The “S” or “SND” terminal connects to the sender, which is the piece of equipment that measures the fuel level or water temperature or oil pressure and sends that measurement to the face of the gauge. The “I” or “IGN” terminal connects to the ignition, and sends power to the gauges. The “G” or “GND” terminal is the ground connection for the lighting circuit. These are the three basic terminals; some gauges may have up to five. If your gauge has more than these three, consult the manual for the equipment and make sure you know what exactly they are for.
Although it is important to properly check all components of the gauge, it is in many cases the sender that is causing the problem, especially if the fuel gauge is the gauge in question. Fuel gauge senders are often notoriously inaccurate because they use a float system to determine the level of fuel, which is problematic because bumping over the waves or the simple fact that the tank may not always be level will change the readings that the sender gives.
The two major problems that gauges usually have is that they are either inoperative or they read incorrectly. Reading incorrectly can be more dangerous, as the gauge appears at first glance to be working.
If the gauge is inoperative, disconnect the sender terminal and turn the ignition on. When the ignition is on, the pointer on the gauge should be all the way to the left. If it is not, you need to check the ignition voltage by connection a multimeter between the Ignition and Ground terminals. If there is no voltage, check the wiring that runs to the ignition and to ground, as this is most likely the problem area. If the pointer is all the way to the left, short the sender terminal to ground. If the pointer does not move all the way to the right, the gauge is faulty and must be replaced. If the pointer does move all the way to the right, the problem is with either the wiring to the sender or the sender unit itself. Check the wiring and run through the test again; if the problem is not solved, you will have to replace the sender unit.
If the gauge is reading incorrectly, your first step should be simply to check the terminals and see if they are loose or dirty. If they are, clean and tighten them, and if that is not the source of the trouble, continue with the rest of the process. Check to see if the gauge calibrates with the test Ohms as set down in the manual. If the gauge does not calibrate, then the gauge is faulty and must be replaced. If the gauge does calibrate, the problem is with the sender and the sender must be replaced.
If you have to replace a sender unit, keep in mind that for a sender to work, it must be grounded and therefore must have metal-to-metal contact with its threads. Use care with thread sealant, or the sender will not function. When replacing a fuel sender, make sure to measure the tank, as senders come in different lengths and you must choose the one that fits your particular need.
If you have to replace wiring, make sure to use the correct colors of wire as dictated by the Standard Marine Wire Code. The colors are used as follows:
Red – Constant Hot
Black – Ground
Purple – Key-on Power (between the gauge and the key switch)
Yellow/Red – Neutral Safety
Tan – Water Temperature
Dark Blue – Gauge Lighting
Pink – Fuel Sender
Gray – Tachometer Light
Blue – Oil Pressure
Brown/White – Trim Indicator
Brown – Bilge Pump
Green/White – Trim down
Blue/White – Trim up
Green/Orange – Independent Tilt Down
Blue/Orange – Independent Tilt Up
Yellow/Black – Choke Circuit
Yellow/Red – Starting Circuit
Black/Yellow – Ignition
When troubleshooting and repairing gauges, remember to take the utmost precautions as you will be working with both combustible materials and materials capable of delivering electrical shocks. Keep a watchful eye on yourself and the tanks and gauges. When buying replacement equipment, have the specs of your existing equipment with you so can be sure that your new purchases are compatible with them.
The more you know about how to fix something aboard your boat, the more comfortable you will be on it and the more satisfaction you will have when out on the water. Learn to understand and fix your gauges, and you will be well on your way to being a seasoned boater!