Troubleshooting Boat Gauges, Instruments
By John Payne
Gauges and meters sit unobtrusively on our boat-engine control panels, switchboards, and navigation stations; they're windows into our boat systems and without them, systems and equipment would operate invisibly and fail without warning. Such devices are crucial to ensuring that boat engines run correctly within normal operating parameters and that we don't overload electrical systems.
These days our gauges and meters may be an array of discrete analog meters, just a couple of basic meters, or a new generation of integrated digital and visual screen displays. I'm one who still favors analog displays and it's for good reason that aircraft still use them as parts of the latest high-tech screen displays. A quick glance is all that's needed to see that your systems are all "go." That is an important point to remember because, when considering meters and gauges, it's not always about the numbers; it's also about monitoring trends, up or down, of the system that the gauge or meter is monitoring.
We're all familiar with engine gauges. While there's a trend to simply have an alarm, I prefer the ability to monitor pressures and temperatures on a gauge or meter. Of course, there are many possible pressures and temperatures to monitor; it all depends on what and how much you want to monitor. The basics are sufficient for most boaters.
Oil pressure is one of the fundamental parameters that need a gauge. The oil pressure sender or transducer unit for the meter uses variable resistance inside a housing that changes in proportion to the pressure applied to it. Low oil pressure readings are caused by low lube oil level or a clogged oil filter or, worst case, by a faulty oil pump. Always believe the meter indication and stop the engine. Many people doubt the meter and learn from the consequences.
Temperature is another vital parameter that we always monitor. The proper monitoring of water temperature is essential to the safe operation of your engine as temperature extremes can cause serious damage. The meter transducer units are resistive, and output is a resistance that's proportional to the temperature. The main causes of high temperatures typically include a faulty freshwater-pump impeller, low engine-cooling water levels, and fouled coolers. In addition, your meter might be telling you of problems with the saltwater (raw-water) cooling system.
The engine tachometer is another essential meter and is indispensable for monitoring engine speed. Observing this information enables us to make informed decisions on fuel consumption and boat performance. There are several tachometer types, based on the sensing system with the meter. The generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading. The inductive tachometer has an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head. This transmits a series of on/off pulses to the meter that are counted and displayed as speed on the tachometer. The alternator tachometer takes a pulse from the DC charging alternator AC winding. This signal is a frequency directly proportional to the engine speed.
Exhaust gas temperature monitoring is commonly used in commercial ships and is becoming more common on powerboats and even on sailing boats. Engine problems are often easier and faster to identify with this instrument than water temperature and oil pressure monitoring. Exhaust temperature sensors are also known as thermocouples or pyrometers. The sensors consist of two dissimilar metals, which, at the junction, generate a small voltage proportional to the heat applied to the sensor, and the voltage is measured in millivolts (mV).
Monitoring fuel and water quantities is essential with a simple electrical gauge. Most tank sensors operate on the same principle of varying a resistance proportional to the tank level volume.
The immersion pipe sensor consists of a damping tube that has an internal float that moves up and down along two wires. These units are generally only suitable for fuel tanks, and the one advantage is that they are well damped; fluctuating meter readings with fluid movement are reduced by the damping effect.
The lever-type sensor comprises a sensor head installed on the end of an adjustable arm. The sensor head has a variable-resistance and float-arm pivot. As the float and arm move relative to the fluid level, the resistance changes, and the meter reading shows the actual level.
The capacitive sensor operates on the principle that the value of a capacitor is dependent on a dielectric between plates. The sender unit measures the capacitance difference between air and the liquid. The sensing circuit outputs a voltage proportional to the level in the typical range of 0 to 5 volts.
Within an electric-gauge system, a voltmeter or ohmmeter is scaled, or graduated, to display the voltage or resistance output from a transducer or sender unit. The VDO range characterizes this type of meter. In a diaphragm-based mechanical system, the meter connects to capillary tubing and operates using vapor pressure to mechanically change the meter via the diaphragm. These types may also incorporate electrical alarm contacts within the meter, and are characterized by the Murphy Swichgage range (www.fwmurphy.com).
Many instrument panels incorporate a voltmeter to show the level of the charging voltage. Charging voltmeters have a colored meter scale to allow a quick view of conditions, a red zone for under- or overcharge, and green zone for proper charging range.
The in-line, or series, ammeter has the main charging alternator output cable connected directly to it and then to the batteries. The long cable run to the meter often causes significant charging system voltage drops and resultant undercharging. Another, more noticeable problem is that the heavy current-carrying cables are run with other cables and this is a cause of radio interference; often you will see other meters "twitching" or pulsing.
The shunt ammeter is essentially a resistance or shunt inserted in the charging cable. The twisted pair of meter wires are connected to the shunt and can be run to any meter location without voltage drop problems, as the output is in millivolts.
Operating hour-meters and counters are necessary to track engine hours for service and maintenance. The meter and counter are only activated when the engine operates. The ignition switch is probably the most practical method of activating the meter, which is simply connected across the ignition positive and a negative so that it operates when the engine is running. The oil-pressure switch-activation method is now uncommon although some older installations activate through the oil-pressure switch.
The simple meter and sensor arrangement does require some basic maintenance:
- Remove oil-pressure sensor units from the engine every year and clean any oil sludge or deposits out of the sensor orifice. These tend to clog, causing incorrect readings. When sensor units are badly grounded on the engine block, it's often found that Teflon tape is incorrectly applied to the threads. Do not use Teflon tape as this creates a high-resistance contact and causes incorrect or even no meter readings. The same advice applies to temperature sensors.
- Check that tachometer sensors that pick up from the engine flywheel are secure. A common cause of failure is damage to the sensor head caused by striking the flywheel when it comes loose.
- Check wiring and ensure it is secure and not chafing or rubbing on any surfaces. Check that the sensor terminal and wire connections are tight and clean.
- For mechanically activated gauges, check capillary tubes for any signs of rubbing or chafing and ensure that tubing is secure.
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