Boat Tachometer Troubleshooting
Imelda Matchett
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.
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.
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.
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.
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).
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, it 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.
John C. Payne is a cruising sailor and professional marine electrical engineer and surveyor with a 28 year career in merchant shipping and the offshore oil industry. He is the author of The Marine Electrical and Electronics Bible, Motorboat Electrical and Electronics Manual and Understanding Boat Batteries all published by Sheridan House.
I glanced down at my tach tonight while cruising along. It was right at 5000 and all of a sudden it just dropped down to nothing. Like I turned the switch off. Any idea why this would happen?....Marine man..are you out there??
I've had the same experience on a johnson outboard, after testing the tach against a shop tach we found the problem with the outboards rectifer and strator (part of the voltage regualator and charging system). It is usually caused by an intermittent connection on your cranking battery from the motor. It's always a good idea to replace the wingnuts on your cranking battery with stainless lock nuts or use a needle-nose plier to crank them super tight.
I had the same trouble 2 years ago on my 70hp Jonny. Replaced the stator, problem solved. If I recall correctly, the fix cost me around $300 +/-. I'd suggest if you have an Jonny or Evinrude, you sit down when they quote you. OMC parts prices have skyrocketed in the last few years. When the present parts supply is gone, it's gone. Good luck, Phred52
I just dropped off at the dealership. He was thinking it's the voltage regulator. He said that if it was the stator, the motor would not run at all. Right now it runs perfect. It is a 1997 75 hp Mariner.
I was out this last weekend and noticed the same thing. I have a '98 Mercury 75. Everything seemed to be running fine, then the tach, fuel gauge and volt meter went hay wire. I'll check it tonight to see if it's still charging the battery. Sure don't want to spend $300 for a new switch box. Let me know what you fine out from the dealer. Thanks!
The bad news... I'm guessing you're still looking at $200 to $300 to replace it... it's a snap to replace, but it's one of the electronic boxes / circuit boards on the motors.. that always spells more money...
No problem Rost... no, not a mechanic, but a mechanical engineer who rigged boats and did a little mechanic work on the side while in college and high school... plus I still try and learn as much as possible on marine stuff from all sorts of sources...
I wouldn't anticipate any more problems with your motor... I wouldn't say the voltage regulator is a real common problem... but it is something that happens every now and then.. it certainly isn't on the same scale as a VRO pump on a older Johnson for example.. just something that happens. You've got a good motor... the 75's are pretty much bulletproof... you should be in good shape.
Ya, thank you both. I took the boat out tonight. Same problem with the tach. The other gauges are acting goofy too. Based on the volt meter on my depth finder, the battery is still charging when the engine is running. I'm somewhat handy with tools, is a voltage regulator fairly simple to replace? Would like to save a few $$ by doing it myself if the part is going to be spendy.
In regard to the VRO pump... I believe it stands for variable rate oil injection... and were notrious for problems (ie - not enough oil mixed with gas) in the 80's and 90's on Johnson and Evinrude motors...
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