The vast majority of recreational boats in service today are still using analogue instrumentation systems. More modern digital, NMEA-networked, and multifunction gauges may be all the buzz, but the reality is that most boats are still equipped with the same analogue set-ups that have been in use for decades.
I recently provided an answer to a visitor's question on boats.com regarding instrumentation wiring. I thought it would be a good idea to expand on that subject a bit more in a bigger article. I hope this information will help you sort out issues you may have with your engine instrumentation.
Engine instruments can get broken down into three broad categories for the purpose of troubleshooting. Most analogue gauges used for the purpose of determining fuel level, engine temperature, and oil pressure fall into one category that utilize a variable resistor sensor mounted on the engine to provide you with a reading on your gauge.
A second gauge group consists of voltmeters and ammeters, and they work a bit differently. With older gauges, an ammeter will typically get connected up via what we call a shunt in a series electrical connection. Essentially this amounts to a break in a primary conductor DC positive wire that is “seeing” full electrical current running to and from your boat’s battery(s). Voltmeters will often be found instead of or in addition to ammeters. These get connected in parallel with one DC positive wire and one DC negative wire supplying power to the gauge and enabling a voltage measurement.
Finally, the third category of instrument we need to look at briefly is the engine tachometer. Tachometers are somewhat unique when it comes to the electrical connections to them.
Besides the electrical connection points on the gauges, the ABYC has for many years had some recommended color codes for the various wires coming from the engine mounted sending units and fuel tank level gauge, as well as the DC positive and ground conductors involved. Not all, but many of the engine manufacturers follow these color recommendations. Those who don’t will provide alternative wiring color codes on a wiring diagram associated with the engine versus the whole boat.
Beginning with the tachometer, Fig. 1 here shows the back of a typical gauge. At the top left of the diagram, we see the cylinder selector switch. Most vendors will use one tachometer head to cover a variety of engines. Make sure this switch is set to the number of cylinders for your engine. The right upper “L” point indicates the internal gauge light. This is most often fed by a jumper on the back of the gauge that is powered by the +12V terminal in the lower right side. This +12 V terminal will be supplied from the ignition switch for the engine and is often purple (PU) in color. In the case where the internal light has power supplied separately, the wire feed will be dark blue (DKBL)
The “G” terminal is the instrument ground and will be either black or yellow in color depending upon the vintage of the boat. Finally, the tachometer sending or “PUL” wire will be dark grey (Gy) in color. Depending on the engine it may go to the ignition system or the alternator on the engine.
Figure 2 illustrates the variable resistor activated gauge connections for fuel level, oil pressure and coolant temperature.
With the variable resistor activated instruments, the L or LT, G or GND, and +12 V wire colors will be the same as already described. The difference will be with the send wires. For oil pressure it will typically be light blue (LTBL) Water temperature will be tan (Tn) and fuel gauge pink (Pk).
For voltmeters, the connections will look as shown, but will not have a separate “send” terminal. Wiring colors will vary slightly depending on where the company takes its readings. Purple for DC + or red or orange are possible. “G or GND” will be either the black or yellow as already discussed.
Finally if your boat is equipped with an ammeter, then the series connected cables will typically be either orange (Or) or red (Rd) depending on where the reading is being measured. Instrument lighting will be accomplished as already discussed.
To troubleshoot the instruments you’ll need a voltmeter and maybe a digital amp clamp. Having the engine service manual in hand will be important to deal with slight variations in troubleshooting techniques and locations. All of these manuals will have a section on instrumentation troubleshooting that should be pretty easy to understand.
For the variable resistor type gauges, with your ignition turned on, pull the wire from the sending unit on your engine and touch it to your engine block. If the gauge then reads full scale, then the problem was with your sender. Get a new one. For the fuel gauge, it’s best to remove it from the tank and manually work the float mechanism to see if the gauge then moves. If not, unplug the pink wire and connect it to a known good ground like your engine block or the negative terminal on your battery. If the gauge goes to full scale then the sender is faulty. In the case of the tachometer, a reading of 2 X or 1/2 X the suspected rpm indicates that the cylinder selector switch is incorrectly set.