Troubleshooting Those Pesky Marine Gauges!

Boat repair can be a frustrating task and at some point the standard issue tap, tap tap is not going to be enough to revive that gauge problem you have been avoiding. Hopefully this will makes things a bit smoother and give you a better understanding of the troubleshooting process. There is also a downloadable guide to assist you!

Gauges are the eyes into the performance of our marine systems, not only do they provide us with normal operating information but can warn us of impending danger. That temperature gauge that seems to creep up a bit more on each trip to the lake may be hinting at a water pump impeller failure. But for those gauges to be useful they have to work. Troubleshooting marine gauges is not as hard as you would think if you are equipped with the proper knowledge, so let?s get started.

Many of the older boats still have the thermometer type temperature gauges, mechanical oil pressure and rpm gauges. In most cases if the temperature gauge fails on these you will have to replace the whole unit. The oil pressure gauges on these older boats have a small hollow line that connect to the back of the gauge and is routed to a fitting on the engine. The line feeds oil up to the back gauge where the pressure is converted to a display on the gauge. In most cases these gauges stop working due to a clog or kink in the line or eventually a leak. The old style tachometers have a medium sized cable fastened to the back. This cable has a smaller cable within it and is routed to the engine normally fastening near the distributor. The most common cause of failure is the inner cable breaking.

The more common systems on recent marine equipment use an electronic gauge connected via wire to a sending unit located on the engine. Many boats also have an alarm systems designed to get your attention while the boat is in operation. The items they monitor depend on the manufacturer of the boat, it is quite common for oil pressure and water temperature to be monitored and on larger boats they may monitor transmission temps etc. These are designed with a sensor mounted on the appropriate piece of equipment that will trigger a switch to activate the alarm.

Electrical power from the battery goes to the ignition switch, from the ignition switch then to the gauges down to the senders to ground.


Attachment 48
Multi-meters
Figure 2 below represents a typical multi-meter. Some multimeters have an auto-ranging function and it is not necessary to select an upper limit scale as shown by arrow B. On this meter however arrow B shows the correct position to troubleshoot a 12 volt system. It sets the range of the meter from 0 volts to 20 volts. Arrow A shows the typical symbol used to indicate DC voltage. Arrow C indicates the symbol for Ohms or resistance. Arrow D indicates the normal setting to measure resistance. For your specific meter consult the manufacturer?s documentation.

Attachment 49
Most gauges have at least 3 terminals located on the back (some may have as many as five). The main ones to be concerned with are marked:

I or IGN - the power feed from the ignition
G or GND - ground connection for the lighting circuit
S or SND - input from the sending unit located on engine

To properly troubleshoot your gauges, you will need a voltmeter or multi-meter. The two key settings you will use are the DC Voltage (Vdc) setting and the Ohm (Ω) settings on the gauge. The Vdc setting will be used to determine if the gauge is getting the proper power. The Ohm setting will be used to check the resistance of the senders/switches. Ensure the battery is properly charged.

General theory for troubleshooting gauges follows these basic ideas. The sending unit on the engine converts temperatures and pressures into a resistance value, in most cases low pressure and low temperature equate to a high resistance allowing only a small electrical current flow thru the sender, the gauges indicate low values (low-scale) at this point. As pressure/temp increases the resistance in the sending unit decreases allowing more current to flow and the gauges to indicate higher (upscale).
Temperature, Oil and Fuel Gauges

If a Gauge is pegged (up-scale) Temperature, Oil and Fuel Pressure:

1. Disconnect wire from the sending unit on the engine, if gauge moves to low-scale the sender is bad.
2. If the gauge does not respond, disconnect the wire from the "S" terminal on the back of the gauge, if the gauge moves to low-scale, the wiring is grounded.

Gauge indicates (low-scale) Temperature, Oil and Fuel Pressure Gauges:

1. Gain access to the rear of the instrument panel cluster, and locate the gauge.
2. Turn ignition switch "ON".
3. Check voltage between "I" and "G", should be between 12-13V.
4. Turn ignition "OFF".
5. Remove wire from "S" terminal on back of gauge.
6. Turn ignition "ON".
7. Gauges should go to the low-scale position (*some oil press gauges operate reversed and go to the upscale position).
8. If gauge indicates correctly, skip to step 10.
9. If gauge shows an upscale reading, remove the wire from "S" on backside of the gauge, if gauge returns to the low-scale position, the sending wire is grounded, if gauge continues to show an upscale reading, the gauge is bad.
10. Disconnect the sending unit wire from the sending unit on the engine, and ground it to the engine block, the gauge should indicate an upscale reading (*some oil pressure gauges indicate reversed), if gauge indicated an upscale reading, skip to step 12.
11. If the gauge did not indicate upscale, connect a jumper wire from terminal ?G? to terminal ?S? on the back of the gauge, if gauge indicates upscale the sender wire is open (cut) or the wiring harness connection at the engine may be corroded or loose, if gauge does not indicate upscale, replace the gauge.
12. If gauge indicated correctly for both downscale/upscale readings you can assume it is okay, now proceed to test the sending switch/unit.

Note: when replacing sending units care must be exercised with thread sealant, the units rely on the metal to metal contact of their threads to complete the ground circuit and too much sealant may inhibit this contact.

Oil Sending Unit

1. Disconnect wire from oil sending unit terminal.
2. Connect ohmmeter to the sending unit terminal and sending unit case, check ohms reading without engine running (zero pressure). Then check ohms reading and pressure with engine running, compare with chart.
3. These values may vary per manufacturer, consult the appropriate Service Manual:
   
   Attachment 780

Water Temperature Sending Unit

1. Remove sender from engine.
2. Connect a digital ohmmeter to sender.
3. Immerse sender and a thermometer in a container of oil, carefully heat the oil over a heat source and monitor the thermometer.
4. Meter should show the correct ohms for the corresponding temperature. Replace sender if not within +/- 7.5% (consult Service Manual for appropriate values)

Attachment 781

Engine Alarm Circuit Testing
Note: Some alarm systems may be different, consult the appropriate service manual or contact the dealer.

1. Turn ignition switch ON, but do not start the engine. Alarm should sound.
2. If alarm does not sound, check for 12V at the positive terminal of the alarm by connecting one lead of the multi-meter to the positive terminal and the other lead to ground. If 12V is not present then check wiring to ignition for an open circuit.
3. If 12V is present, connect a jumper wire from the negative terminal of the alarm to ground. If alarm does not sound the alarm is faulty.
4. If the alarm sounds, the problem is in the wiring to the switches on the engine, or the switches themselves.

Attachment 945

1. Remove the wire from the terminal on the switch.
2. While the engine is off, connect one lead of the multi-meter to the sender terminal and connect the other to a ground on the engine. At low oil pressure the switch will be closed and the meter should show continuity (low resistance).
3. Start the engine and check the resistance again. The pressure switch should open when oil pressure exceeds appx. 5psi. The meter should indicate a resistance of infinity (max scale).

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Water Temp Alarm Switch

1. Remove the switch from the engine.
2. Connect one lead of a multi-meter to the switch terminal and the other to the base.
3. Immerse in a container of oil with a thermometer, carefully heat oil and observe the temperature. The meter should indicate infinity until a temperature of approximately 203 deg F, above this temperature the meter should show continuity (low resistance).

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Transmission Fluid Temperature Alarm Switch

1. Remove switch from the transmission.
2. Connect multi-meter to both switch terminals.
3. Immerse in a container of oil with a thermometer, carefully heat oil and observe the temperature. The meter should indicate infinity until a temperature of approximately 230 deg F, above this temperature the meter should show continuity (low resistance).

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Ignition Switch Testing

If the ignition switch appears to be bad, first check fuses and/or circuit breakers. Also check the neutral safety switch and the shifting lever.

1. Turn ignition switch OFF, there should be no continuity between any terminals.
2. Turn ignition switch ON, there should be continuity between the "BAT" and "IGN" terminals, there should be no continuity between the "SOL" terminal and any of the others.
3. Turn the ignition switch to the START position; there should be continuity between the "BAT" and "IGN" terminals, and between the "BAT" and "SOL" terminals.
4. If the ignition switch test bad, remove the switch and repeat the test. If the switch test good, then the wiring harness is bad.

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Testing a Voltmeter Gauge

1. Ensure battery is fully charged.
2. Turn ignition switch ON.
3. Check voltage at terminal "I" and terminal "G". The reading should be within +/-1 volt of the reading at the battery.
4. Check indication of front of the gauge, it should indicate the same reading as the multi-meter.

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Testing a Tachometer

1. Check the cylinder selection on the rear of the gauge, select the appropriate setting. It may help to exercise the selector to remove any dust or corrosion.
2. Some multi-meters have a RPM setting, dwell meters may be used or connect a service tachometer to the engine and compare readings at various speeds.
3. Tachometer should be within +/- 150 rpms, if not try adjusting the calibration on the rear of the tachometer.
4. If unable to adjust to within limits, replace tachometer.