troubleshooting pesky marine gauges

8/12/2019 Troubleshooting Pesky Marine Gauges

http://slidepdf.com/reader/full/troubleshooting-pesky-marine-gauges 1/12

Troubleshooting Those Pesky Gauges!

At some point the standard issue tap, tap tap is not going to be enough to revive that gaugeproblem you have been avoiding. Hopefully this will makes things a bit smoother and give you abetter understanding of the troubleshooting process. There is also a downloadable guide to assistyou!

Gauges are the eyes into the performance of our systems, not only do they provide us withnormal operating information but can warn us of impending danger. That temperature gaugethat seems to creep up a bit more on each trip to the lake may be hinting at a water pumpimpeller failure. But for those gauges to be useful they have to work. Let’s get started.

Many of the older boats still have the thermometer type temperature gauges, mechanical oilpressure 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 linethat connect to the back of the gauge and is routed to a fitting on the engine. The line feeds oilup to the back gauge where the pressure is converted to a display on the gauge. In most casesthese gauges stop working due to a clog or kink in the line or eventually a leak. The old styletachometers have a medium sized cable fastened to the back. This cable has a smaller cablewithin 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 viawire to a sending unit located on the engine. Many boats also have an alarm systems designed toget your attention while the boat is in operation. The items they monitor depend on themanufacturer of the boat, it is quite common for oil pressure and water temperature to bemonitored and on larger boats they may monitor transmission temps etc. These are designedwith a sensor mounted on the appropriate piece of equipment that will trigger a switch toactivate the alarm.

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

Figure 1 Sending Unit Schematic



For troubleshooting a basic understanding of a Multimeter is required.

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. Onthis 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.

Figure 2 Typical Multimeter

©2009-12-18 www.boatinghowto.com 2



Location of typical switches and sending units on a Crusader Marine Engine.

Figure 3 Switch/Sender Locations on Crusader

Most gauges have at least 3 terminals located on the back (some may have as many asfive). 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




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 isgrounded, if gauge continues to

show an upscale reading, the

gauge is bad.

10. Disconnect the sending unit wirefrom the sending unit on the

engine, and ground it to theengine 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” toterminal “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 orloose, if gauge does not indicate upscale, replace the gauge.

12. If gauge indicated correctly for both downscale/upscale readings you can assumeit is okay, now proceed to test the sending switch/unit.

Note: when replacing sending units care must be exercised with thread sealant, the unitsrely 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:

Oil Pressure (psi) Ohms Reading (Ω)

0 227 - 257

20 142 - 162

40 92 - 114

80 22 - 49

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 toomuch sealant may inhibit this contact.

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. Replacesender if not within +/- 7.5% (consult Service Manual for appropriate values)

Water Temp (°F) Ohms (Ω)

100 °F 404 – 495

130 °F 158 - 202

160 °F 119- 140

200 °F 63 -79

Water Temperature Sending Unit




220 °F 22 – 24.5

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.

2. Alarm should sound.

3. If alarm does not sound, check for 12V at the positive terminal of the alarm byconnecting one lead of the multimeter to the positive terminal and the other leadto ground. If 12V is not present then check wiring to ignition for an open circuit

4. If 12V is present, connect a jumper wire from the negative terminal of the alarmto ground. If alarm does not sound the alarm is faulty.

5. If the alarm sounds, the problem is in the wiring to the switches on the engine, or

the switches themselves.

Figure 4 Typical Alarm Schematic




Oil Pressure Alarm Switch

1. Remove the wire from the terminal on the switch.

2. While the engine is off, connect one lead of the multimeter to the sender terminal

and connect the other to a ground on the engine. At low oil pressure the switchwill 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 ofinfinity (max scale).

Water Temperature Alarm Switch

1. Remove the switch from the engine.

2. Connect one lead of a multimeter to the switch terminal and the other to the base.

3. Immerse in a container of oil with a thermometer, carefully heat oil and observethe temperature. The meter should indicate infinity until a temperature of

approximately 203°F, above this temperature the meter should show continuity

(low resistance).

Transmission Fluid Temperature Switch

1. Remove switch from the transmission.

2. Connect multimeter to both switch terminals.

3. Immerse in a container of oil with a thermometer, carefully heat oil and observethe temperature. The meter should indicate infinity until a temperature of

approximately 230°F, above this temperature the meter should show continuity

(low resistance).




Ignition Switch Testing

If the ignition switch appears to be bad, first check fuses and/or circuit breakers. Alsocheck 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 switchtest good, then the wiring harness is bad.

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

multimeter.

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 multimeters 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.




Figure 5 Troubleshooting Flowchart




Typical Wiring Color Codes

Wire Color Function

Engine Alarm Lt Blue/Tan

Starting Circuit/Neutral Safety Switch Yellow/Red

Water Temp Sender TanAlarm Buzzer Lt Blue/Tan

Ground Black

Ignition Instrument Feed Purple

Bilge Blowers Brown with Yellow Strip or Yellow

Nav Lights or Tachometer Dark Gray

Generator, Alternator, Pumps Brown

Accessory Feed Orange

Cabin Instrument lights Dark Blue

Oil Pressure Light Blue

Fuel Gauge Pink

Tilt down or in Green with stripeTilt up or out Blue with stripe


troubleshooting pesky marine gauges

Documents