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Old 11-11-2003, 07:46   #1
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Basic Engine Gauge Theory and Testing

ENGINE GAUGES (Fuel, Pressure, & Temperature Instruments):

Basic Theory of Operation:

There are two basic types of Engine Instrument Gauges:
a. The VOLTMETER, which requires an outside Voltage Source. This is the most common type Engine Gauge used on boats.
b. The AMMETER, which does NOT require external power.

The standard Voltmeter type Gauge receives its reference voltage from a Sender unit, mounted on the Engine. The Sender is a device, whose resistance varies with changes in Level, Pressure, or Temperature. As the sender resistance increases, the reference voltage transmitted to the Gauge decreases.

Temperature Senders are often Bi-Metalic Thermo-Couple devices, whose resistance decreases as Temperature rises; resulting in a higher reference voltage at the gauge. Other Senders (Level, Pressure, Vacuum, etc.) employ differing methods of varying their resistance (ie: Varisters) in response to the measured stimuli.

Alarm Switches are "On-Off" devices, either Normally-Open or Normally-Closed; which switch 'On' or 'Off' in response to a pre-set condition. Switches are utilized to operate Warning Lights and/or Buzzers.

All Resistance values noted are for Single Station Senders.
Dual Station Senders, operating two Gauges, will have One-Half the Resistance values noted below.
If one Gauge (of a Dual Station Pair) failes, the Other Gauge will to read much Higher than expected.

Pipe Extensions, Tees, etc., utilized to connect both a Gauge Sender & a Warning Light/Buzzer Switch to the same Engine Block Port are NOT recommended. Sender values could read Cooler or Lower Pressure than exists, and the added weight & moment could cause fatigue failures.

Basic Troubleshooting of Gauges:

All Gauges:

1. Remove Sender Wire from S Terminal on Gauge, and Turn ON Instruments Circuit.

2. Test Voltage between I Terminal (External Power Supply) and Ground Terminal G.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery Voltage.
If no voltage - check Ignition or Instrument Switch(s) and/or Fuses.
If tests less than battery voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.

If a Sender fails Open (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails Closed (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Temperature Gauges:

1. Disconnect Tan Sender Wire from Gauge (Terminal S) and turn Instrument Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).

2. Short the Gauge Sender Terminal S to Ground Terminal G.
The Gauge should deflect to FULL Scale (> High Temperature).

3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 1000F)
or 29.6 Ohms (Engine Hot @ 2500F )
European Temp. Senders will read: 281 Ohms (Cold @ 400C) and 22 Ohms (Hot @ 1200C)

4. Disconnect Tan Wire from Sender on Engine Block.

5. Test continuity of Tan Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.


6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.

Pressure Gauges:

1. Disconnect Dark Blue Sender Wire from Gauge, and turn power ON.
The Pressure Gauge should read ZERO Scale (Low Pressure).

2. Short the Gauge Sender Terminal to Ground.
The Gauge should deflect to FULL Scale (>High Pressure).

3. Measure Resistance from Dark Blue Sender Wire to Ground.
American Press. Sender: 240 Ohms (Zero Pressure) 33.5 Ohms (Full Pressure @ 80 psi)
European Press. Sender: 10 Ohms (Zero Press.) & 180 Ohms (Full Press. @ 150 Bar)

See 4, 5, & 6 above (but Dk. Blue Wire)

Fuel Gauges:

1. Disconnect Pink Sender Wire from Gauge, and turn power ON.
The Fuel Gauge should read ZERO Scale (< Empty)

2. Short the Gauge Sender Terminal to Ground.
The Gauge should read FULL Scale (> Full).

3. Measure resistance of Pink Sender Wire to Ground.
American Floats: 240 Ohms (Empty), 103 Ohms ( Full), & 33 Ohms (Full)
European Floats: 10 Ohms (Empty), 95 Ohms (Half), & 180 Ohms (Full)
Automotive & other Float Switches may use other resistances such as 0 - 90 Ohm, 73 - 10 Ohm, & etc.

See 4, 5, & 6 above (but Pink Wire)

See also the graphics posted at:
Wiring Diagrams: http://cruisersforum.com/photopost//...php?photo=1622
Testing Instructions: http://cruisersforum.com/photopost//...php?photo=1623
Sender Resistance: http://cruisersforum.com/photopost//...php?photo=1624

A few Notes on Wiring:

under construction ...
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Old 01-02-2006, 06:54   #2
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I've just posted diagrams etc, which illustrate the Gauge Testing instructions
(urls corrected April 12/07)

Wiring Diagrams:
http://www.cruisersforum.com/gallery...=500&userid=79


Testing Instructions:
http://www.cruisersforum.com/gallery...=500&userid=79


Sender Resistance:
http://www.cruisersforum.com/gallery...=500&userid=79

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Old 11-04-2007, 14:58   #3
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Hi Gord,
I'm trying to access the diagrams you posted for basic engine gauge testing on 01-02-2006, and the links aren't working for me.
Is there somewhere else I can see them?
thanks
liz
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Old 12-04-2007, 01:37   #4
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Sorry liz - had an incomplete url. corrected now.
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Old 12-04-2007, 21:12   #5
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Thanks Gord!
liz
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Old 08-03-2011, 01:08   #6
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Re: Basic Engine Gauge Theory & Testing

Found this gem on Page 70 of 79!
It should be sticky'd, but I can't do that.
So bang we bump!

Thanks Gord!
Extemp.
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Old 10-03-2011, 07:43   #7
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Re: Basic Engine Gauge Theory & Testing

Gord MAy, Do you have any ideas on how to test a Tach? My tach registers and then cuts out. The old did the same thinm, this one new volvo as of Feb 2011. I have a volvo 2040. Thanks
Dave
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Old 10-03-2011, 08:52   #8
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Re: Basic Engine Gauge Theory and Testing

I don't know anything about Volvos.

Check out the Shop Manual
Volvo Workshop Manual (MD20 - 10/20/30/40)
php captcha script
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Old 12-03-2011, 18:18   #9
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Re: Basic Engine Gauge Theory & Testing

Quote:
Originally Posted by landonshaw View Post
Gord MAy, Do you have any ideas on how to test a Tach? My tach registers and then cuts out. The old did the same thinm, this one new volvo as of Feb 2011. I have a volvo 2040. Thanks
Dave
Tachometer Calibration
Don't know if that will help or not?

Cheers,
Extemp.
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Old 24-11-2013, 13:38   #10
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Re: Basic Engine Gauge Theory and Testing

I had teleflex tach cut out intermittently as well. I was fed by a hamilton ferris alternator. I was told by Ham Ferris that when the batterys are topped the alternator stops putting out intermittently to maintain the float voltage. This causes the tach to read and then drop back to 0.
If you need the tach to read all the time then put a load on the 12 volt system. ex. turn on inverter and plug in a space heater
Hope this helps. Rob
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Old 20-10-2015, 07:54   #11
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Re: Basic Engine Gauge Theory and Testing

Quote:
Originally Posted by GordMay View Post
ENGINE GAUGES (Fuel, Pressure, & Temperature Instruments):

Basic Theory of Operation:

There are two basic types of Engine Instrument Gauges:
a. The VOLTMETER, which requires an outside Voltage Source. This is the most common type Engine Gauge used on boats.
b. The AMMETER, which does NOT require external power.

The standard Voltmeter type Gauge receives its reference voltage from a Sender unit, mounted on the Engine. The Sender is a device, whose resistance varies with changes in Level, Pressure, or Temperature. As the sender resistance increases, the reference voltage transmitted to the Gauge decreases.

Temperature Senders are often Bi-Metalic Thermo-Couple devices, whose resistance decreases as Temperature rises; resulting in a higher reference voltage at the gauge. Other Senders (Level, Pressure, Vacuum, etc.) employ differing methods of varying their resistance (ie: Varisters) in response to the measured stimuli.

Alarm Switches are "On-Off" devices, either Normally-Open or Normally-Closed; which switch 'On' or 'Off' in response to a pre-set condition. Switches are utilized to operate Warning Lights and/or Buzzers.

All Resistance values noted are for Single Station Senders.
Dual Station Senders, operating two Gauges, will have One-Half the Resistance values noted below.
If one Gauge (of a Dual Station Pair) failes, the Other Gauge will to read much Higher than expected.

Pipe Extensions, Tees, etc., utilized to connect both a Gauge Sender & a Warning Light/Buzzer Switch to the same Engine Block Port are NOT recommended. Sender values could read Cooler or Lower Pressure than exists, and the added weight & moment could cause fatigue failures.

Basic Troubleshooting of Gauges:

All Gauges:

1. Remove Sender Wire from S Terminal on Gauge, and Turn ON Instruments Circuit.

2. Test Voltage between I Terminal (External Power Supply) and Ground Terminal G.
Tested voltage must match nominal voltage of the Gauge (ie: 12V Gauge must test between 10 - 16VDC), and the Battery Voltage.
If no voltage - check Ignition or Instrument Switch(s) and/or Fuses.
If tests less than battery voltage, repair or replace (Violet) wire from Ignition or Instrument Switch.

If a Sender fails Open (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails Closed (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Temperature Gauges:

1. Disconnect Tan Sender Wire from Gauge (Terminal S) and turn Instrument Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).

2. Short the Gauge Sender Terminal S to Ground Terminal G.
The Gauge should deflect to FULL Scale (> High Temperature).

3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 1000F)
or 29.6 Ohms (Engine Hot @ 2500F )
European Temp. Senders will read: 281 Ohms (Cold @ 400C) and 22 Ohms (Hot @ 1200C)

4. Disconnect Tan Wire from Sender on Engine Block.

5. Test continuity of Tan Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.


6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.

Pressure Gauges:

1. Disconnect Dark Blue Sender Wire from Gauge, and turn power ON.
The Pressure Gauge should read ZERO Scale (Low Pressure).

2. Short the Gauge Sender Terminal to Ground.
The Gauge should deflect to FULL Scale (>High Pressure).

3. Measure Resistance from Dark Blue Sender Wire to Ground.
American Press. Sender: 240 Ohms (Zero Pressure) 33.5 Ohms (Full Pressure @ 80 psi)
European Press. Sender: 10 Ohms (Zero Press.) & 180 Ohms (Full Press. @ 150 Bar)

See 4, 5, & 6 above (but Dk. Blue Wire)

Fuel Gauges:

1. Disconnect Pink Sender Wire from Gauge, and turn power ON.
The Fuel Gauge should read ZERO Scale (< Empty)

2. Short the Gauge Sender Terminal to Ground.
The Gauge should read FULL Scale (> Full).

3. Measure resistance of Pink Sender Wire to Ground.
American Floats: 240 Ohms (Empty), 103 Ohms ( Full), & 33 Ohms (Full)
European Floats: 10 Ohms (Empty), 95 Ohms (Half), & 180 Ohms (Full)
Automotive & other Float Switches may use other resistances such as 0 - 90 Ohm, 73 - 10 Ohm, & etc.

See 4, 5, & 6 above (but Pink Wire)

See also the graphics posted at:
Wiring Diagrams: http://cruisersforum.com/photopost//...php?photo=1622
Testing Instructions: http://cruisersforum.com/photopost//...php?photo=1623
Sender Resistance: http://cruisersforum.com/photopost//...php?photo=1624

A few Notes on Wiring:

under construction ...
Gord, nice to provide the info for the OP. There are other types of gauges too, like tachometer, and some meters are replaced by idiot lights to indicate problems rather than the actual level, e.g. low oil pressure vs. actual pressure. There are also often alarms connected to water temp and oil pressure which can add complexity to a given panel. I have also noticed that the traditional resistor based ammeters are going away to avoid having large wires having to go to the panel and back. Definitely a variety of panels and how they work.

And some on/off senders can be NO (normally open) or NC (normally closed) depending on what you are measuring and how it is handled on the panel or with alarms. I say all this because it was quite the learning experience for me in the early days. What has always helped me is to diagram the system "as is" first so that I physically trace all the wires (hopefully color coded all the way from the panel to the senders in some fashion which may or may not follow the "standard" coding) to the engines from the panels. I use that to identify anomalies, e.g. why does one wire go to only one engine and not the other. And then I draw a "to be" diagram to show how I want to reinstall, improve, and/or fix the circuits.

Nice basic diagrams to get the OP started.
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Old 20-10-2015, 09:11   #12
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Re: Basic Engine Gauge Theory and Testing

Gord,

Thank you for taking the time to elaborate in such detail.

Very useful.

Cheers!

Bill
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Old 20-10-2015, 09:54   #13
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Re: Basic Engine Gauge Theory and Testing

Quote:
Originally Posted by GordMay View Post
ENGINE GAUGES (Fuel, Pressure, & Temperature Instruments):

Basic Theory of Operation:

There are two basic types of Engine Instrument Gauges:
a. The VOLTMETER, which requires an outside Voltage Source. This is the most common type Engine Gauge used on boats.
b. The AMMETER, which does NOT require external power[/i]

Both meters use the d'arsonval meter movement. There is no difference between the 2 just how they are connected. Most analog meter movements use this style meter.
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