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Old 07-10-2011, 15:58   #1
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Bad Oil Pressure Gauge

I finally had the time and was up to the task of getting my critical engine gauges working today, tremp, oil pressure and alarm for the boats Perkins 4.108. Boat was built 1976 and has what appears to be trhe original VDO gauges. The Perkins I have in hte boat now is not the original and never had a oil pressure sender just alarm switch. When I went through the prior owners spare parts collection there, was a brand new VDO oil pressure sender. So as part of the getting the gauges working project today I swapped the oil pressure switch module to a different position on the fitting at the block and added the the oil pressure sender as the fitting is designed to accomidate the two sensors.

To my surprise and dissapointment when I started the motor the oil pressure gauge only is reading 20psi and when I give the motor some throttle to increase the revs the gauge starts to decrease. Now this engine has had some fairly significant work performed on it prior to the installation, motor had 5000hrs on the clock and when the mechanic tore it down he indicated it was in great shape. Prior to taking the engine back from the mechanic I had him run it for me with the gauges that came originally with this motor and that gauge showed 45psi.
The motor sounds great through all rpm range, no knocks.

So the question is considering the age of the gauge and the fact that it sat idle for years can the gauge be bad. Would it be reasonable to expect that if I had the oil pressure alarm wired in correct that it should of sounded at 20psi. I am also considering that maybe the new VDO sender that was found on the boat is the wrong one.

I was hoping to take the boat out on the river for its first motor since I have the boat and am now leary that I may be having a real problem.
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Old 07-10-2011, 16:27   #2
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Re: Bad Oil Pressure Gauge.

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 it’s 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)

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

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
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Old 07-10-2011, 17:30   #3
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Re: Bad Oil Pressure Gauge.

[QUOTE=GordMay;791971]ENGINE GAUGES (Fuel, Pressure, & Temperature Instruments):

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.


Well Gord that is exactly what I have going on, I will remove and plug the switch port and see if the gauge reads correctly.
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Old 07-10-2011, 17:34   #4
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Re: Bad Oil Pressure Gauge.

you checked all the wiring already..........right?
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Old 07-10-2011, 17:46   #5
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Re: Bad Oil Pressure Gauge.

All new wiring, will see if it's the two sensors on the same fitting thing.
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Old 07-10-2011, 20:20   #6
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Re: Bad Oil Pressure Gauge

While doing some more research on the net tonight I came across a post from a 4.108 owner that said he had changed his oil from a straight 40 weight oil to a 15w-40 multi vis and experienced a drastic drop in oil pressure. When he went back to the straight weight the pressure went back to normal. Well I just changed the oil recently and the only weight oil I could find was 15-40, could this be the issue. I have been having real problems in the NY area finding oils for naturally aspirated diesels in the auto supply shops. They are all pushing the 15w-40 products. My manual makes no mention of this type of oil being speced for this engine.
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Old 08-10-2011, 05:56   #7
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Re: Bad Oil Pressure Gauge

Any members have any thoughts on the grade of oil I am using affecting causing the low oil pressure reading?
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Old 08-10-2011, 06:12   #8
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Re: Bad Oil Pressure Gauge

Not sure that the oil would make a big difference but only one way to tell but first...... you need to find, buy or borrow a mechanical pressure guage and install in your tee fitting ...then you will know if you really have a issue Dont waste time chasing a problem you may not even have
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Old 08-10-2011, 06:17   #9
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Re: Bad Oil Pressure Gauge

I agree Motion, game plan is to take the tee and alarm switch out of the loop this am and put the sender directly on the block. Think before I destroy all the nice wiring I will run the electrical tests on the gauge outlined by Gord above.
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