1994 mercruiser 7.4 oil pressure question

jsweeden

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Jun 13, 2012
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I am having little to no oil pressure on the gauge, no alarm going off. Am going to check sending unit, can anyone tell me where the unit and switch is located on a 1994 7.4? Also, an advice on how to check them would be appreciated.
 

jsweeden

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Re: 1994 mercruiser 7.4 oil pressure question

Oops, sorry, newbie mistake... I reposted Thanks.
 

04fxdwgi

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Re: 1994 mercruiser 7.4 oil pressure question

Look back on the block The sender is there with the light blue wire on it. That's your sender, screwed into the block.

Here is a trouble shooting guide for the oil pressure gauge:

1. To test the gauge, voltage from “I” to “G” terminal must be 10-16 volts DC, with no wire
on “S” terminal. Gauge pointer should rest below the “0 PSI” mark. Next, connect the “S”
terminal to the “G” terminal (leave “I” and “G” terminals connected). The gauge pointer should
rest above the maximum pressure mark.

2. Sender resistance can be measured to determine the sender’s correct operation.
Remove wire to gauge. Connect an ohmmeter to terminal of sender and to engine block.
Approximate pressure sender resistance values are:
Zero pressure = 240 ohms
1/2 gauge reading = 103 ohms
full gauge pressure = 33 ohms.
(You will get half these values on a dual gauge sender.)

3. If sender is shorted (0 ohms) gauge will read above full gauge pressure reading.

4. If sender has infinite resistance (open) gauge will read below 0 PSI.

5. If gauge reads lower than expected, was sealer used on the sender threads?

6. The accuracy of the system (gauge & sender) can be ?5 PSI at 40 or 50 PSI.

7. Use of pipe extenders to plumb both a sender and pressure switch (for horn or warning light)
from one port is not recommend ed. Pressure readings will be accurate, but the weight added
to the extended fitting of a vibrating engine may cause fatigue related break age.

NEVER use teflon tape on the threads. I use a dab of Permatex "HiTac" and it seals 1st time, every time and doesn't insulate the threads
 

jsweeden

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Jun 13, 2012
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Re: 1994 mercruiser 7.4 oil pressure question

Thanks, that was very helpful. I have the gauge here with me, can i check it with a omh meter or jumper without power on it?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Re: 1994 mercruiser 7.4 oil pressure question

plumb in a mechanical gauge to see what the motor actually has for oil pressure. then troubleshoot your gauge and sender
 

04fxdwgi

Senior Chief Petty Officer
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Re: 1994 mercruiser 7.4 oil pressure question

plumb in a mechanical gauge to see what the motor actually has for oil pressure. then troubleshoot your gauge and sender

BBC engines make a heck of a valve train racket if there is no oil pressure. If his engine was quiet, then I suspect the oil pressure is present. Even if it is the minimum needed to run the engine and I'm pretty sure the '94 had the low oil pressure warning buzzer (my '92 does), which would have singing loudly if there is no oil pressure.

The trouble shooting guide I sent can mostly be done w/ out engine running.

To JSWeeden.... You'll need 12 VDC on the gauge to test it. If you have a 120 volt to 12vdc power supply handy, even a little wall plug type, you can test the gauge sitting at your computer desk, using the power supply, just like your boat's electrical system. Just make sure the + and - are connected correctly. Just need to have a little "Mc Giver" in you to do some things, sometimes.

If everything checks out OK and still no indication of pressure, then yes, the mechanical gauge would certainly be my next step. But I bet he'll find an electrical problem 1st. As normal oil pressure is 30 - 70 PSI on these engines (Mine holds right on 40 PSI at all times from idle to WOT, hot or cold) and believe it or not, 4 PSI is absolute minimum at idle w/ engine hot (factory spec), seems as though any oil pressure at all will keep engine fairly quiet.
 

jsweeden

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Jun 13, 2012
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Re: 1994 mercruiser 7.4 oil pressure question

To: 04fxdwgi Thanks for the info. I am going up to test everything this afternoon, I will let you know what I found out.
 

jsweeden

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Jun 13, 2012
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Re: 1994 mercruiser 7.4 oil pressure question

Went up and tested the gauge and sending unit, both tested good. Got worried... But I noticed that there was some corrosion on the stud where the wires plugged into the gauge, so before tapping in a manual gauge, I took a wire brush and cleaned all conections plugged it all back up, crossed my fingers and cranked the engine... Had better pressure than I have had since I bought the boat 6 years ago. 35-40 PSI Thanks everyone for your input and help...
 

04fxdwgi

Senior Chief Petty Officer
Joined
Jun 10, 2011
Messages
754
Re: 1994 mercruiser 7.4 oil pressure question

Went up and tested the gauge and sending unit, both tested good. Got worried... But I noticed that there was some corrosion on the stud where the wires plugged into the gauge, so before tapping in a manual gauge, I took a wire brush and cleaned all conections plugged it all back up, crossed my fingers and cranked the engine... Had better pressure than I have had since I bought the boat 6 years ago. 35-40 PSI Thanks everyone for your input and help...


Glad it worked out and on the cheap. Funny how a little corrosion will gum up the works.. Like I said "But I bet he'll find an electrical problem 1st". A little grease on the electrical connections will help keep the corrosion to a minimum, also.
 
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