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Old 11-21-2020, 11:52 AM   #1
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Oil Pressure gauge actually an idiot gauge ?

Sorry if this has been covered previously...After owning our 2004 Mountain Aire for 9 years, I never gave much thought to the oil pressure....always seems to stay right @ 55 psi. Recently, after running for several minutes, the gauge went full scale high and and oil buzzer and light went off. I immediately shut the engine down, thinking perhaps the oil pressure relief/bypass valve was stuck. Restarted after several minutes and all seem normal except for the fact the gauge showed 55 psi the moment the ignition was turned on, prior to starting. My Silverleaf VMSpc showed the same 55 psi. I located the sensor and pulled on the wire slightly (have not figured out how to remove connector yet). Upon the next start sequence, the gauge did not show 55 until after the engine was started, thus Im assuming the connector or wire, not the sensor is the issue. It is a single wire connector, though there is a unused 3 wire connector in the same harness. Cummins ISL 400hp, Allison 3000MH, Freightliner Chassis.
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Old 11-21-2020, 12:06 PM   #2
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There may be differences is wiring and sensors , both for engine type and chassis manufacturer , could you add more info to your post .
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Old 11-21-2020, 12:34 PM   #3
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Thanks....guess that would help. My oversight.
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Old 11-21-2020, 01:02 PM   #4
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Quote:
Originally Posted by reosw View Post
Thanks....guess that would help. My oversight.
Thanks for the adding of info , have you considered setting up a signature line .

If your gauge and Silverleaf showed the same reading prior to start up , then an internal issue with the sending unit is a distinct possibility .
Most sending units are a variable resistance to ground, thus only a singe wire , with the electrical section separated from the oil by a diaphragm , if the diaphragm leaks oil into the electrical section then any thing can go wrong with the readings .
There have been posts about the Cummins ISC, using two oil sensors ; one for the ECM another for the gauge , hence your extra connector ( pretty sure that didn't carry over to the ISL) and others where , coach builders have added their own sensors to the engine for their gauge packages.
I'll see if I can track down a photo of the oil pressure sensor wiring connector to see if there is a trick to disconnecting the wire.
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Old 11-21-2020, 01:31 PM   #5
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Well I found a post that relates to the sending unit on an ISL . With pictures .

http://www.irv2.com/forums/f104/oil-...re-326980.html

Post #6

As far as disconnecting the wiring connector , it would appear the connector has a double lock , the sliding lock red part in this photo ( your connector could be black and the slide red or blue ) has to be slid sideways in order to squeeze the primary release to pull the connector . This photo shows the secondary lock in the released position. I'll look for a picture that shows both primary and secondary locks.
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Old 11-21-2020, 01:40 PM   #6
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I ran into similar problem this spring on our 2002 MADP ISL with my sending unit failing.
After a lot of research found that the “sending unit” was only a low oil pressure switch. The ECM and gauges always read 55psi.
There were some early actual sending units but apparently they had issues with those and went to the switch only.
Some chassis mfg do add a second sending unit for gauges only.

As Skip mentioned you have to move the red slide then press to remove. Pain in the rear to remove connector and sending unit. I ended up doing it from top side through the hatch.

My adventure with diagnosing and replacing.
https://www.irv2.com/forums/f103/200...-a-485454.html
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Old 11-21-2020, 01:49 PM   #7
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Many thanks....I had read that some sensors function simply as an on/off device, lacking variable resistance. What I find odd is that the ECU is sending a 55 psi signal to my Silverleaf system. After a bit of tugging on the connector (thanks BTW for the diagram) neither the gauge or silverleaf had an indication until the engine was started. Previously, both would show 55 simply by turning on the ignition, which makes me suspect the wiring....but Ill replace both connector and sender to eliminate those as the cause. If the sender is simply on or off, the next question, given the change in gauge behavior after fiddling with the connector, is it N/C or N/O. Thanks again for the input.
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Old 11-21-2020, 01:54 PM   #8
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Oil Pressure gauge actually an idiot light ?

$250 for an oil pressure switch is crazy. I paid $32 for the replacement switch.

https://www.ebay.com/itm/OEM-CUMMINS...-/133254051983
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Old 11-21-2020, 02:00 PM   #9
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My error.....found one on ebay for 25.00. On its way. Edited my post. BTW, what size socket do I need. Thanks
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Old 11-21-2020, 02:12 PM   #10
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I purchased a 27mm oil pressure sender socket OTC-4673-3

Mine was tight, tried with normal tools and just not enough room to work. Socket was as much as pressure switch.
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Old 11-21-2020, 02:50 PM   #11
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Found this. https://www.tooldiscounter.com/produ...0aAifsEALw_wcB
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Old 11-21-2020, 02:59 PM   #12
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What you describe about an oil gauge really being nothing but a fancy idiot light is more common than you'd expect in the auto industry. Many cars use the same setup - the gauge either shows the needle in the 'ideal' spot or it shows it in the 'trouble' spot. Guess the engineers didn't figure people needed real gauges or would know what to do with them.
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Old 11-21-2020, 03:05 PM   #13
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Looks like the one I purchased!
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Old 11-21-2020, 03:09 PM   #14
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..... or would know what to do with them.

Correct, from some of my research it sounded like too many people were freaking out when gage showed normal low oil pressure at idle. So instead of displaying they replaced with a switch and programmed ECM to only show 55psi.
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