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Unread 01-09-2014, 10:12 PM   #1
Gante
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Oil pressure gauge goes up and down with rpms

Well, I have noticed that the oil pressure gauge on my 1991 Grand Wagoneer goes up and down with the rpms. As I accelerate or run te truck the pressure goes up. Once I stop and iddle the pressure goes down.

Any idea what's going in here? What to check for or troubleshoot?

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Unread 01-10-2014, 01:12 PM   #2
govmule84
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That's kind of how oil pressure works - it goes up with engine RPM on any properly-functioning engine.
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Unread 01-10-2014, 01:37 PM   #3
Gante
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Quote:
Originally Posted by govmule84
That's kind of how oil pressure works - it goes up with engine RPM on any properly-functioning engine.
I was doing more reading in the topic. I will replace my oil pressure switch since I am sure it has not been replaced in a long time plus it is not that expensive to do.
My truck has shown up to 80psi which seems to be a little high compared to what other members would see on theirs.
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Unread 01-10-2014, 01:57 PM   #4
muddy93
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your 91 should be about the same as my 87 in the way the gauge works.
The first thing I would check would be the wiring between the sender and gauge.
You mention that it has "shown up to 80psi....." The gauge works off of resistance, and when there is none (like when the wire becomes disconnected from the sender compleatly), the gauge will go to the highest position. Similarly, if you were to ground the sender wire (remove it from sender, and touch to frame), the gauge would go to the zero position.
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Unread 01-10-2014, 02:06 PM   #5
Gante
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Quote:
Originally Posted by muddy93
your 91 should be about the same as my 87 in the way the gauge works. The first thing I would check would be the wiring between the sender and gauge. You mention that it has "shown up to 80psi....." The gauge works off of resistance, and when there is none (like when the wire becomes disconnected from the sender compleatly), the gauge will go to the highest position. Similarly, if you were to ground the sender wire (remove it from sender, and touch to frame), the gauge would go to the zero position.
I'll check that first. Thank you!
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Unread 01-10-2014, 02:08 PM   #6
RockRollin
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If I disconnect the pressure sending unit wire, my guage goes to zero(90 year model). The oil pressure should correspond with engine RPM. When cold my guage stays at 60 (the max it should ever read, mechanically limited, bypass valve). When warmed its at 16 idle, 45@2500rpm. Also will note when my sending unit was going south, at times my 2500rpm oil pressure would be at 20 ( had me scared for a bit)
if its showing 80psi, I believe you have a restrictive filter and/or increased pressure bypass valve, try to put 80psi into an oil filter and see what happens
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Unread 01-10-2014, 02:33 PM   #7
muddy93
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yes, like everyone else is saying, the oil psi WILL fluctuate with engine rpm. If you have reason to believe that the pressure is actually to high or to low, the best way to check is with a mechanical oil pressure gauge.

RockRollin,
do you know when they switched the gauges(reversed the ohms)?
when my oil gauge pegged at 80, I found the wire had broken, and was just hanging there not touching anything. When I grounded it, the gauge pegged at zero.
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Unread 01-10-2014, 03:18 PM   #8
RockRollin
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88 model year many things changed in the electrical system. The 87 was the last of the AMC (maybe Cryco had issues buying more Ford parts)
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Unread 01-18-2014, 09:30 PM   #9
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I found a little issue with my oil pressure switch. The wire coming from the pressure switch is in pretty bad shape, just holding on by a few strands of wires. The wire is too close to the exhaust manifold and probably burned a few strands. This may explain the higher resistance reading that would give me the occasional higher pressure reading on the gauge.

It seems that the previous owner at one point added a different type of oil pressure switch which was added with a T fitting to the existing oil pressure switch. The newer pressure switch only has one wire connection while the original switch uses two wires. I would like to eliminate one of the two, at this point I think it would be easier to eliminate the original switch with the wire connection since i already have wired for the one wire switch.

Does any one see or know of any advantages of having the original 2-wire switch vs the 1-wire switch?
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Unread 01-18-2014, 09:31 PM   #10
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Quote:
Originally Posted by gante
i found a little issue with my oil pressure switch. The wire coming from the pressure switch is in pretty bad shape, just holding on by a few strands of wires. The wire is too close to the exhaust manifold and probably burned a few strands. This may explain the higher resistance reading that would give me the occasional higher pressure reading on the gauge. It seems that the previous owner at one point added a different type of oil pressure switch which was added with a t fitting to the existing oil pressure switch. The newer pressure switch only has one wire connection while the original switch uses two wires. I would like to eliminate one of the two, at this point i think it would be easier to eliminate the original switch with the wire connection since i already have wired for the one wire switch. Does any one see or know of any advantages of having the original 2-wire switch vs the 1-wire switch?

image-3871736254.jpg



image-3229748768.jpg
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Unread 01-18-2014, 09:40 PM   #11
RockRollin
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I belive the two wire sending unit is for the electric choke. Basically it sends power that powers the choke. No oil pressure, no high idle choke.
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Unread 01-19-2014, 11:30 AM   #12
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Just curious my 90 has the same setup is one a pressure switch for an idiot light and the larger I know is the pressure sending unit.
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Unread 01-19-2014, 04:22 PM   #13
RockRollin
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I pretty sure single wire is oil pressure, two wire is choke switch.
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Unread 01-19-2014, 08:10 PM   #14
Gante
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Quote:
Originally Posted by RockRollin
I pretty sure single wire is oil pressure, two wire is choke switch.
So, is this the way this car came from the factory with both switches? I thought this was a modification. If so, then my chocke switch is not wired...how is this chocke switch supposed to be wired?
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Unread 01-19-2014, 10:44 PM   #15
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http://oljeep.com/gw/elec/GW_wiring-1990.html
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