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Oil pressure sender ohms.

7.7K views 62 replies 9 participants last post by  agear  
#1 · (Edited)
So I’m trying to figure out why my 84 cj 7 150000 mile has 10 psi oil pressure at idle but when I try my multimeter I can’t get an ohm reading when the engine is running. It gives me the ohms . When engine is off. I got the 246 ohms but I can’t get a reading running . I know my 1 multimeter switches to volt reading when it detects voltage which prompted me to get a 6 $ unit from harborfreight. But can’t get a measurement
 
#2 ·
You can "Tee" this into your hole (then have both this and the OEM) or you can use it by itself as a test.

A mechanical gauge will be much more accurate than an electric one.

As a test, you can hook the tube up and weave it from under the hood and up front so you can see it while underway.

Once satisfied you can put it back the way it was.

I run a mechanical one full time in my dash.


They make nicer ones too!

---JEEPFELLER
 
#4 ·
Correct!

Your volt meter provides the juice to run thru the sender and back.

The same kinda setup you use on an installed fuel sender to

see what ohms are being generated (as to how full your tank is, in a perfect world)

----JEEPFELLER
 
#5 ·
Agear, the Technical Service Manual states normal oil pressure is 13 psi at 600 RPM. 37 psi max at 1600 RPM.

With 150K miles on the engine, I think 10 psi at idle is probably normal. Does it jump up to around 30 psi with more RPMs? If so, I wouldn't worry about it.

Matt
 
#6 ·
Agear, the Technical Service Manual states normal oil pressure is 13 psi at 600 RPM. 37 psi max at 1600 RPM.

With 150K miles on the engine, I think 10 psi at idle is probably normal. Does it jump up to around 30 psi with more RPMs? If so, I wouldn't worry about it.

Matt
gotcha . on the interstate it’s higher.. I’ll confirm in the morning on the ride to work and follow up
 
#7 ·
Ok this morning it was about 13 psi at idle before it got up to temp. . Driving down the interstate with it warmed up it was 10. I leaned over and looked at it straight on.
I know you can’t see the foggy picture very good . That’s the highway one but I could see it ,
Pictures or it didn’t happen.
 

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#8 ·
If the oil pressure gauge is connected to the gauge with the engine running, the gauge is supplying a voltage to the sending unit causing the ohm reading to be incorrect.
You need to disconnect the gauge to get any good reading.

I would, at this point, attach a mechanical gauge and verify the reading is accurate before proceeding. Then you can either get into the "Matching the correct sending unit with the right gauge" game or tear your engine down replacing perfectly fine bearings. A cheap gauge like this is all you need for a quick check.

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With a Tee, you can leave it connected permanently.
It could save you a lot of money.
 
#9 ·
If
You need to disconnect the gauge to get any good reading.

I would, at this point, attach reading is accurate before proceeding. Then you can either get into the "Matching the correct sending unit with the right gauge" game or tear your

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With a Tee, you can leave it connected
yea ordered it.Jusy gotta remember not to force it to fit . If you break that threaded piece of block off it’s a bad feeling
 
#10 · (Edited)
John is right , as always. To test the ohms of oil sender disconnect the sender from the gauge. Run your ohms test from sender connection point out to a good ground and see what it reads. However the thing you want to know is oil pressure, not just ohms…that is a gauge question. So as John and others have said, start it using Jeep gauge and record reading. Then install mechanical gauge and compare. I’ll wager that 10-13 at idle is accurate and is reflected on both gauges. I run zinc added Castrol 20/50 and my oil pressure idles at 10 and cruises at 40ish.
 
#11 ·
Not meaning to muddy this issue and the mechanical gauge should be here Wednesday - I disconnected gauge and got a reading of 19 ohms idling. that’s roughly 50 psi. According to 1 of the sending units available that I know of. This jeep has 3.31s and drives like my old one that had 4.10s and efi. I think I scored on this jeep. If I could I’d give it back I would. Nice jeep . Except cosmetics. Just wanted to put this out there.
 
#16 · (Edited)
If somebody is lost on the specs

Here's a pic from my 1979 paper factory manual of the resistances.

----JEEPFELLER
This is great. I was looking for a correlation of gauge to sending unit. Wonder why I never saw this before.

Notice the back of the gauge in the upper right.
These resistance is for that style oil pressure gauge.
Here's a little better picture:

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They were 2" deep and had 4 posts on the back.
You can see the whole needle in the gauge.

there was also a 1" deep gauge that used a different sending unit.
They had 3 posts on the back and the bottom part of the needle was hidden.
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0 PSIG = 68-78 ohms
40 PSIG = 21-25 ohms
80 PSIG = 7-13 ohms


Here they are side by side to see the difference in the depth.
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#18 ·
Nvm I see to the best of my ability that 3 terminal is the later one in question. But that would mean my gauge is off . And I already checked the gauge it pegs to 80 when grounded briefly , there must be a more in depth check
 
#23 ·
Ok I got the new sending unit in . I looked up at advance auto as ps113.
With the jeep fully warmed up I would say it was at 13 psi. Before it was hard to say it was at 5.
It doesn’t fit as good. I ain’t forcing nothing though because I chipped an engine block last time and had to have brazed back on. But b anyway we’ll see how long this lasts and I ll have to find a tee for the mechanical gauge . That might space my sender off the block too,
 

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#24 · (Edited)
Here’s some pictures of my old sending unit . It seems to be smaller than my new 1. The numbers on the unit are 80 on one side of the hex and e7 on the other side of the hex
 

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#30 ·
I believe it was Keith460 who came up with the brilliant idea of drilling and tapping the sending unit mounting fitting to accept a mechanical gauge.

One of these days I'm going to do this, as well.

Matt


View attachment 4189427
If you want it, you can have this elbow I have capped off - I'll find a non-drilled & tapped elbow to replace it.
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#31 ·
From really close to my mechanical gauge all the way down to my fitting on the block

I have my copper line sleeved with 1/4" fuel line.

I ain't done it yet, but I plan on making a "stand-off" hanger to attach to that nearby bell housing bolt

and then to my "sleeve"

This may sorta keep my oil line steadier on the engine end as the engine torques.

If I have a failure it will be at the gauge or at the fitting on the block---if it's in between, it will

pour out of the hose near the block.

I've used plastic and copper line in the past for the gauge, both of 'em will fail at some point.

At present, it's a copper line.

So far, when these tubes to the mechanical gauges start seeping or full blown break loose

I have detected the oil leaking/ spraying and have shut down the Jeep to fix it.

I haven't lost all of my oil, but I usually have 2 quarts of oil onboard to get by for a bit.

board.

The little yeller vacuum cap contains a 1/4" brass plug.

I drilled the top edge with a small drill bit and inserted a piece of safety wire through it and

tied it to my block fitting.

I'm now prepared to plug the hole if I'm not in a position to repair the oil line!

There's the plug--In my face!

No searching!

No having to go get one!

A 1/4" ball valve and a short nipple would work too!

-----JEEPFELLER



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#32 ·
I have seen and like the idea of an electrical gauge on dash with a me h in the engine bay. Same with temp gauge. Nice when working on motor to see those gauges inside engine bay.
Agree that mechanical can leak, but installed well and maintained I have not had any problems with leaks.
I can say that I once, years ago, I had pulled out the oil sending unit to replace. Forgot about it a few days later and jumped in and started the Jeep. Was like an oil well exploding! Had the top off and it went all over my cloth seats!!! What a mess.
 
#34 ·
You can’t tell much in the picture it’s the best visual I could get where it is with my camera but it looks like the ps113 I got from advance auto is accurate to my mechanical gauge. At 18 psi just under 20 at idle before it gets to operating temperature, but I’d have to set up a camera under my hood to see while driving .
I guess the question now is I have a high volume oil pump , which I could install. Should I ?
 

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#36 ·
Just increase the rpm next time you have the hood open and see what your mechanical gauge reads.
It doesn't make a difference if your moving or standing still. it's all dependent upon RPM.

I just put in a HV oil pump in mine. Cold it's reading >80psi at operating temp it reads 30 psi.
Previously it would only reach 20 psi at idle it was scary.
 
#35 · (Edited)
2 things .
1 my jeep doesn’t go up to 30 psi going down the interstate. I’m in 5th gear with3.31s.
2. My ohm meter said I have like 19 ohms at idle . If I have the .gauge and sending unit mentioned 2nd then that would put me closer to 40 psi when my gauge is at 18
 
#39 ·
Ty . Now everytime I buy one of those gaskets they’re in the box with them . 4 blue ones. But yep fighting gravity sucks and that will clip it up there , i guess since it’s not leaking that bad I wouldn’t necessarily have to replace the rear main seal.