97 TJ 4.0 Oil Pressure Sender Bench Test - JeepForum.com
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post #1 of 35 Old 07-06-2012, 09:25 PM Thread Starter
jeepCJTJ
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97 TJ 4.0 Oil Pressure Sender Bench Test

I have a 97 TJ 4.0 Auto, Stock, only 70k original miles. This week in the hot weather, I found the dreaded oil pressure drop off at idle, the condition seemed to be consistent with water temp reaching 210. Normally with A/C off and average 70-80 degree ambient, my temp gauge is running just to the left of 210, lets say 195. In these hot temps, this week (100-105), I actually used the AC and found that the water temp reached 210. Essentially the factory gauge was reading 40 at cold start up, 25ish at speed when hot and once hot, will peg to 0 at idle. It pegs fast, very fast, but wakes back up with accelleration.

Here's what I've done so far:
  • Checked with a brand new mechanical gauge.
  • Replaced sending unit.

Here's the results:

Mechanical gauge is reading 60 psi at cold start and never drops below 16 at idle even when as hot as 210. Typically I'm seeing about a 20 psi difference bettween the factory gauge and the mechanical gauge. When the factory gauge is pegged at 0, the mechanical gauge is at least 16psi.

I've read for hours on numerous forums, there is a lot of info out there, but surprisingly no information on how to verify if you have a good sending unit or not. I hate throwing parts at things and being that I have verified good pressure and the new sending unit didn't solve the innacuracy, I'd love to know what makes a good sender.

I hooked up a compressed air line with a regulator and found the following readings with my multi meter set to 200ohms


OHMS
PSI "original sender" "New NAPA Replacement"
0 open open
10 open open
20 80 91
30 75 81
40 65 73
50 60 68
60 61 64
70 56 58
80 50 51

Both of these senders result in inaccurate readings. So what OHM readings would be considered a good sender? Has anyone ever tried testing a sender?

Could my issues be in wiring or PCM or the gauge cluster itself? This Jeep is pretty clean and the wiring appears to be untuched. The low miles are the result of this having been someone's weekend driver. It had as little as 50k miles on it as of 2010.

thanks,
-Jeff


UPDATE: Tried another sensor from NAPA, this time their $60 unit. It read even higher resistance at lower pressures. NAPA was nice enought to return the sensors. I guess my next stop is to the dealer to get an OE sender, but I am skeptical if that it will work too.

I also verifiied that there is no additional resistance present on the wiring harness for the sender both on the the ground wire and to the PCM connector.


Last edited by jeepCJTJ; 07-07-2012 at 02:35 PM. Reason: Update
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post #2 of 35 Old 07-15-2012, 07:06 AM Thread Starter
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Recieve the new factory sending unit. The results are not good. Ohms rating is nearly the same as the NAPA sending units which I had last week. The result is that the factory gauge averages about 15-20 PSI lower than what the mechanical gauge reads. This means that at a hot idle, the factory gauge pegs to zero, while the mechanical gauge reads a comfortable 18psi.

Anybody have any thoughts on what else I can do? I'm ready to give up and just wire in a resistor where the sending unit would be so the gauge looks happy and then just use the mechanical gauge.

thanks,
-Jeff
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post #3 of 35 Old 07-15-2012, 08:04 AM
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I am having the same issues you are. So far I have: changed sending unit, changed oil and dropped back 1/2 qt, changed to a different oil filter that I read has worked, sprayed contact cleaner on connectors, taken dash bezel apart and sprayed the contacts there as well. All of this and I still have the same problem. I was actually going to go and purchase a OEM sender tomorrow but after seeing you had no luck with it I am thinking twice about it.

Hopefully someone will chime in and point us in the right direction.

As a side note, my jeep runs great, it is just very aggravating to see the pressure drop to 0 and the gauge light come on. I am meticulous and this is driving me crazy.
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post #4 of 35 Old 07-15-2012, 12:52 PM
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Do you know who manufactures the NAPA and/or OEM sender?

Maybe we can get some more tests results and find a better (non-Jeep) application.


http://www.datcon.com/Tables/SDRprs-ta.htm

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post #5 of 35 Old 07-15-2012, 07:42 PM Thread Starter
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DanZ51, that's a great thought, but I'm unsure of who makes them. No idnetifiable markings on the senders. My general theory is that if I could get the 10psi output to be right around 75ohms, everything would read accuratley.

mtbrchad, You didn't mention a mechanical gauge. If you havent put one in yet to verify that the sender is the problem I'd highly reccomend it. $20 of cheap insurance. I picked up a brass Tee at the hardware store and have both installed right now. Helps to see how wacky the factory setup really is. Oddly, I believe the sending units are compeltly different bettween 97,98,99,00 yet we still have the same issue. Seems like Jeep was chasing issues year after year.
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post #6 of 35 Old 07-16-2012, 11:18 AM
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IIRC a lot of NAPA brand products are Standard Motor Products (SMP). Do you have the part number?

I didn't know Jeep changed sender part numbers during those years,,,odd.

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post #7 of 35 Old 07-16-2012, 05:00 PM Thread Starter
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The NAPA part numbers are OP6751SB for the $27 unit and OP6751 for the $68 unit
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post #8 of 35 Old 07-16-2012, 05:05 PM
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Quote:
Originally Posted by mtbrchad View Post
I am having the same issues you are. So far I have: changed sending unit, changed oil and dropped back 1/2 qt, changed to a different oil filter that I read has worked, sprayed contact cleaner on connectors, taken dash bezel apart and sprayed the contacts there as well. All of this and I still have the same problem. I was actually going to go and purchase a OEM sender tomorrow but after seeing you had no luck with it I am thinking twice about it.

Hopefully someone will chime in and point us in the right direction.

As a side note, my jeep runs great, it is just very aggravating to see the pressure drop to 0 and the gauge light come on. I am meticulous and this is driving me crazy.
Have you installed a temporary mechanical guage to verify? The pressure might actually be at zero. Mine was. The oil pump pressure relief spring would not seat all the way, so at idle it was bypassing. Above idle, and there is enough flow to register enough pressure to not trip the light. A new oil pump corrected it. An easy install, and $125 shipped with gasket.

Currently:
79 CJ7 "CJ Express" 4.2L, NV4500, D44 rear
69 800 with SBC, SM465, NP205, 14BT
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post #9 of 35 Old 07-17-2012, 07:46 AM
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Quote:
Originally Posted by jeepCJTJ View Post
The NAPA part numbers are OP6751SB for the $27 unit and OP6751 for the $68 unit
The first one, OP6751SB, is Mileage Plus Electrical part number which is owned/associated with Standard Motor Products. The second one OP6751 is an Echlin part (which I believe also supplies the Standard Motor Products line).

from NAPA Echlin
Quote:
In May of 1998, Echlin agreed to be acquired by Dana Corp. At the time, the Dana deal created an auto parts colossus but five years later in February of 2003 Dana agreed to sell a significant portion of the Engine Management operations of its Automotive Aftermarket Group to Standard Motor Products, and the Echlin brand joined the SMP family. Standard Motor Products continues to enhance and support the Echlin brand by strengthening the partnership with NAPA and positioning NAPA Echlin as the premier aftermarket automotive brand.
Echlin (and Blue Streak) is supposed to be NAPA's premium line, so I'm told.

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post #10 of 35 Old 07-17-2012, 02:22 PM
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Quote:
Originally Posted by BioTex View Post
Have you installed a temporary mechanical guage to verify? The pressure might actually be at zero. Mine was. The oil pump pressure relief spring would not seat all the way, so at idle it was bypassing. Above idle, and there is enough flow to register enough pressure to not trip the light. A new oil pump corrected it. An easy install, and $125 shipped with gasket.
I will go and get one to make sure. The PO told me that he had replaced the oil pump trying to fix the problem himself so I have not explored that. A question on that though, the oil gauge works when the engine is cold, even at idle, just after it has ran for a few minutes does the gauges start to go down. That being said wouldn't that lead to the pump actually working?
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post #11 of 35 Old 07-17-2012, 03:15 PM
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Quote:
Originally Posted by mtbrchad View Post
I will go and get one to make sure. The PO told me that he had replaced the oil pump trying to fix the problem himself so I have not explored that. A question on that though, the oil gauge works when the engine is cold, even at idle, just after it has ran for a few minutes does the gauges start to go down. That being said wouldn't that lead to the pump actually working?
Mine did that exact same thing. Only went to zero when the engine was up to temperature. When the oil is cold, it is thicker, so the pressure is greater than when it heats up and thins out. If the relief spring is slightly stuck open, the thicker oil has a harder time bleeding completely off, so you still register enough pressure to not trip the CEL.

As the oil thins, the pressure drops because the thinner oil can more easily bleed off any pressure since it flows more freely. it will trip the CEL.

If your engine mains, rods and cam bearings are in good shape, I can think of no other thing to cause such a drop in pressure. Just verify with a mechanical guage to be sure it is actually at zero.

Currently:
79 CJ7 "CJ Express" 4.2L, NV4500, D44 rear
69 800 with SBC, SM465, NP205, 14BT
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post #12 of 35 Old 07-17-2012, 03:49 PM
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I have been having this same issue with my daughters 99 XJ lately ( hot weather ) . Her friend offered to change her oil at Mastercraft where he works and the first almost 100 degree day she called me on the phone and said Check Gauge light was on and OP was at 0, Oh Great ! When she came home I checked the oil, it was a bit over full. I then looked at a tiny oil filter that was installed and removed that and put on a MOPAR . The filter so far is the only thing that has made a difference . It no longer goes to zero and turns on the check gauge light. I seems to go lower than I like to see or remember ever seeing on it or on my just sold 99 TJ . I have also changed the oil out of desperation , I used Valvoline VR-1 10w30 , no effect in the least just lighter wallet . I have also switched OP sending units, I have a NAPA Echlin #OP6185 that changed nothing but my wallet some more . It never goes to 0 anymore but the MOPAR filter seemed to of stopped that but either way the gauge isn't right . I had read a lot about this on NAXJA forum also but no one has found a cure that I have found .
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post #13 of 35 Old 07-17-2012, 05:01 PM Thread Starter
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Originally Posted by Gary2 View Post
I have been having this same issue with my daughters 99 XJ lately ( hot weather ) . Her friend offered to change her oil at Mastercraft where he works and the first almost 100 degree day she called me on the phone and said Check Gauge light was on and OP was at 0, Oh Great ! When she came home I checked the oil, it was a bit over full. I then looked at a tiny oil filter that was installed and removed that and put on a MOPAR . The filter so far is the only thing that has made a difference . It no longer goes to zero and turns on the check gauge light. I seems to go lower than I like to see or remember ever seeing on it or on my just sold 99 TJ . I have also changed the oil out of desperation , I used Valvoline VR-1 10w30 , no effect in the least just lighter wallet . I have also switched OP sending units, I have a NAPA Echlin #OP6185 that changed nothing but my wallet some more . It never goes to 0 anymore but the MOPAR filter seemed to of stopped that but either way the gauge isn't right . I had read a lot about this on NAXJA forum also but no one has found a cure that I have found .
I'd really reccomend the mechanical gague to verify if you are dealing with a real oil pressure issue or a sending unit issue. Another thing you can try is to connect your sending unit to a compressed air line with the reguator set to 0. Connect the sending unit to the wiring harness under the hood. Turn the key on (don't start the engine). Then slowly turn the pressure up in 10 psi increments and verify what the gauge is reading. If it is a close match to what PSI of air you are putting in, then your factory sending unit and gauge are in good shape. In my case, i have a consistent 15-20psi variance.
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post #14 of 35 Old 07-17-2012, 05:21 PM
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if it was a month ago when this started and I had not run across so many threads in the last month (since it got hot ) with the same complaints and results on many different forums dealing with the 4.0 I would. At this point I will live with it , if something was as wrong as the gauge says mine and alot others would be dead now. seems way to common in the last month or so. Hers never drops to 0 any more since the new oil filter it just reads very low and if it is about 16psi higher than the gauge says its good enough for me . When it read 0 and idled a long time it never sounded any different than it ever has . 0 Oil pressure or close to it for 5-10 minutes will create some ticks or knocks . Both OE and NAPA sending units produced the same reading on the gauge . I'm done with this issue other than reading about it .
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post #15 of 35 Old 07-17-2012, 06:45 PM
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Quote:
Originally Posted by BioTex View Post
Mine did that exact same thing. Only went to zero when the engine was up to temperature. When the oil is cold, it is thicker, so the pressure is greater than when it heats up and thins out. If the relief spring is slightly stuck open, the thicker oil has a harder time bleeding completely off, so you still register enough pressure to not trip the CEL.

As the oil thins, the pressure drops because the thinner oil can more easily bleed off any pressure since it flows more freely. it will trip the CEL.

If your engine mains, rods and cam bearings are in good shape, I can think of no other thing to cause such a drop in pressure. Just verify with a mechanical guage to be sure it is actually at zero.
Will do, I will try and check it tomorrow. Thanks for the information.
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