oil pressure gauge issue-remixed

[Joshooha]

1988 MJ, 4.0 renix. 5 speed manual with full set of gauges.


When I purchased this fine piece of equipment the oil pressure gauge was pegged with engine running. I popped the hood and noticed immediately that the sending unit was definitely original with 193k. So being the super smart person I am I replaced it! Of course it didn't fix the issue. So I decided I should step up my brain power and check the wiring to the sending unit.

Wire to sending unit with KOEO is 5 volts, then with the KOER it reads 11.88 volts. Pretty standard I would imagine. So I began to think to myself "their is no way in hell I have been driving this thing with a pegged gauge for this long and haven't blown anything up, if the gauge is in fact accurate"

I then hop in the car and take a ride to advanced auto and pick up a sunpro oil pressure gauge to test what we got for oil pressure! I plugged the new gauge into the new sending unit and fire the engine up. Engine cold we got 40psi! perfection, I am happy with it. We draw the line from there into the interior...
Kicking the brain into 5th gear now, before removing anything I inspected all the wiring going into the gauges and the gauges themselves for tampering or burning and everything seems to be legit.

I then take out and replace the oil pressure gauge with a junkyard one (as a matter of fact I have 3 junkyard gauges to try). I do the KOEO test and the gauge sweeps to zero, I then fire the engine. Pegs back to 80 psi! So, I figure "okay, bad junkyard gauge." -Of course that's not the case, I replace it with all 3 gauges and still the same thing.

Lets move back to the engine bay area where we started. I do a little test. KOEO with the sender plugged in, the gauge goes to 0 (where it should be) I then unplug the sender and the gauge pegs to 80psi still with the KOEO. I then take a jumper and jump the oil pressure sender wire to the battery ground (koeo still) the gauge goes back to 0. This indicates to me that I have a bad sending unit. Being brand new I find this hard to believe.



IN CONCLUSION: the oil pressure gauges sweeps properly with engine not running. While engine running the gauge pegs immediately. Put new sending unit in and still had same issue. checked oil pressure at new sending unit with a manual gauge and reads 40 psi when cold which is normal. Checked wire into sending unit and voltage is fine. Swapped out oil pressure gauge and still have same issue. Rechecked brand new sending unit with simple test of unplugging and grounding the circuit and the unit failed the test.

any further tests/opinions/complaints/mockery anyone has to offer would be greatly appreciated.

Without any further opinions I will replace the sending unit with a dealer issued part and/or go deeper into the wiring schematics of this pig.

:hang:

 

[Pelican]

Bear with me -- I'm blind in one eye (literally}, presumably temporarily, and I'm having trouble following this. One thing is I'm not sure what "KOEO" is and too lazy to try and figure it out.

That aside are you saying you have a mechanical gauge and have removed the sending unit and plugged the mechanical gauge into the oil pressure port? What are the specific readings engine cold idle and at speed and engine hot idle and speed.

Then I take it that you have taken readings with one of the replacement senders plugged into the oil pressure port and wired directly to one of the junkyard gauges. Give us the same readings as above (hot/cold, idle/speed).

We can go from there.

 

[old_man]

Are you sure you didn't get the switch version instead of the gauge version of the sender?

 

[Pelican]

^^Good question.

 

[Scorpion8]

One thing is I'm not sure what "KOEO" is and too lazy to try and figure it out.

KOEO - Key ON Engine Off
KOER - Key ON Engine Running

 

[Sidewinder CC]

I don't know anything about '88 MJ's, but looking at the wiring diagrams for your truck I have a few comments:

How long have you had this truck? Was it built with a gauge package or was it retrofitted?

"Wire to sending unit with KOEO is 5 volts,"

I don't see where 5 volts would be coming from with KOEO. See what you get with KOff EOff.

Try seeing if you also have 5 volts at the coolant gauge sending unit connector with KOEO. If so, I can't figure out why. If you don't get 5 volts at the coolent temp sender then there may be a short to power somewhere to your oil pressure wire (LT BLU).

 

[Manic Mechanic]

KOEO - Key off/engine off? (EDIT: By the time I hit post, everyone was on it already...too slow...)

I'm in the same boat myself, haven't gotten around to chasing down what the problem is, but am curious as to what yours is, may be helpful info when I get around to dealing with mine.

-Eric

 

[Joshooha]

Are you sure you didn't get the switch version instead of the gauge version of the sender?

Yes, I checked at advanced, they did offer both and I made sure by looking at both that it is in fact the one for the gauges.


Sidewinder CC-How long have you had this truck? Was it built with a gauge package or was it retrofitted?

I have had this truck for about 4 months and yes I believe it was built with the gauges, if not that is the best aftermarket work I have ever seen, dust and all!


Pelican-That aside are you saying you have a mechanical gauge and have removed the sending unit and plugged the mechanical gauge into the oil pressure port? What are the specific readings engine cold idle and at speed and engine hot idle and speed.

yes I unscrewed the new sending unit and plugged in a sunpro gauge to get a reading. Cold idle its 40psi steady, and fluctuates between 35 and 40 when warm.

Pelican-Then I take it that you have taken readings with one of the replacement senders plugged into the oil pressure port and wired directly to one of the junkyard gauges. Give us the same readings as above (hot/cold, idle/speed).

In all honesty that's a great idea, didn't think about doing that. Don't know how to though. I will do some research and get back with you on that.

Sidewinder CC-Try seeing if you also have 5 volts at the coolant gauge sending unit connector with KOEO. If so, I can't figure out why. If you don't get 5 volts at the coolent temp sender then there may be a short to power somewhere to your oil pressure wire (LT BLU).

I will try that next. I understand that there are 2 coolant temp sensors on these renix motors. which one is for the gauge? the block sensor, or the head sensor?

 

[Sidewinder CC]

Disregard the coolant temp sdensor for now.

Is there three wires or two wires going to your oil PSI sending unit?

Later model XJ's had three wires going to the sensor, one of which brought 5 Volts to the sensor from the PCU. The other two wires are Sensor Ground (through the PCU), and the other is oil pressure sensor signal to the PCU. These systems, such as TPS & MAP used 5 Volts to transmit data to the PCU.

The '88 wiring diagrams show two wires, but it also shows the one wire from the sensor, LT BLU, going to connector "C6" then to the instrument cluster connector, Pin A2, and the other, BLK, going to ground.

If you go to pin A2 at the instrument cluster connector and read out to the sensor, see if there is 5 volts present. If so it's coming from that connector pin C6, The only connector pin C6 I could find was on the ECM, but that's "MAP Output" from the MAP sensor.