Scorpion8

My Jeep lives again, but still has a lot of issues. Not the least is a "Check Gauges" light. All the gauges check out fine, but ..... Voltage meter fluctuates, and the oil pressure gauge is well-not-kinda-completely-dead. I just changed the oil, and put in a new oil pressure sender. Fired her up, and the oil pressure went right up to mid-gauge, and then died to just above zero. But it is above zero, and when I turn the engine OFF, it goes below zero. So it is showing "something", and the new sender unit didn't fix that.

So I'm thinking something electrical, not mechanical. Engine gets good oil pressure, and oddly shows no codes on my scanner. ECU issue? I haven't driven it except to turn it around in the driveway and to put it on ramps to change the oil, but neither time did the oil pressure gauge change.

Help, please!

tjwalker

Best thing to do with goofy oil pressure readings is to verify.

Good data = good decisions

Get a mechanical gauge (can be rented from many parts stores; call around) and temporarily install it where the oil pressure sending unit currently resides.

This way, you'll know for sure. There can be inaccuracy between the oil pressure sending unit and the gauge. It is not impossible that the gauge is the cause here.

Oil pressure spec for the 4.0 is as follows:

*Not less than 13 psi at hot idle

*Between 37-75 over 1600 rpms

Scorpion8

Thanks, I'll try that. But there are quite a few electrical "issues" from the previous owners self (POS) who started the rebuild, that I'm wondering if a lot of my issues are electrical. My voltage meter goes up, drops off, then while seemingly randomly fluctuate up and down. Ideas there?

tjwalker

Grounds can be the root cause of many electrical gremlins. Refreshing grounds is NEVER a bad idea!

You can't just check or look at these grounds. You must remove, scrape, clean until shiny the cable/wire ends and whatever they bolt to.

One of the main ones is on the back corner of the head, and where it attaches to the firewall. Scrape the paint from the firewall. Best to replace that woven cable with a #4 or #2 gauge cable. You can attach the one end to the intake manifold if you would like.

Next go over to the engine dipstick tube stud. Remove the nut and clean the wire ends and scrape the block until shiny at the stud. Reattach tightly.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support. Scrape the paint off first.

Scorpion8

Update: I check the oil pressure with a hard gauge and have 40~50psi at idle, so I know I'm getting good oil pressure. Just that the sender isn't showing it at the instrument cluster.

Okay, the oil pressure sender on the 98's (and most others) for a system with gauge is a two wire system, so it "should" be easy. One wire, brn/yel is a sensor common ground. If I check continuity from the sensor plug to the engine block (where this sensor line grounds) I get good strong continuity. The other wire, grey/yel goes from the oil sender to the ECU, cavity B23. I check continuity from the sensor plug to the ECU multiplug, I get a good continuity signal. When running, if I pierce the grey/yel line and read voltage I get a fluctuating .5~.4 volts at the ECU pin. There is no line from the ECU to the instrument cluster, its all placed on the data bus.

So, does anybody know what the voltage level range "should be" at the ECU for any sensor?

All my other instrument cluster gauges work fine, so it's not the cluster unless this individual oil pressure gauge isn't working right. And since the other gauges read properly, they are getting the signal from the data bus. Only the oil pressure acts like this.

Ideas?

thakmfb

what kind of oil pressure sensor did you use? I have heard some of the cheaper ones are hit or miss and that MOPAR is the way to go on this particular part

Scorpion8

Two different ones, one from NAPA and one from O'Reilleys.