First of all, If you have a '97 '98 or early '99, there is a common fault in this circuit, and most dealers will replace the sending unit and the gauge if necessary for free, saving you the trouble of fixing it yourself. For other years, especially earlier models, read on...
Since the oil pressure gauge gets its 12 volt power from the same source that the voltmeter does (the same trace on the instrument circuit board), it is not likely that the problem is in the 12 volt side of the circuit unless your voltmeter is also acting up. A faulty reading usually means there is a problem in the ground circuit, which is the sending unit and the wire to the gauge.
For vehicles with an oil pressure gauge, the sending unit (located next to the oil filter) is essentially a variable path to ground for the gauge. When the is engine off, the sender is shorted to ground, and the gauge reads "0" psi (or close), as the oil pressure increases, the sender offers resistance to ground, providing a mid scale readings on the gauge. If the wire is removed altogether, the gauge will read off the scale to the high side (3:00 position) all the time. This is a quick and simple test you can perform to help isolate the problem. With the key ON - engine OFF, pull the wire off the sending unit. The gauge should read full scale. Now short the wire to ground (this will not harm anything), and it should read "0". If this is correct, then your gauge, and the wiring is good, and your trouble is likely to be the sending unit. If this test has little or no effect on the gauge, then your problem is in the wiring or the gauge itself.
For vehicles with an indicator light, the sending unit is like a simple switch. When the engine is off, or the pressure is too low, the switch closes, and illuminates the lamp. Under pressure, the switch is open and the lamp goes out. The only difference in the above test, is with the wire disconnected, the light is out, and with the wire grounded, the light is on.
To test the sender, you will need an ohm meter. Disconnect the wire, and measure from the connector of the sender to ground with the engine off. The reading should be "0" ohms (or close). Now start the engine, and the reading should increase to over 100 ohms (infinite for the idiot light sender). As you rev the engine, the reading should increase some (this will vary from engine to engine), but the readings should be somewhat stable, and not erratic.
Now if you have determined that the gauge and sending unit are good, yet your gauge reads high all the time with the sender hooked up, you have an open in the wiring. If it reads low all the time, then the wire is shorted somewhere. If you have an erratic reading, there is an intermittent problem in the wiring, ....If it jumps from normal to the high side its opening intermittently, if it jumps to the low side, it's shorting.
Make sure the connection to the sender is clean and secure, and inspect under the rubber boot. This is the number 1 trouble area in the circuit. It gets subjected to vibration, high temps, oil, mud and water, and can become corroded, or the wire can come loose or break inside the boot. If these connections are good, You'll need to trace the wiring back to the gauge, looking for the trouble. The sender wire (light blue - can appear green with age) goes through a couple of connectors before it arrives at the instrument cluster. The first one is a large flat 6 conductor connector located just to the rear of the battery, partially under the front edge the relay cover (on late '80 and early 90s models). You can pull this apart, and inspect the pins and sockets (pin "B") for corrosion, and tug on the wires to be sure they are firmly connected. The next connector is the massive one located on the firewall, below and to the left of the steering column, however, this one is not easy to troubleshoot, but is seldom the source of electrical problems, so on the the connector at the rear of instrument cluster (pin #2). From here it travels on a circuit board trace to the pressure gauge. The gauge is secured with 3 studs, through the circuit board, these are also the electrical connections for the gauge. You can check the nuts for tightness, and corrosion around the connections.
If the connections all look good, and the sender has been replaced, (or tested good), but you still have a problem, I would suspect the circuit board had a small crack in the trace(s) connected to the gauge. This is a very common problem with the newer "flex" type boards, and very difficult to see, or repair. It would be possible to run a new wire, by-passing the connector and trace on the board, directly to the gauge. Otherwise, the printed circuit board will need to be replaced. It is not vary expensive, however as of this writing (8/99), the instrument circuit board is no longer available from the dealer, and only a few dealers have any left in stock.
On another note, a cheap oil filter (even Fram) can cause problems with your oil pressure. The XJ oil filter sits upside down on early models of the 4.0, and sideways on later models. The filter needs to have an "Anti-Drainback valve" to prevent the filter from emptying out every time the engine is shut off. Cheaper filters use an inferior design for this valve (or no valve at all), that do not provide a good seal, and allows the oil to seep out when the engine is off. This is very bad, since the oil pump has to fill the canister every time you start the engine, and this can take several seconds, during which time your engine is running with little or no lubrication at all. Start up is already the hardest condition your engine will ever experience (most ware takes place during this time), so it is very important that the Drainback valve is of a good design, and is working properly. Another problem with low end filters, is the reduced surface area of the filter element, and the more porous material used in construction. These can give either very high or very low readings, that are not reliable, or a true indication of the oil system itself. I know it's hard to justify spending extra money on a disposable item like an oil filter, but the bottom line is not money here, it's engine life. Even though it is disposable, the oil filter is an integral part of the lubrication system, so this is not a corner you do not want to cut!
I hope this info will
be of some value, you may e-mail me privately if you have any questions,
or would like more details on rewiring the gauge around a defective circuit
board.
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