bought a 99 cherokee classic last month, the oil pressure gague in the cluster is pegged all the time (shows 80lb pressure) weather the engine is running or not. does anyone know the nominal resting resistance of the oil pressure sending unit is so that i can check to see if the sender is nuked like on a chevy or VDO brand gague, if there is no wire hooked to a VDO the gague will pegg and also if the sender is toast. if the resistance checks out it'll probably be the connection in the dash or somewhere along the circuit, and also what is the average running pressure range of a 4.0?

 

EXACT thing happened to me. I just replaced the sending unit and it was good to go. I was very confused why it stayed pegged even after disconnecting the wire also. I had managed to crack the plastic from the metal during an oil change and the thing was pegged no matter what I tried to fix it.

I know you want to diagnose it first, but the part is $20 and not worth testing, imo.

Just make sure you get the sending unit, some places try to sell you a part that looks identical but is a (switch?) that goes on the other side of the engine.

Edit: the oil pressure should run between 40-60 when heated up, and can drop to about 20 at idle.

 

totally awesome thanks man, yea since i bought my xj i figured it was the sending unit but $20 is cheap, thanks for the average running pressures too. the other paret "switch" must be for an idiot light or buzzer.

 

yeah I learned the hard way, bought the switch and it fit in perfect, wire plugged right in, but my pressure stayed pegged. I was scratching my head until i came here and someone told me the possible mistake. So i paid $20 twice lol

 

whoops, stuff like that happens all the time just makes you wish you had the $20 back tho lol

 

I've had best luck with the sending units directly from Jeep. Some (not all) aftermarket sensors are a bit off in accuracy and not long in the longevity department. Worth the extra $20 in my opinion.

Oil pressure spec for the 4.0 is as follows:

*Not less than 13 psi at HOT idle
*Between 37-75 over 1600 rpms

Good luck!