Oil Light Malfuctioning???

[Shane]

Hi I have an 89 MJ and I believe that something may be wrong with my oil light or the sending unit. My mj has the idiot lights. Tonight on the way to the movies the light came on. I checked it when we got there and when we left. It was a little low so I added a quart. The light stayed off for a while then it started coming on when I mashed the gas, then it went off as I let off the gas. Checked the oil again and it read full on the dipstick. What could the problem be? I thought maybe the sending unit was going out. Next thing in time I would like to switch out the gauges to where they have the needles on everything. What is involved in doing that? Again thanks for the advice it is very appreciated. Thanks Shane

[Wildman]

You could have an oil pump that's starting to fail, or the oil pickup screen is plugged with sludge.

 

Get a mechanical oil pressure gauge, and plumb it in place of the sending unit, you should be reading 13psi at idle (600rpm) and 37-75psi at 1600rpm+ {this is from the '89 FSM for a 4.0}

 

Yes, it could be just a bad sending unit too, a $6 - $12 part.

 

I would check with the mechanical gauge first, and see what the reading is, and not drive around to much with the oil pressure failing like that.

 

If it's the oil pump, it would be a good time to change the rear main seal, the better one piece pan gasket.......and other little items that go with the process of changing out the oil pump.

 

As far as changing all the idiot lights with gauges........there are several articles posted on that subject that you can find.

 

And........How was the movie?????

[BLHTAZ]

Definitely get a check the actual pressure. Mine had oil pressure issues and was barely able to hold 25 psi at 2000 RPM. Mine ended up being worn rod & main bearings on the crank shaft so I also repalced the oil pump at the same time when doing the bearings.

 

While it could be just a switch going bad...my fear for you is that it's not that simple as the switches in these things rarely ever go bad. The guage senders go bad much more often on vehicles with guage clusters.

 

I would definitley make the switch to the guage cluster. It's not a hard thing to do and it enables you to see problems coming instead of waiting until the idiot light comes on and the issues is already bad.

[Pete M]

What brand oil filter are you using?

[Eagle]

+1 to the above. Check with a mechanical gauge, and be sure NEVER to use a FRam filter.

 

But ... you don't seem to understand what the oil light does. It's not a warning on low oil level, it's a warning of low oil pressure. The dipstick tells you the level, not the light.

[BLHTAZ]

What brand oil filter are you using?

Yep...didn't think of that. I have seen the FRAM filters cause low oil pressure problems many times.

[Shane]

I checked the oil pressure. It was @ 14 psi at idle. I use Motocraft oil filters. I know that the light doesn't tell you when the oil is low, but is it not true that low oil causes low oil pressure? Mine was low @ first. I drove it a little today and the light did not come on. What is a realistic diagnosis since the oil pressure checked out. Could any one find a thresd of the gauge swap. Thanks again Shane

[BLHTAZ]

14 psi at idle does not tell me that it's OK. What was it at 2000 RPM? Mine would sit at 12-14 at idle, but not come up past 20-25 at 2000 RPM with the engine warm. You need to make sure that the engine is at full operating temperature when you do pressure testing.

[Shane]

I do not remember exactly, but the guy that was checking it with me, the one that had the tester, said that it was good I believe it was around 30-35 psi @ 2k rpm. So what could it be? The light has not came on @ all Idrove it for about 5 miles the 1st time today and the 2nd time a bit ago was about 25 miles and it has not came on at all today. It never hit me to check the oil pressure though. I guess that was a blonde moment. Thanks a gain for the advice. If there is any more please let me know. Thanks Shane

[Eagle]

... is it not true that low oil causes low oil pressure?

No. Not true. Not until it gets so low the pump can't pick up oil. At that point you don't have low oil pressure, you have no oil pressure.

 

I do not remember exactly, but the guy that was checking it with me, the one that had the tester, said that it was good I believe it was around 30-35 psi @ 2k rpm.

Those numbers do not indicate "good." The factory spec is 13 psi minimum at 650 RPM idle, and 37 to 75 psi above 1600 RPM. As BLHTAZ noted, that's at normal operating temperature. Most 4.0L XJs and MJs run between 50 and 55 psi on the highway at the speed limit, which is around 2000 RPM (or lower if you have a 5-speed). Your pressures are marginal, at best.

 

What weight oil are you running?

[BLHTAZ]

Yeah...sounds to me like you are running on the boarder line of pressure. As stated before...it is possible that the switch is having issues and is grounding out when it shouldn't be. If you continue to have trouble, try changing it if you wish as it is not and expensive thing so it wouldn't be a big waste.

 

I personally think you have a bearing and/or oil pump job in your not to distant future.