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Not Getting An Oil Pressure Reading; Used 2 Senders


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Hello again, a few of you recently helped me out with my '90 Cherokee. On the same vehicle( 4.0 5spd), the oil gauge doesn't read anything, though I'm almost positive the oil pump is still good. It died on a trip this winter. I tried ohming it out, and got erratic readings. Also tried using another working sender from an MJ, and I still got no gauge action. I'm curious if the sender feeder, or whatever it's called, can get plugged since it's only the size of a pinhole? Only idea I got.

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Check your wiring. Ignition ON (engine can be running, but does not need to be).

 

1. Remove wire from sender. Leave it hanging in air and observe gauge. Should be pegged at 80 psi.

 

2. Ground wire to a known good ground. Observe gauge. Should be pegged at zero psi.

 

If this test fails, you have a broken circuit between the sender and the gauge.

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Take the sensor out and screw in a mechanical pressure gauge to see your actual oil pressure. Sounds like you have one (or more) of 3 problems as noted.above:

Wrong sensor - make sure you have the sensor for a gauge, not for a light

Bad circuit - if you get a positive reading on the mechanical gauge (you should since your engine hasn't locked from lack of lubrication), check the circuit as Eagle explained

Bad gauge - pull your cluster out and switch the gauge for one from another cluster. There may be a way to test the gauge but I don't know what it would be.

My theory is you should have a spare cluster for when all or part of yours craps out.

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The gauge should be good. It was working when I first got the vehicle, then quit midway through a couple hundred mile trip this winter. The sender itself is indeed the correct one for real gauges, so that's not the issue either. I'm thinking I either have a plugged feeder or a broken circuit between the sender and the gauge. I'm going to go check it out right now. Thanks for the input.

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Okay, I just unscrewed the brass hexagonal fitting and it was clean, no sludge at all. So, while it was off I had my roommate crank it over and oil made a fountain a couple feet high, ergo the pump does indeed work. So I then checked the plug to Eagle's specs, and the needle didn't move. It barely twitched, though it did move a hair's breadth. Definitely a wiring problem. Any idea where I go from here?

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Yes, you will need to pull the cluster. Not sure if the 90 speedo cable is mechanical or electrical. Make sure you have enough slack to get your hand behind the cluster to disconnect the speedo. Good time to check all the lighting, replace bulbs as needed

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Alright, it does indeed have continuity up to the instrument connector. So what now, I have a bad gauge and it needs replacing, or can I remove it and clean it, or does the Russian method of beat it with a wrench work?

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I pulled the gauge out of the cluster and visually inspected it. It looked ok, the tiny copper filaments weren't broken, nothing else seemed broken, so I put it back in and made sure the screws on the back were tight. Still no cigar. This means I honestly have a bad gauge? Didn't think gauges went bad...

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I pulled the gauge out of the cluster and visually inspected it. It looked ok, the tiny copper filaments weren't broken, nothing else seemed broken, so I put it back in and made sure the screws on the back were tight. Still no cigar. This means I honestly have a bad gauge? Didn't think gauges went bad...

 

Visually inspecting an electronic gauge is only the first step. If no obvious breaks are detected, touch a 1.5V or 9V battery across the + and - terminals (observing polarity) momentarily and look for instant meter deflection. If no needle deflection, the meter movement mechanism is shot. Not an uncommon problem with any electronic gauge.

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There is also a possibility that the printed circuit on the back of the cluster has been damaged. Shouldn't happen unless you've been working under the dash and maybe slipped with a tool, but it's a possibility.

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You could try hot-wiring it. Take a 12-volt wire from any circuit that's switched by the ignition and run that to the positive terminal on the gauge. Run a second wire direct from the negative terminal on the gauge to the sender.

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Ok I'll see if I have to resort to that after I've put it all back together. Now I realize that the sender itself is self-grounded, but are there any other grounds that could be the issue? Oh, and the continuity of the printed connector sheet on the back of the cluster is good, so that isn't the issue either.

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Ok I'll see if I have to resort to that after I've put it all back together. Now I realize that the sender itself is self-grounded, but are there any other grounds that could be the issue? Oh, and the continuity of the printed connector sheet on the back of the cluster is good, so that isn't the issue either.

If the gauge is good and the printed circuit has continuity, then the problem HAS to be in the wiring between the sender and the printed circuit, or in the power feed TO the gauge cluster. But the other gauges (temperature and fuel) work the same way, so if they're giving you readings you have 12 volts to the cluster. Did you check the printed circuit for continuity of the power feed from the wiring connector to the gauge?

 

If you have a multimeter, you can run another check on the sender. The ohm rating is 0 to 88 ohms. According to the FSM, 0 psi should return a reading of 1 ohm, 40 psi should return 46 ohms, and 80 psi should return 87 ohms. So if you put the multimeter on the sender connector and the other probe to ground, you should be reading 0 or 1 ohm with the engine off. Idle is about 25 psi (typically) so by interpolation if you start the engine and let it idle, you should read something like 20 to 25 ohms, and if you rev it to around 2000 RPM (cruising speed) you should get 46 ohms or better.

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I just fixed it. I cleaned all connection points on the printed back, the 3 screws holding the gauge in, as well as pronouncing the spring on the jack that plugs into the back of the gauge cluster. Somewhere between those 3 things it was resolved. Thank you for your help everybody, especially Eagle. Thanks  :bowdown:

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