yungcas

The oil pressure in my 2002 Jeep Grand Cherokee Laredo kept dropping to 0 while i was idling at red lights, but would jusmo back up to 40 when i took off. Can anybody tell me what caused that? & How I can fix it?

5-Speed

Verify the pressure with a mechanical gauge before doing anything else. The psi sending units are known to fail.

dave1123

Welcome to CF!

Somewhere in the 2002 production run, the oil pressure sensor stopped being an actual pressure sensor and was just a slide switch that gave a 40lb reading or a 0lb reading. If you change it, go for a 99-01 sensor. I bought one at NAPA and they only stock the newer one anymore. Changed it for a Mopar for my year. They have different part numbers.

Bugout4x4

Well that is dumb as hell. Why did they even put a gauge in it then? An idiot light would be just as useful. lol

dave1123

Chrysler cars always had a full set of gauges and the thought was they would look cheap if they did away with them and sales would fall off. Besides that, the gauge clusters would have to be changed and the WJ was scheduled for another 3 years run. At least that's the explanation I got from a salesman.

When my oil pressure sensor failed, it broke taking it out. Inside there was a pressure sensitive chip and a printed circuit. it was probably cheaper to just make it a switch with a resistor to limit the output.

One other thing I discovered by reading my FSM. All the information displayed on the gauges do not come directly from the sensors, but are interpreted by the PCM and then displayed.

Yesterday morning my battery went dead. When I tried to start it, the speedo and tach needles would swing wildly around in circles. I read 7.8 volts on the posts. After charging for 2 hours, it struggled and fired up, but my speedo and tach weren't working. I noticed both needles were on the wrong side of the stops! After putting a new battery in it, everything returned to normal.

Bugout4x4

I had the hardest time explaining this to old school truck drivers who used to jump "sending units" with a paperclip to try and make them run. I had to explain that they don't work like that anymore. They are like a modem sending a radio signal on a modified wave to another modem. There is no resistance involved at all and if the computer doesn't "see" the chip on the other end it's broke until it sees it again.

I had to use a cell phone analogy, The computer is trying to call a phone that is broke. And until you repair the phone on the other end the call is not going to be completed to then share the information. "But there is a wire?" Yes...but it could actually be done wireless, it is just cheaper and more reliable to send the radio signal through a wire. Then I would ask them if a call from one cell phone to a broken cell phone can be forced to go through and work if you put a paper clip between them? "well no.." Of course not, so stop trying to do that... lol

dave1123

Another funny item. My brother worked for a short time for a heavy construction company involved with the "fracking" industry in PA. His job was to organize the maintenance of the equipment. He related a story about a bucket loader that sent him a text message on his cellphone that it needed an oil change! Now THAT'S integration!

He found 6 wiring harness kits that were used to update some of these machines that had about a dozen or so color-coded 16ga wires attached to a gang plug, left over from the project. I've been using them for my wiring "fixes" since then. Some of these wires were 8ft long!

Bugout4x4

It gets better Dave. You can track them and monitor all the systems in real time as they are being used. I tracked my trucks and my reefer unit systems on the go by satellite from my office computer. It was almost like a video game. lol

dave1123

Yeah, I read an article about the police being able to shut down the engines in Onstar equipped GM cars after getting authorization. Again, what's next?

Bugout4x4

They are all this way now... In fact they can be hacked by unauthorized entities. It's even happened to commercial aircraft. We have gone too far with this technology.

dave1123

Yeah, give me a CJ with breaker-point ignition and a carb and I'm happy!

My dad once said the technology is there for pilotless airliners, but nobody in their right mind would fly in one. They also toyed with the idea of a transcontinental rocket powered airliner that would cross the Pacific in 2 hours, but would require passengers to withstand a 2G acceleration for 15 minutes.

extrashaky

Funny thing about that. The temperature gauge on the dash of my Subaru looks like a real gauge, just with no numbers. It has marks for cold, normal and hot along a curve that the needle follows.

I run Torque as an engine monitor, and it shows me the actual coolant temp the computer is receiving before it "interprets" it for display on the dash. I was watching and noticed that the needle seems to follow the actual temp until it gets to about 180°. At that point the needle has reached the "normal" point and stops moving while the actual temp continues to climb. With the air conditioner on, the normal operating temp is between 190° and 195°.

However, if you turn the air conditioner off, the ECM changes the behavior of the fan so that it allows the coolant temp to continue rising to 220°. The needle on the dash stays right at the same "normal" position through a range of at least 40°, possibly much more. I don't know how hot it has to get before the needle starts moving again.

It scared the crap out of me the first time I saw it do that, so I took it to the dealership to have it checked. The tech told me that was normal behavior. The car actually does have a "normal" coolant temperature range of more than 40°. The gauge sticks at the same spot every time to keep people from doing exactly what I did, see the temp go up and take it in to be checked unnecessarily. They were nice about it, though.

People on the community message board for this car were convinced it was a real gauge. There were even people who had built them into track cars without ever noticing that the gauge wasn't real. I argued with people until some of them went out and tested it for themselves. It seems most manufacturers are doing this now, so it's really nothing more than a glorified idiot light.

dave1123

Right! They are for glorified idiots, which is what they think WE are! I'm the type of guy who scans my gauges all the time so I don't get flatfooted when something goes wrong. I've been caught by not checking my oil level in my Chevy truck often enough to see the gauge suddenly drop when going around a sharp corner, stopping and checking, and finding out it's 2-1/2 quarts low! I had a bad oil plug gasket and didn't know it. I could have toasted a perfectly good 350! That's when you stick a couple of quarts of pisswater convenience store oil in it to get it home and fix it.

extrashaky

Well, to be fair, there are enough idiots out there that it's not a bad idea for the manufacturers to build for the stupid.

Bugout4x4

This sounds like another case where it was purposely engineered to kill engines.