1990 Suburban oil pressure gauge only reads 30 PSI

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haromaster87

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1990
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Suburban R1500
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350 TBI
Hey everyone,

I am working on the dash refresh for my 1990 R1500 Suburban and I'm trying to sort out some odd gauge behavior it has, specifically the Oil Pressure gauge. Since I got the truck, the OP gauge has always behaved more like an idiot light. It reads 0 PSI when the truck is off, and reads right dead in the middle on 30 PSI when the engine is running. Regardless of oil temp or RPM, the gauge sits right on 30 PSI and doesn't budge until I shut the truck off.

First I want to make sure, is that abnormal behavior for the 1988-1991 square bodies? I noticed when I pulled the cluster that it's different from 81-87 trucks(electronic speedo, different bulb connectors) and wondered if they changed the gauge behavior to just sit in the middle as long as pressure is above a certain amount? Or should the gauge vary with the oil pressure like the earlier trucks?

If the latter, any clue what would cause the gauge to still work, but only read that single reading? After googling every word combo I can think of, I've found two other folks that described this same "issue" on their truck. One of them said his gauge just went back to normal(variable readings) one day with no intervention, and the other never mentioned if it was solved or not. I pulled the gauge and measured the resistor on the back of it, it measures ~84 ohms when off the gauge, and ~42 ohms when installed back on the gauge. Not sure if that's normal either, it's hard to find the correct values for those. I don't remember the exact numbers now, but the sending unit seemed to test okay in the sense that it varied its value with engine rpm.

Thanks for any info or ideas! -Tony
 
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haromaster87

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Can any one with a 1988-1991 Suburban tell me their oil pressure gauge behavior? Does it move around at all or sit at a fixed value regardless of engine conditions? I have tested every thing I can find on this gauge.

I have confirmed the ohm value varies at the actual connection contacts in the cluster housing. But regardless of what ohm reading is showing on the contacts, the gauge immediately snaps to 30 PSI as soon as the posts touch the contacts.

I tested the ceramic resistor on the back of the gauge, it is not open and shows around ~82 ohms. I have also cleaned the contacts on the resistor and reinstalled it, no change. At the same time, I'm also fighting an issue of the temp gauge not moving off of 100, regardless of engine temp. I have also confirmed the resistor on the gauge is fine and that the contacts are showing a correct ohm value that decreases with a rise in engine temp.

Initially I thought the issues were unrelated since the volt gauge reads fine and the speedo and gas gauge work fine as well. But now I'm wondering if there could be a common issue.
 

haromaster87

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Maybe someone will know this, should there be continuity between the +12v signal and the sensor signal on any of the gauges when the key is on?

I found that on both the oil pressure and temp gauge, the sensor wire/connector shows continuity with the pink/black wire/connector that supplies +12v to the gauges. Is that normal? It's not what I expected, but while digging around I came by reply #2 in this thread: https://www.gmsquarebody.com/threads/temp-gauge-issues.19919/

He states "So one thing you can do is pull the sender wire and measure the voltage between it and battery negative" and that it should be near battery voltage. So do the sensor wires normally have +12v on them? That's a bit counter to my previous understanding of how the sensors function, but I'll be the first to say my understanding was already kind of poor haha.
 
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haromaster87

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Well I got the oil pressure gauge working, after reseating the oil pressure sensor connector again, it started working normally. Reads ~40 PSI at cold idle and goes up a bit when I rev. Still have to figure out this temp gauge issue, but based on this thread, I might seek out another forum to get some advising on that one lol.
 

gmbellew

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Did you pull the temp gauge and reseat it? Might just be another issue of bad contact between the gauge and the cluster housing
 

Bextreme04

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Well I got the oil pressure gauge working, after reseating the oil pressure sensor connector again, it started working normally. Reads ~40 PSI at cold idle and goes up a bit when I rev. Still have to figure out this temp gauge issue, but based on this thread, I might seek out another forum to get some advising on that one lol.
First time I've seen this thread pop up on the recent list... sorry you haven't gotten any responses before now. The gauges work by sending a voltage signal down the sensor wire that is then grounded with a variable resistance to the engine block through the sensor. This works the same way on the oil pressure sensor and the temp sensor. Reading full hot means you have a short to ground in the wire or sensor. You should be able to get 12v(ish) between the sensor wire and the battery ground with the key on and the sensor wire disconnected. disconnecting the wire should also send the gauge to zero with key off if the sender is bad. If it stays high, you probably have a sensor wire pinched under a valve cover or melted to an exhaust manifold.

Here is a thread with the expected sender values.
https://www.gmsquarebody.com/threads/temp-gauge-issues.19919/
 

haromaster87

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First time I've seen this thread pop up on the recent list... sorry you haven't gotten any responses before now. The gauges work by sending a voltage signal down the sensor wire that is then grounded with a variable resistance to the engine block through the sensor. This works the same way on the oil pressure sensor and the temp sensor. Reading full hot means you have a short to ground in the wire or sensor. You should be able to get 12v(ish) between the sensor wire and the battery ground with the key on and the sensor wire disconnected. disconnecting the wire should also send the gauge to zero with key off if the sender is bad. If it stays high, you probably have a sensor wire pinched under a valve cover or melted to an exhaust manifold.

Here is a thread with the expected sender values.
https://www.gmsquarebody.com/threads/temp-gauge-issues.19919/
Ah no worries, these gauge issues are non stop on this forum so I know how it is haha. Thanks a bunch for the response though, you filled in my last bit of curiosity about the gauge functions. I completely overlooked the fact that there would need to be positive voltage on the sensor wire, makes perfect sense. I did test the sensor wire today and confirmed I get a solid +12v signal from it with the key on or truck running.

In my case of the temp gauge, it's the opposite, the gauge will not go more than 1mm above the 100 mark, regardless of temp. I have two gauges I'm using for testing and both exhibit the same behavior. The gauge maxes out when the sensor wire is grounded, and the ceramic resistor on the back of both gauges I have test okay, showing around 82 ohms. I removed the resistor from each gauge and plugged them in temporarily and the gauge maxed out to hot as expected.

I have also measured the resistance values being provided to the gauge at the cluster housing with the truck running. It starts around 1.3k ohms and goes down to about 450 ohms, which indicates ~160 water temp. I would think this would push the gauge up to around the mark between 100 and 210, but either gauge I try just barely comes off the first line. Even as I watch the resistance value at the cluster come down, the needle just sits in the same spot.

If I manually move the needle higher/lower before plugging the gauge in, it will move it right back to the spot I've described, so it's not a matter of the gauge sticking as far as I can tell. Both gauges can move freely, there's just something about the signal they get that parks the needle in that spot.
 

haromaster87

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Did you pull the temp gauge and reseat it? Might just be another issue of bad contact between the gauge and the cluster housing
Sadly I've removed/reseated it more times than I can remember haha. I even sanded the contacts on the cluster housing and both gauges I'm testing with to see if it was poor conductivity, but the issue remains.
 

Bextreme04

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Ah no worries, these gauge issues are non stop on this forum so I know how it is haha. Thanks a bunch for the response though, you filled in my last bit of curiosity about the gauge functions. I completely overlooked the fact that there would need to be positive voltage on the sensor wire, makes perfect sense. I did test the sensor wire today and confirmed I get a solid +12v signal from it with the key on or truck running.

In my case of the temp gauge, it's the opposite, the gauge will not go more than 1mm above the 100 mark, regardless of temp. I have two gauges I'm using for testing and both exhibit the same behavior. The gauge maxes out when the sensor wire is grounded, and the ceramic resistor on the back of both gauges I have test okay, showing around 82 ohms. I removed the resistor from each gauge and plugged them in temporarily and the gauge maxed out to hot as expected.

I have also measured the resistance values being provided to the gauge at the cluster housing with the truck running. It starts around 1.3k ohms and goes down to about 450 ohms, which indicates ~160 water temp. I would think this would push the gauge up to around the mark between 100 and 210, but either gauge I try just barely comes off the first line. Even as I watch the resistance value at the cluster come down, the needle just sits in the same spot.

If I manually move the needle higher/lower before plugging the gauge in, it will move it right back to the spot I've described, so it's not a matter of the gauge sticking as far as I can tell. Both gauges can move freely, there's just something about the signal they get that parks the needle in that spot.
Sounds like the sensor is bad or you have poor grounds from the engine to the battery. Its also common to have corrosion or too much sealant/tape on the sensor when it is screwed into the head and that can cause the sensor to not be able to ground properly through the threads.
 

haromaster87

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Sounds like the sensor is bad or you have poor grounds from the engine to the battery. Its also common to have corrosion or too much sealant/tape on the sensor when it is screwed into the head and that can cause the sensor to not be able to ground properly through the threads.
Sadly I've replaced the sensor as well with a new Delco unit and made sure to not use any sealant. The old sensor behaved the same way so that was seemingly ruled out. I will definitely do some checking on the ground connection between the block and battery though, see if I missed anything there. Thanks for the info!
 

YakkoWarner

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My oil pressure gauge (1989 Suburban) does give variable readings but I always wonder if it is telling the truth or not...I've seen it fluxuate between 30 and 45 while cruising down the highway at steady speed and unchanging temperature, sometimes see up to 40 at idle and then lower to around 30 while driving, just doesn't behave like I would expect. I'm used to engines where oil pressure is mainly a function of RPM for a given temperature. I do have the factory oil cooler in front of the radiator but that shouldn't be making the indicated pressure swing randomly. I replaced the sending unit with no effect.
 

haromaster87

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My oil pressure gauge (1989 Suburban) does give variable readings but I always wonder if it is telling the truth or not...I've seen it fluxuate between 30 and 45 while cruising down the highway at steady speed and unchanging temperature, sometimes see up to 40 at idle and then lower to around 30 while driving, just doesn't behave like I would expect. I'm used to engines where oil pressure is mainly a function of RPM for a given temperature. I do have the factory oil cooler in front of the radiator but that shouldn't be making the indicated pressure swing randomly. I replaced the sending unit with no effect.
lol That sounds about correct based on how I've heard the "accuracy" of the factory gauges described. Honestly even now that mine seems to be working correctly, it works about how you described. So I guess they're working as designed at least, for whatever that's worth haha.
 

YakkoWarner

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lol That sounds about correct based on how I've heard the "accuracy" of the factory gauges described. Honestly even now that mine seems to be working correctly, it works about how you described. So I guess they're working as designed at least, for whatever that's worth haha.

Possibly - I bought and was planning on putting in a 3 gauge "A" pillar housing with Autometer tach, temp and oil pressure but unfortunately the person who was helping me with that passed away so it sitting in the house...I don't know what happened to the actual sending units that went with it during all the confusion, or where they would go since removing the factory units would mess up whats left of the ECU systems.
 

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