Oil Pressure Switch

mjlimo82 · June 21, 2010

I put a rebuilt 350 in my truck this past weekend (runs great, by the way) but for some reason the new oil pressure switch I put in doesn't seem to be working right. Before it was replaced it sat at 0 and the Check Guages light would stay lit always (for over 6 years)... now it sits at about 30 and will drop low enough to kick the light back on when you are sitting at an idle while in gear. I will say that it works better now than it did before, but its still not working right. Any idea what I could check to see what's causing it not to read right? It's a Delco switch and it matches perfectly. Not sure what the issue could be unless the guage doesn't work right... it's a 90 Sierra 4x4 automatic. Thanks in advance for any help!

txab · June 21, 2010

I'd look into the gauge. Also check your wiring. Are you sure you've got the correct replacement part with the correct pressure range?

mjlimo82 · June 22, 2010

I'll look and see if there's a way I can test the guage's functionality. As far as the switch itself I believe it to be correct; I know they changed from year to year (the 89 the motor originally came out of had a two terminal switch; mine has 3) but as far as range, I don't know if it's right or not. Something to look into apparently. As big of a pain it is to change I almost don't want to mess with it, but at the same time I'd like to know what's going on with my engine especially since it's new. I'll look into the range of the switch and see if it's the wrong one. I should still have the part number somewhere...

Thanks for the advice!

JJP442 · August 24, 2010

Did you ever figure out what the problem was?

On my 97 Yukon (5.7) the oil gauge acted the same as yours.

When fully warmed up it dropped to zero and the "check gauges" light would come on at idle.

Any increase in RPM would raise oil pressure and turn off the light.

Recently, I swapped in a new sending unit hoping to cure the problem.

Now, the pressure reads lower and drops to zero as before.

Anybody have any idea what else I can do to make this annoyance go away

Big John NYC

I put a rebuilt 350 in my truck this past weekend (runs great, by the way) but for some reason the new oil pressure switch I put in doesn't seem to be working right. Before it was replaced it sat at 0 and the Check Guages light would stay lit always (for over 6 years)... now it sits at about 30 and will drop low enough to kick the light back on when you are sitting at an idle while in gear. I will say that it works better now than it did before, but its still not working right. Any idea what I could check to see what's causing it not to read right? It's a Delco switch and it matches perfectly. Not sure what the issue could be unless the guage doesn't work right... it's a 90 Sierra 4x4 automatic. Thanks in advance for any help!

mjlimo82 · September 7, 2010

No, it's still doing the same thing and I've yet to figure out what it is. Yesterday on my way home from NW Arkansas ALL the gauges except for the speedo went haywire. The voltmeter dropped to around 9, temp was fluctuating back and forth from one side of 210 to the other, the oil pressure sat at 0 constantly, and the gas gauge was reading lower than it should with a full tank. Killed it going down the interstate and started it back up and all was well. Did it maybe 3 or 4 times but it hasn't done it since. I'm having intermittent knock sensor problems and I thought it might be related to that but I have no proof behind my theory - while driving home at 75 mph with the cruise set, the SES light came back on and it lost power as usual, but the gauges remained normal. At this point I'm guessing its the gauge but unfortunately I haven't been able to find one to replace it with. At least not new anyway.

In the truck the motor came out of it did the exact opposite - sat maxed out at 80 no matter what. It was an 89 so the leads were different therefore I wasn't able to test the switch in my 90. With that said, can I try swapping clusters from the 89 to my 90 and check it that way to determine if it's the gauge? Is there any difference between the two?

ncaaref · September 8, 2010

I am kind of the same issue, even whenthe truck is off the guage sits at 30 and them when cranked it is pegged at 60. The Temp guage has a similar issue but it isn't pegged high. It just never goes to 0. I just replaced the Dizzy, rerouted most of my vac lines, replaced the sending unit, switch and fitting. By the way I have an 85 GMC Sierra 5.7l 350

mjlimo82 · September 8, 2010

I decided last night I'm going to tear the cluster out of the 89, swap odometers, and put it in the 90 to see what happens. I'm curious about the oil pressure gauge AND the fuel gauge since it doesn't work right either. That will at least help me determine whether it's the sending unit or not that doesn't read right (full tank = approx 1/4). And not only that, it's in better shape anyway. The plastic cover is near flawless; mine's all scratched up. Wish I could find a 91 tach cluster... that'd be even better!

ltm0807 · September 17, 2010

without the Oil Pressure Switch to provide the essential data to the engine computer, some engine parts might end up damaged. This switch is one of the parts that car owners must check on a regular basis

mjlimo82 · September 17, 2010

I'm not sure this one is new enough for the oil pressure to matter to the computer. Before I replaced the motor, it had NO oil pressure according to the switch (sat at 0 all the time) and it ran like that for over 7 years without a problem. This was also a motor that had been driven with the number one cylinder dead for over 15 years. This thing went through hell but was still running - and probably still would - without a hitch or a hiccup ever. But it was slow, shaky, and would fog any mosquito colony within a 5 mile radius.

Interesting factoid for the day: so this morning on my way to work, no less than 100 yards from the house, it hiccuped real bad - nearly died - and I noticed at that time the idiot lights and gauges were going crazy, and the voltmeter was reading about 10V (it was the only one working right). It was also idling higher than normal. I killed and restarted it two or three times and it didn't fix it like it had before... until it warmed up. I got almost to work; sitting in traffic I decided to try one more time and afterward I had 14V like normal and all was well. Left to go to lunch and it did the same as it did this morning. It even died this time when going from Park to Reverse. Kept on all the way to the restaurant; after I got there I shut it off and restarted and it was fine, and it did fine on the way back after sitting for about 45 minutes or so.

Long story short, I think it's a wiring problem somewhere but where I don't know. I put a new starter on back in July and no problems until two weeks ago so I'm wondering if I have a loose wire on it somewhere. But that doesn't explain the temperature thing to me - it doesn't do it when it's fully warm, only when it's cold. I also have a knock sensor issue but have been too lazy to crawl under it and replace it yet... surely that wouldn't cause this issue though I wouldn't think. Anybody got any ideas or seen this one before? I haven't ruled the alternator out either, FYI. The only thing that makes me think it isn't the alt is it's been a sporadic thing, however I'm beginning to think that's changed now. I've got an extra one so if it comes down to it I'll just swap them out no harder than it is.

Reminder - 1990 truck with a 1989 TBI 350 that's been rebuilt but still completely stock, and hasn't had any issues until this came about. It's still drivable, it just worries me. I'm going to look over all the electrical connections this afternoon after I get it home to make sure nothing's worked its way loose since so many of the clips broke when I swapped motors.

ChRoNo16 · September 27, 2010

Definitely watch yourself my friend. My 91 GMC started doing that stuff early this summer. it is currently completely dead and Im out of ideas.

mjlimo82 · September 28, 2010

Not cool. What was it doing chrono? It's doing OK now; I guess it was just a fluke? Been running fine the last week and a half or so, although the voltmeter now sits well above 14. It isn't getting into the red though so I guess it's OK.

mjlimo82 · October 24, 2010

Quick update... so I was under the truck the other day and happened to notice that there is an oil pressure switch located on the oil filter housing that I sure don't remember seeing when we swapped the motor. Now... interesting thing is, my truck is a 90, the motor came out of an 89, but mine's 4WD where the 89 is a 2WD so I had to take the oil filter housing off of my old motor and put on this one because of the torsion bar. That switch was already on there which means that there has to be a wire somewhere that goes to it, only trouble is I can't seem to find it. The wiring diagram in Chilton's is NO help; basically useless if you ask me. I can plainly see the wire on the harness of the 89, but it's nowhere to be found in that same location on the 90 (but there's not a 350 in the way in the 89 either). Anyone have a wiring diagram, know where I can find a decent one, or perhaps know where this wire might be hiding?? It's a flat-blade connector similar to the temperature sensor, and on the 89 harness it's tan/brownish colored...