Oil Pressure Gauge Problems

[Jake Chudovan]

Hello Everyone,

I just joined this forum because I have had a problem with my oil pressure gauge on my 1986 Suburban K20 that has really been puzzling me.

I bought the truck about 2 months ago and the seller said it had some problems with the electrical and charging system. I ended putting in a new alternator, battery, and replacing the engine wiring harness with another used one because the old one had a lot of butt connectors and exposed wire that I was worried ruined the alternator. It turned out the big 30 amp fuse for the power locks and tailgate window lock was drawing about 25 amps but not blowing the fuse.

Long story short when I replaced the engine wiring harness everything worked great except for my oil pressure gauge which was now pinned at about 3 o’clock way passed the max PSI. It worked great before hand for the little driving that I did. After that happened I figured I maybe messed up plugging in the 2 prong plug that was behind the distributor (Which is the style that mine has, not the style near the oil filter) so I did everything to try to get it to make a good contact with the sensor and flipped the plug to face the other direction. Still nothing.

Last night I went ahead and cut off the old plug and soldered in the one from the original harness. I had a suspicion the the “new” harness’ plug was the problem because when I got it, it had a jumper wire connecting the two prongs like someone else had a problem with it in the past. But after plugging it back in still no luck

I did some testing with a multimeter before and after soldering the new connector in, and when the truck is keyed on I got a 12.5 volt reading from the choke wire that comes off of that oil pressure plug. So that means some voltage is running through the two wires that come off of the firewall wiring block. I’m just not sure which wires are what on that two prong plug and which one goes to the choke etc.

I also did pop off the dash cluster and move the needle on the gauge to make sure it wasn’t stuck. It was super light and could be turned to face any direction which seemed odd because the other gauges would return to a 0 reading after I would turn them a bit.

Id really appreciate any help that you could offer and information that would help me solve this problem!
Thanks in advance!

 

[C10MixMaster]

Hello Everyone,

I just joined this forum because I have had a problem with my oil pressure gauge on my 1986 Suburban K20 that has really been puzzling me.

I bought the truck about 2 months ago and the seller said it had some problems with the electrical and charging system. I ended putting in a new alternator, battery, and replacing the engine wiring harness with another used one because the old one had a lot of butt connectors and exposed wire that I was worried ruined the alternator. It turned out the big 30 amp fuse for the power locks and tailgate window lock was drawing about 25 amps but not blowing the fuse.

Long story short when I replaced the engine wiring harness everything worked great except for my oil pressure gauge which was now pinned at about 3 o’clock way passed the max PSI. It worked great before hand for the little driving that I did. After that happened I figured I maybe messed up plugging in the 2 prong plug that was behind the distributor (Which is the style that mine has, not the style near the oil filter) so I did everything to try to get it to make a good contact with the sensor and flipped the plug to face the other direction. Still nothing.

Last night I went ahead and cut off the old plug and soldered in the one from the original harness. I had a suspicion the the “new” harness’ plug was the problem because when I got it, it had a jumper wire connecting the two prongs like someone else had a problem with it in the past. But after plugging it back in still no luck

I did some testing with a multimeter before and after soldering the new connector in, and when the truck is keyed on I got a 12.5 volt reading from the choke wire that comes off of that oil pressure plug. So that means some voltage is running through the two wires that come off of the firewall wiring block. I’m just not sure which wires are what on that two prong plug and which one goes to the choke etc.

I also did pop off the dash cluster and move the needle on the gauge to make sure it wasn’t stuck. It was super light and could be turned to face any direction which seemed odd because the other gauges would return to a 0 reading after I would turn them a bit.

Id really appreciate any help that you could offer and information that would help me solve this problem!
Thanks in advance!

if you unplug the oil pressure sensor wire you should show max or above max pressure on the gauge. if you ground that wire it should read 0 psi on the gauge.

 

[Turbo4whl]

Welcome

Couple questions:

Was the replacement harness the same year truck?

Pressure switches, such as the one for the electric choke, have 2 wires. This would be and on off switch. Pressure senders have one wire, should be tan in color. Are you sure there is not a second sender for the gauge?

Was the wire harness donor from a truck that did not have gauges?

 

[Jake Chudovan]

if you unplug the oil pressure sensor wire you should show max or above max pressure on the gauge. if you ground that wire it should read 0 psi on the gauge.

Thanks I’ll give that a try

 

[Jake Chudovan]

Welcome

Couple questions:

Was the replacement harness the same year truck?

Pressure switches, such as the one for the electric choke, have 2 wires. This would be and on off switch. Pressure senders have one wire, should be tan in color. Are you sure there is not a second sender for the gauge?

Was the wire harness donor from a truck that did not have gauges?

I am not exactly sure what year truck the harness was from, but I made sure to tell the guy I bought it from what year my truck was and it seemed to match up with the other one, except the new one didn’t have a wire that went to the grounding bolt on the back of the passenger side head.

As I went through and unplugged the harness from the truck I only found the one two prong plug behind the distributor, nothing with only one.

Unfortunately I don’t know about the donor truck, the guy I bought it from had a huge collection of parts trucks and parts but I think he would have noted something special like that.

 

[79Square]

The fact that it is pegging full, most likely is a grounded circuit through the gauge. The oil sensor is a variable resistor that with full resistance hen the gauge is at 0, and no resistance is at full peg. The problem could be a short in the sensor, but usually when they go bad, it would create an open circuit instead. The problem might be on the gauge side. The gauge should have a 12v+ connection that is shared and spliced with other gauges. The other connection should be a clean connection to the sensor. This could be connected to ground incorrectly or grounded somehow. That would cause a full peg. There may be another connection on the gauge that is a ground, but that would be for a light if the gauge has one. Take a tester and check two things. Test the gauge sensor connection to a common ground and see if you get a beep or connection reading. If so, it's grounded. Then check and test the connection from your oil sensor connector on the engine side to the gauge sensor connection and see if you have a beep or connection reading. If it is not grounded, and the connection between the gauge and engine are good, then it would be a bad sensor. Hope this helps.

 

[42Grand]

I have a similar situation. Just swapped my 77 cluster with one from a 78 so I could use the tach. My 77 had a mechanical oil pressure gauge and hard line but the 78 has an electric version. I re-pinned my wiring harness and since I had to run a new wire, I made sure it was pinned correctly. I asked the guys at AutoZone for a sending unit out of a 78 and after installing it my gauge was pegged past 60 as well. I looked on my receipt and it says I purchased a switch. It’s got a single terminal so I’m not sure how that’s supposed to work. I thought switches have 2 terminals. Anyway, I haven’t tested my wiring, sensor / switch, the gauge or circuit board yet but I’ll jump under the hood tomorrow after work and fill y’all in with the results from my testing.

 

[AuroraGirl]

a switch should be on off signawl, so a dummy light for example.

 

[AuroraGirl]

@Catbox do you have a page in your 78 manual for oil pressure sender?iirc the 1980 manual shows both senders switches on a single page under instrumentation

 

[Bextreme04]

Gauges were optional in squares, so you can get either a switch or a sensor for oil pressure. You have to look through the descriptions and pick the right thing, Napa has good descriptions for whether they are for a light(switch) or gauge(sensor). The one from autozone that is for a gauge is the more expensive one(~$38) and is part number PS140

 

[42Grand]

Ok, thanks for the info guys. The first part I bought was apparently the right part because it was HUGE and I would have had to lift the distributor up some to make room for the unit.
What has everyone done in that circumstance? It looks like there is enough clearance once the distributor is loose, but will it clear after it is tightened back up? Has anyone put an elbow or T up there?

 

[AuroraGirl]

mine has a 45 degree. auto stores have em, brass NPT threads, no idea size

You must be registered for see images attach

 

[42Grand]

mine has a 45 degree. auto stores have em, brass NPT threads, no idea size

You must be registered for see images attach

Awesome! I'll look for that. I'll ask the guys there for a same thread sized 45 degree adapter. I think it'll be fine once installed. Thanks AG!

 

[WebMonkey]

my 85 k20 has a 't' with both idiot light AND gauge 'sending units' attached.