Crazy acting oil pressure gauge

[HotWheelsBurban]

Okay guys, got one for y'all. I suspect what it may be but would like some opinions/confirmation before I start tearing into it.
Rawhide (97 C3500, L31 5.7) started doing this on my way home from work today. Oil pressure gauge needle is sitting where it normally does (35 at idle, 50-60 at speed depending on RPM) and then it the needle just flops over to way past 80, full right angle. No noise, truck still runs great, no codes. Then it goes back to "normal" for a few minutes, then goes back to cuckoo.
Gauge bad, or sending unit bad?

 

[Drunkcanuk]

I would put my money on the sending unit.
Time to break out the multi meter.

 

[99xcss4]

the sending unit might be going bad I know it is a tight fit back there but I would try unpluging it and pluging it back and make shore that it is dry if your gauge is going bad you have that really nice 120mph cluster to put good luck figuring it out

 

[HotWheelsBurban]

the sending unit might be going bad I know it is a tight fit back there but I would try unpluging it and pluging it back and make shore that it is dry if your gauge is going bad you have that really nice 120mph cluster to put good luck figuring it out

Yup I had thought about that, just to check the gauge....
And swapping out the cluster is the easiest fix too.

 

[99xcss4]

Yup I had thought about that, just to check the gauge....
And swapping out the cluster is the easiest fix too.

now you are talking you go to the yards when ever you can to find things to sell and help out your fellow members that need it or ask for it it is time that you treat your self to some of your findings I say to check if it is dry because I have seen them read wrong when they leak

 

[Road Trip]

Okay guys, got one for y'all. I suspect what it may be but would like some opinions/confirmation before I start tearing into it.
Rawhide (97 C3500, L31 5.7) started doing this on my way home from work today. Oil pressure gauge needle is sitting where it normally does (35 at idle, 50-60 at speed depending on RPM) and then it the needle just flops over to way past 80, full right angle. No noise, truck still runs great, no codes. Then it goes back to "normal" for a few minutes, then goes back to cuckoo.
Gauge bad, or sending unit bad?

Hello HWB,

Your symptoms suggest that there is an intermittent open in either the sending
unit, or the TAN wire between the sending unit and the instrument cluster.

The reason why:

* 0 ohms = 0 psi.
* 90 ohms = drive pointer right to 80 psi mark.
* Open = infinite ohms = driving pointer as far as possible past 80.

The possibilities include the sending unit, loose connection between TAN wire
and sending unit, bad (open) spot in TAN wire, or even a dirty connection between
the connector and the instrument cluster. Or even the gauge.

Taking the possibillities and figuring out the highest probability is where the magic happens.

My conservative take would be to inspect/clean each connection in the path first. If the problem
persists, then the easiest would be to swap in the other cluster and see if the problem clears or
persists.

In the attached are the '98 FSM electrical pages that should work for your '97. (The fuel quantity
was moved to the VCM, but as far as I can tell the oil pressure circuit was left 'as is'.)

****

Your detailed observations narrowed down the possibilities -- that is, we're looking for an intermittent
open circuit instead of a chafe to ground.

Hope this proves helpful.

 

[HotWheelsBurban]

Hello HWB,

Your symptoms suggest that there is an intermittent open in either the sending
unit, or the TAN wire between the sending unit and the instrument cluster.

The reason why:

* 0 ohms = 0 psi.
* 90 ohms = drive pointer right to 80 psi mark.
* Open = infinite ohms = driving pointer as far as possible past 80.

The possibility includes the sending unit, loose connection between TAN wire
and sending unit, bad (open) spot in TAN wire, or even a dirty connection between
the connector and the instrument cluster. Or even the gauge.

Taking the possibillities and figuring out the highest probability is where the magic happens.

My conservative take would be to inspect/clean each connection in the path first. If the problem
persists, then the easiest would be to swap in the other cluster and see if the problem clears or
persists.

In the attached are the '98 FSM electrical pages that should work for your '97. (The fuel quantity
was moved to the VCM, but as far as I can tell the oil pressure circuit was left 'as is'.

****

Your detailed observations narrowed down the possibilities -- that is, we're looking for an intermittent
open circuit instead of a chafe to ground.

Hope this proves helpful.

Thank you very much for the information from the manual! I have the '99 books for Burb, but I think they are with it (at the shop building 10 miles away). The Escalade cluster is also in the shop....
I'll look at the wiring harness and plug in the morning; just got home from work and the truck is still rather warm to go sticking my hands back there!
The Burb had a similar situation a few years ago, and I had the shop (that I no longer trust) replace the sending unit. Either the new one was defective, or they broke it installing it. So they said they could fix it on their dime, but it would be the next week before they could do it. Was Saturday afternoon by that point and they were about to close, and I needed to be somewhere else too. Two weeks later our schedules meshed; was a bit nerve-wracking driving an old Chevy that you know has low oil pressure, but have no gauge to see what it is! And during this time of bad sending unit, the needle went all the way to 3 o'clock....way past 80psi like the crew cab is doing now. And most $#!+ on my vehicles is rarely the "simple" option....
So we'll see what everything looks like in the morning!

 

[scott2093]

was a bit nerve-wracking driving an old Chevy that you know has low oil pressure, but have no gauge to see what it is!

after installing a mechanical gauge I'd feel better with my chances calling the tv station and asking the weatherman if it's going to rain on my street today than trust my gauge in the dash.

 

[HotWheelsBurban]

after installing a mechanical gauge I'd feel better with my chances calling the tv station and asking the weatherman if it's going to rain on my street today than trust my gauge in the dash.

Yup I want to install one eventually. Dad had them on our squares. How much of a PITA is it to do on a 5.7 Vortec?

 

[scott2093]

How much of a PITA is it to do on a 5.7 Vortec?

My 93 was comparable to maybe replacing the distributor and plug wires...with 3 stuck plug wires..not sure about the vortec.....

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