Oil pressure gauge not working correctly on 87 convertible

[Jim S]

My oil pressure gauge sits at the halfway mark with the key off.  With the key on, and engine running or not the gauge pegs to zero.  Would this most likely be a bad sending unit?  Read the GM 305s had a pressure sender below the engine, but this being an electric gauge, I assume the sender is the hard to reach one behind the distributer.

[Avanti83]

Traditional position for the SBC oil sensor is behind the head on the drivers side that can be accessed from above the engine. Look straight down to see it.

There is usually room to get a hand down to pull the wire and use a socket to remove the sender unless it's the large bodied one used on some engines. IAC, it can be removed.

[popeyesteve]

I believe that in 1987 the oil pressure sending units are on a boss on the engine block, right above the oil filter. A can shaped unit about 2” diameter and looks like this...

[Jim S]

I found both units.  There's one on the block.  I disconnected it and it made no change.  Then I put 12 volts to the wire and again no change.  There is also a sender I found under the distributer.  It has two contacts.  I shorted them and still no effect on the gauge.  This is a very peculiar problem I have never encountered before!

 

[popeyesteve]

The one in the block should power the gauge. I had to replace one laying in the snow in Colorado in winter years ago. The gauge was reading really low pressure and I was 800 miles from home so I didn’t want to chance it for the drive back. Replacing the sender fixed it.

[Jim S]

2 hours ago, popeyesteve said:

The one in the block should power the gauge. I had to replace one laying in the snow in Colorado in winter years ago. The gauge was reading really low pressure and I was 800 miles from home so I didn’t want to chance it for the drive back. Replacing the sender fixed it.

Thank-you!  I'm on it!

[IndyJimW]

On 4/20/2021 at 1:25 PM, Jim S said:

I found both units.  There's one on the block.  I disconnected it and it made no change.  Then I put 12 volts to the wire and again no change.  There is also a sender I found under the distributer.  It has two contacts.  I shorted them and still no effect on the gauge.  This is a very peculiar problem I have never encountered before!

 

I posted wiring diagrams for the 1987 Avanti, page 32 is for gauges. 

There is an oil pressure sender and an oil pressure sensor. The sender is for the gauge. The sensor is used to by the ECM to tell if the engine is running,

90 Ohms full, 0 Ohms empty. Unplugged will peg the needle full, grounding the wire will peg the needle empty.

Don't apply voltage. If no movement problem could be in wiring or gauge. Check for voltage at the sending unit.

Good luck,

Jim Wood

[Jim S]

Thank you Jim.  I also had a local mechanic tell me that there's a small motor in the gauge that usually goes bad causing my current symptom?

[IndyJimW]

2 hours ago, Jim S said:

Thank you Jim.  I also had a local mechanic tell me that there's a small motor in the gauge that usually goes bad causing my current symptom?

Newer cars use stepper motors to operate the gauges. I've replaced a lot of them.

This should be a Steward Warner gauges with Avanti face plates.

If memory serves me no motors, just field windings to operate the gauge.

Pretty bullet proof, usually sender or wiring.

Good luck,

Jim Wood

[Jim S]

I replaced the sender and now the gauge pegs past 60 PSI.  I hope that is what my engine is putting out!

 

[Avanti83]

Put a mechanical gauge in the system temporarily to check the true pressure. They're fairly inexpensive from Ace Hardware to Amazon.

Edited April 29, 2021 by Avanti83