Oil Pressure Gage Not Reading Correctly With SBC

[Twisted46]

So pretty simply as soon as I turn the key to the ON position the oil pressure gauge (OEM 260z) goes off the scale past 90 and stays there.

Once I actually start the car and it is running it pegs over to 0 and stays there no matter what. 

Now I know I am getting at least some oil pressure as I left the mechanical fuel pump shaft bolt out and it sprayed oil all over the front of the engine bay... sigh.

 

Anyways I have also watched the oil valleys in the heads fill and drain when I pull the valve covers after running it. 

 

I followed the JTR book, any ideas what may be happening?

[Miles]

Check your wiring to the oil pressure switch, oil pressure safety switch etc.

 

If you followed JTR then your pressure switch and oil pressure safety switch should look like the attached picture.

 

Post the  wire list you used for the oil pressure switch etc and where each wire is connected.

Edited May 29, 2019 by Miles

[Twisted46]

Hi Miles, 

 

I actually do not have the oil pressure safety switch, I rely on a kill switch and inertia switch. My ignition and fuel are on dependent switches, my key doesn't do much lol.

 

Anyways right now I only have the yellow-black wire hooked up to the oil pressure sensor. 

Edited May 30, 2019 by Twisted46

[Miles]

Sounds like you have altered the stock wiring.  So the following may or may not help you, but may serve as a process of elimination as you trace your electrical problem.

 

Possibilities:

 

• Wrong oil pressure sensor?  
• A wiring alteration is energizing the oil pressure sensor circuit when the ignition switch is in the start position and then de-energizes when the switch is released to run mode.  Check the ignition, fuel pump and starting circuit wires. Check the circuits that have been altered to see if they have been cross connected to the oil pressure sensor.
• Test: connect a volt meter to the yellow and black oil pressure sensor wire. Note what the volt meter does as you turn the ignition switch slowly to each position on the switch.  This will narrow down which wires to inspect.

        

Look at/test these wires at each position on the ignition switch :

 

• Yellow-black - oil pressure sending unit
• Black-white (without protective sleeve) - tach terminal on HEI distributor
• Black-white (with protective sleeve) - battery terminal on HEI distributor
• Black-yellow - S terminal on starter
• Black-blue - energizes the fuel pump while cranking (260z and 280Z only).  Note: 240z uses a green-white wire to run the fuel pump while cranking (energized  just before start position on ignition switch.

 

This is just a starting point.  You have some detective work ahead of you.

Edited May 31, 2019 by Miles

[Twisted46]

I was able to confirm 12v to the oil pressure sensor in run mode only. The wiring diagram I have shows only the yellow black wire is used for the oil pressure gauge. I will double check but IIRC I just used the sensor that was already on the L6 when I took it out and used a brass fitting to adapt to the chevy block. 

 

I check other things you mentioned this weekend.

[Miles]

Looks like you have the correct sensor. The sensor could be defective.

 

 

Edited May 31, 2019 by Miles

[Twisted46]

I tried a new sensor from ZCar, same problem. Maybe the gauge?

[Miles]

Try starting the car with the wire to the oil pressure sensor disconnected from the sensor and observe the oil pressure gauge.  With the engine running and the wire removed from the sensor what does oil pressure gauge do?

[Twisted46]

Thanks Miles, I'll try that. Car has been down waiting for me to change the water pump so I will give it a try this weekend.

[Twisted46]

I just got around to testing this. The gauge did not move at all.

[Miles]

21 hours ago, Twisted46 said:

I just got around to testing this. The gauge did not move at all.

 

There is your clue. 

 

The oil pressure sending unit is just a variable resister.  If I recall correctly, the sending unit basically grounds the oil pressure gauge (moving the needle) as oil pressure increases.

 

So, if when the wire to the oil pressure gauge is connected,  and the   gauge needle goes all the way to full pressure,  what does that imply:

• The wire going to the sending unit is shorted out? However, when you removed the wire from the sending unit,  the needle  did not move. So you can rule this out.
• Or the sending unit is bad.

 

Another test:  while watching the oil gauge, with power on, short the sending wire to ground.  What happens?  Does the needle move to full pressure?  If the needle moves the gauge is ok.

 

 

 

[Twisted46]

Thanks Miles,

 

I test this out when we get another nice day here in Ohio..... So next April 😝