Help - Strange Oil Pressure Gauge Behavior Options

[Tom1394racing]

I am working towards first start on the 3.2 Short Stroke installed in my -6 GT project.

While cranking the engine over with the fuel pumps and MSD disconnected and the spark plugs out, I am getting no oil pressure reading on the dashboard gauge. To verify if I do really have oil pressure, I connected a mechanical gauge in place of the sending unit and cracked open the cam oil line. I got 30 psi on the gauge and plenty of oil leaking from the cam line.



The gauge registered around 30 psi, I just did not capture it in a photo.



So, it seems like the engine is making good pressure while cranking. The problem must be in the sender or in the gauge. To check these, I connected the sender to a variable air pressure source and checked the gauge.



The gauge showed the following readings

Key off, no pressure applied to the sender:



Key on, no pressure applied to the sender:



Key on, 50 psi applied to the sender:



Note the engine was not cranking for this test. All looked good for both the sender and the gauge.

Stay tuned.

[Tom1394racing]

Next, I reinstalled the sender in the crankcase, connected the sender wire and repeated the test to see how the gauge behaved.

Key off:



Key on:



So far, just like when I tested the sender with external pressure.

Next cranking the engine:



When cranking the engine, the dial on the gauge moves back to the key off position???

I know I am making oil pressure in the engine and I know that the sender and gauge respond correctly to applied pressure. However, when I apply the starter to crank the engine, the gauge moves back to its key off position.

I am stumped at the moment. Any Ideas?

[Superhawk996]

When cranking the engine, the dial on the gauge moves back to the key off position???

Any Ideas?

When cranking the ignition switch cuts power to some circuits to give maximum current to the starter motor, ignition coil, ECU. Basically only the things needed to start the engine.

I believe the red/white circuit that powers all dash gauges is on that ignition switch “x” terminal that gets cut during cranking.

[Tom1394racing]

When cranking the engine, the dial on the gauge moves back to the key off position???

Any Ideas?

When cranking the ignition switch cuts power to some circuits to give maximum current to the starter motor, ignition coil, ECU. Basically only the things needed to start the engine.

I believe the red/white circuit that powers all dash gauges is on that ignition switch “x” terminal that gets cut during cranking.

Yes...That's what the wiring diagram shows. Different than my early 911 experience where cranking the engine over and watching the gauge to achieve oil pressure prior to first start has been my standard practice. This one has had me stumped for a few days of head scratching.

I can't thank you guys enough for all your help!