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?

[troth]

Have you tried jumping the starter with the key in the run position? I'll defer to the experts but don't think all accessories are powered with the key in the start position. This might include your gauges. Doesn't seem like it's powered at all in the start position since both the brake warning and gen lights are off.

You could also pull the gauge and put a test light on the power wire with the key in the start position.

[sixnotfour]

leave mechanical gauge and fire it up....thats what you want.....wiring another day....

[r_towle]

For what it’s worth, my 84 911 3.2 registers no oil pressure while cranking, only after it starts.

[Tom1394racing]

Thanks for all your replies. It seems like there is no power to the gauge when the ignition switch in in the cranking position (temperature gauge also drops to zero power reading).

I guess this is normal for a 914. Another difference between the 911 and the 914.

I will plan to install both the mechanical gauge and the sender for initial start-up.

[930cabman]

leave mechanical gauge and fire it up....thats what you want.....wiring another day....

(IMG:style_emoticons/default/agree.gif)

I always run a mechanical gauge, with an electrical gauge for backup.

Could be the sender or the gauge or the wiring.

Was it working recently?

[rgalla9146]

Tom after your early tests the cam supply hollow bolts were tight correct ?
I'd install a mechanical OP gauge for start up.
I agree it's better to go after the dash gauge later.

[Tom1394racing]

leave mechanical gauge and fire it up....thats what you want.....wiring another day....

(IMG:style_emoticons/default/agree.gif)

I always run a mechanical gauge, with an electrical gauge for backup.

Could be the sender or the gauge or the wiring.

Was it working recently?

I checked the sender/gauge function with applied air pressure, and they worked as expected.

[Tom1394racing]

Tom after your early tests the cam supply hollow bolts were tight correct ?
I'd install a mechanical OP gauge for start up.
I agree it's better to go after the dash gauge later.

Yes Rory, I did retighten the cam oil line banjo bolt after the test.

[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.

[Cairo94507]

@Superhawk996 That makes complete sense; good information. (IMG:style_emoticons/default/beerchug.gif)

[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!

[mb911]

Thanks for all your replies. It seems like there is no power to the gauge when the ignition switch in in the cranking position (temperature gauge also drops to zero power reading).

I guess this is normal for a 914. Another difference between the 911 and the 914.

I will plan to install both the mechanical gauge and the sender for initial start-up.

That is not my experience. Hmm sounds like wiring and or ignition switch

[sixnotfour]

I see your gauge has oil idiot light, you could add that sender while your messing with wiring...its less than 10 psi..

[Tom1394racing]

I see your gauge has oil idiot light, you could add that sender while your messing with wiring...its less than 10 psi..

That is in the plan.

[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.

Based on Ben’s comment I went and checked the Haynes schematic.

The red/white circuit coming off the Ignition switch X terminal cuts the head lights and the fresh air blower motor up front.

It appears that I was mistaken about that red/white circuit being the same Red/white circuit that powers the gauges. Confusing - yes given red/white is the same color for two different circuits. Also it’s been a couple years since I’ve had wiring in my hands sorting circuits. But . . . I was wrong about where that X terminal was feeding to.

For OP - check your wiring - perhaps someone has switched it erroneously?