My oil pressure gauge has become erratic. It starts out at 40 psi then quickly drops to 5psi and fluctuates around the 5 mark.
I changed the sender which made no difference, the wiring seems fine, that leaves the gauge. Does anybody have any info on the voltage output for various pressures.
I am getting readings of 60 psi at 14.7 volts showing 2 amps, 40 psi at 12 volts showing 1.6 amps, 20psi at 7.1 volts showing 0.9 amps.
this test was performed at the gauge varying the input voltage.

Is there a simple way of getting a direct oil pressure reading from the running engine. I have ordered some fittings to do this using the port the sender sits in. This will take weeks to get here (Canada Post)
The engine runs well with no overheating even on long runs.
any comments would be welcomed.

 

The oil, fuel and temperature gauges are all the same device, with different faces. That gives you a hint on how to test for a bad gauge. The oil pressure system is a catastrophic design. All gauge systems have to be fed with constant voltage...the two other gauges are powered using a 10V instrument voltage regulator, which is located on the fold-down panel. But the oil gauge relies on a voltage regulator that's built into the sender. It's been difficult to source modern replacements for the sender because of this. And it's why many owners opt to install a mechanical gauge.

The sender port is 1/4 BSPP, so you will need a proper adapter to set up a mechanical gauge, either for permanent or test use. There's no substitute. Installing a different fitting will damage the threads in your filter head, and cause other problems.

 

Thanks for the reply. The root of the problem turned out to be wiring and grounding.
Once they were corrected the problem went away.
I have ordered all of the BSPP fittings and intend to install a permanent, under the hood, pressure gauge for back up and maintenance.
The car goes it on Tuesday for a complete harness replacement. There have been far to many issues with faulty wiring and grounds.
I may eventually install a direct reading capillary gauge, to replace the original Smiths gauge.
Your description of the instrument is perfect, I think the numbers I recorded are pretty accurate, they show the gauge is working just fine.
for interest I opened up the original sender, I have attached a photo of it below, the ground wire from the bimetallic strip did not survive, ot must have been less than .010 dia and very brittle