90-94 Miata Oil Pressure Sensor

Here's where the sensor and its connector is located on a 1990 miata with a 1.6 liter engine.

It will look the same on a 94 miata with a 1.8 engine, except there is more engine plumbing to restrict your view.
   

And this is another view on an engine that's out of the car.


What about 95-97 miatas?

The 95-97 miatas use a oil pressure switcj, located in the same position as the above pressure sensor. On these cars, the
gauge does not show the true oil pressure. They only indicated that the pressure is "safe, i.e., enough to run the engine.
   

What's the right oil pressure?

  
It will vary with the engine temperature and spped. On the right is
the specification from my 94 shop manual.
 15-28 psi @ 1000 rpm
43-56 psi @ 3000 rpm
In general, when the engine is warm, expect to see the needle between 20-30 psiat idle, and around 60 at highway speed. When the engine oil is cold, you might see it above 30 at idle, and around 90 when moving. It is best not to race the engine when the oil is cold.
   
How does it work?  

The oil pressure gauge is an electrical meter that senses the resistance inside the oil pressure sensor.

The resistance is 115 ohms when the engine is off, The resistance goes down as the oil pressure goes up. Its lowest value is around 13 ohms which corresponds to a reading of 90 psi on the oil gauge.

Electrically minded readers will recognize that this design is susceptible to stray resistance. A dirty connector at the sensor can throw off the dash gauge and make it read low by as much as 10-20 psi. I've seen this happen on my 90 miata.
 

Caution and Disclaimer.

A low or zero oil pressure reading can also be caused by lack of oil or a bad oil pump. Running your engine with low or zero oil pressure can and will destroy it in minutes.

Don't assume it is a connector problem until you have eliminated the oil pump or lack of oil as reasons for your low oil pressure reading.

Not a certifed auto mechanic. Not a Mazda technician. The information on this page is my own interpretation and may be flat-ass wrong. Use at your own discretion.