Miata Oil Pressure Gauge

From notes on how the oil pressure gauge should work to how to fix it when it doesn’t work this section covers everything related to oil pressure. This ended up being one of the most annoying problems our Miata had when we purchased it. Basically it kept my wife from being able to enjoy the car for a good bit of time. Hopefully it will help others out some.

The howto further down on fixing the oil pressure sender is for Miata MX-5 Models up  until 1994. In 1995 Mazda changed the sender to a go or no go design thus making the oil pressure gauge a glorified idiot light and not a real oil pressure gauge. So keep in mind that the rebuild of the oil pressure gauge in this howto is only for 1989 to 1994 models.


Handy notes for me to refer to from time to time and might help others

Oil pressure sender on 1989-1994 models :

1990 manual says that for a sensor resistance to chassis ground of 52 ohms, the gauge should read 0 psi. For 41 ohms to ground, the reading should be 30 psi, and for 16 ohms to ground, the pressure reading should be 90 psi.

Keep in mind that the block is tapped with BSP threads (British Standard Pipe) and 99% of the ‘replacement’ senders you’ll find are not . . . . . which means you’ll have to screw anything else into an adapter to make it fit the block.

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.

Another chart from the manual

0 = 52 Ohms
30 = 41 Ohms
90 = 16 Ohms

Typical readings :

15-28 psi @ 1000 rpm
43-56 psi @ 3000 rpm







Oil passages



I’ve had several people ask me after I made this howto what pressure tester I was using that had the british thread adapter included with it. This is the tool I used.

Advanced Tool Design Model ATD-5550 Automatic Transmission and Engine Oil Pressure Gauge Kit



Rebuilding/Repairing the Oil Sensor (1989 to 1994 ONLY)


How to rebuild the oil pressure sensor or at least how to fix it when you hulk out on it with a socket in place of using the crows foot wrench like your supposed to.

This is what happens when you use a socket and the sensor bottoms out or gets too tight while tightening it or just sticks when trying to remove it, the outer housing slips. When this happens the small wire that runs from the backside of the clip you removed the sensor wire from breaks away from the inner sensor. Dead sensor and you just kissed off $193. Way to go 🙂

Ok $193 for a damn oil sending unit is beyond ridiculous in my opinion. So I figured I had nothing to lose trying to fix it.

First you need to open the thing up to get inside of course. Start by putting the sensor in a vise. Don’t get carried away and crush it. I’ve seen beer cans that had more tensile strength than the outer housing has.

Now use a screwdriver to pry the flange up around the base. Once its lifted a bit you can use a pair of pliers to finish straightening it out like this :




Now slowly pull the end that is threaded out of the housing. It will look like this :




Now before we get to fixing stuff lets drill a hole were going to need later. Look down into the housing from the side you removed the sensor from and notice a small indentation. Use a drill bit large enough to allow the wire you choose to repair this thing with to pass thru. I had 16 gauge wire on hand which was overkill but its what I had. This is the spot to drill :




Now its time to solder the wire. Feed the wire thru the hole you drilled and strip the end to solder onto the sensor. Here is where to solder it to :




Here is what it looks like being soldered :



After the wires soldered then slide the sensor back into the outer housing and crimp it back in place. After that cut the wire so its just long enough to solder to where the sensor wire clips to like this :



That’s it you just saved yourself $193 or more. Cool huh ? 🙂



Never underestimate the power of a moron to screw stuff up

I just spent a week trying to fix the oil pressure gauge on my wife’s Miata. This should of been easy. First I tested the gauge itself. I shorted the sensor wire to ground and confirmed the gauge swung full sweep. Ok that means the gauge works and the wiring to the sensor is good. No brainer right? Bad sensor.

I tested the sensor with a volt meter and sure enough no reading.

Well removing the sensor with a socket caused the housing to spin without the sensor actually coming out of the motor. It spun very easy too. So I’m guessing that is what killed the sensor. The previous owner must of done that. But I will fully admit I could of done the same thing. I priced a new sensor. $193……  NOT!

So I enlisted the aid of my stepdad (I don’t own a vise) and off we go. After repairing the sensor and getting good readings with the volt meter I figured this puppy is done. I borrowed a set of crows foot wrenches from my stepdad and installed the sensor. Got a good reading after installing the sensor. Hooked up the sensor wire. Cranked the car. NOTHING!


Ok trying to remain calm and being super stubborn I kept at it. The sensor uses some funky British pipe thread. So I’m looking at having to order a special adapter to be able to hook up my oil pressure tester to the thing to even get close to figuring this out. I checked the cams by looking into the oil fill hole on the valve cover with the engine running and they are bathed in oil nicely so it looks like it has great pressure. Motor makes no ticking sounds or anything. Still I wont let the wife drive it till I KNOW the oil pressure is safe. So the car will stay parked for now.

Tonight after days of messing around with the sensor to see if that was the problem in some funky way I noticed the manual for my pressure tester has the British 28 adapter that the sensor uses. Hot dog were cooking with gas now. So I go thru ALL the fittings(none of the stupid things are labeled). Nothing will start in the hole I removed the sensor from. Grrrrrr.

So I went back in the house and I was looking at the pictures I had put on this page of the sensor and the side of the block. Something just didn’t seem right. So carefully noting how it looked in the picture I went back out to the car and sure enough. Some moron had put the sensor in the wrong hole. Not only that they put some bolt in the correct spot for the sensor. Here is what they did.



So after a week of trying to get oil pressure to show I now have a working oil pressure gauge…….

Oh and out of idle curiosity I just had to know where that bolt came from. Sure enough I found it.



All said and done I’m just glad its done. Course this is the same rocket scientist that cut the exhaust manifold downpipe with a saws-all in place of removing the pipe with the bolts. Then used 2 large hose clamps and dr pepper cans as sleeves to fix it……

I ended up using the used sensor I bought just because I was to lazy to take it out and use the one I rebuilt. So I have a spare now at least.

I hope this helps others out some day. Moral of the story, never take anything for granted on a used car.



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