by Neil Slavin

The following article recommends the use of a J.C. Whitney oil pressure sender to replace the Mazda sender and provide the Miata with a real oil pressure gauge. Unfortunately, there have been numerous reports of problems with that sender. Instead, the author now recommends that a VDO sender, available at all NAPA stores, be used for this modification.

If the VDO sender is used, it is not necessary to do the "optional" part of the following procedure. Instead, it is advisable to re-position the needle on the Miata's gauge to provide the best indication.

Please refer to the postings on for the proper VDO part number and for instructions on how to reposition the gauge needle.


When I learned that the oil pressure gauge in my '99 Miata was just an "idiot light" masquerading as a gauge, I decided to do something about it. Then I read Drake Daum’s account of his doing just that – a great idea, I thought, but scary. Dremel tools, silicone glue – yuk! I had visions of going through more than one set of parts before I got it right – and there just aren’t enough Miatas in my local junkyard.

For those of you with the requisite mechanical skills – more power to you. I’m sure Drake did a fine job and is justifiably proud of his results. But I thought there had to be an easier way.

There is! The secrets revealed below will lead you to a real, analog, oil pressure gauge, for your Miata, requiring only a (relatively) few dollars, a couple of hours, and some fairly basic mechanical abilities.

Please note that everything I say here has only been tested on one Miata – a '99 model. While it probably works on all other '99s, and may even be applicable to the M1s that came with idiot gauges, I offer this advice without warranty of any kind. And who knows what it may do to your official Mazda warranty? So proceed at your own risk.


Those of you who are bored by theory can skip this section.

In olden days, almost all cars with pressure lubrication systems (yes, there were some that didn’t have them) had gauges to give the driver an indication of the engine’s oil pressure. A thin tube actually carried high-pressure oil to a mechanical gauge in the dashboard.

These gauges provided an accurate, real-time indication of oil pressure. They could warn the driver of a failure in the lubricating system or a major oil leak, and they could provide some useful information about the general health of the engine. Unfortunately, however, they also introduced an extra failure mode – when the fitting behind the gauge came loose, the driver’s legs were treated to a bath of hot, dirty oil.

As engine designs improved, it came to be that the gauges were less reliable than the lubrication systems they were intended to monitor. Carmakers moved in two directions: toward electrically-operated oil pressure gauges and toward lights that warned of oil pressure failure.

The electrical gauges consisted of transducers whose electrical resistance varied with the oil pressure and instruments – much like fuel gauges – that responded to the variations in resistance. These continued to report on the engine’s health, but they weren’t much good when the oil pressure dropped all the way. In fact, a GM Service Manager I know calls them "get-out-your-checkbook gauges," because by the time you notice the gauge on zero, you’ve cooked your engine. At least they don’t leak.

The idiot light, on the other hand, provides a clear and quick indication of oil pressure failure, but no information at all about the state of the engine. And since, these days, most engines last longer than the bodies in which they’re installed, most drivers don’t care much about engine health. An idiot light is also much cheaper than a gauge. That’s why most cars today have the lights.

In the recent Miatas ('95 to the present), Mazda has chosen the worst of both worlds. They use an on-off switch like those used with the idiot lights coupled to an analog instrument. In this way, Miata drivers get none of the engine-health information that a gauge can provide and also none of the rapid response provided by a warning light.

As Drake discovered, it’s not sufficient to get an analog sender from an older Miata and use it with the newer car’s gauge. Due to the electrical characteristics of the parts, if you want an all-Mazda solution, you need to change the sender and the gauge.



1. Get everything ready. It may be possible to do this all from the top of the car if your arms are triple-jointed, but I found it much easier to work from below. Put the front end of the car up on ramps. Cover the right front fender with something soft. Remove the plastic cover below the engine (lots of small screws and nuts hold it to the car, but it should be obvious how to get it out). Remove the brace that connects the intake manifold to the engine block (two bolts on the manifold and one on the block).

2. Remove the wire from the terminal on the existing oil pressure switch. This is a bit tricky, because you have to squeeze a release tab while you pull on the connector and you can’t see what you’re doing. It may help to get a peek at it with a mirror and a flashlight before you start, so you can see how the tab is oriented.

3. Remove the switch with a 24 mm or 15/16" deep socket. It may be possible to reach it with an open-end or adjustable wrench, but the socket makes it really easy. Only a few drops of oil will spill from the hole in the block.

4. Open the adapter kit you got from J. C. Whitney, which contains five brass adapters. Identify the adapter that goes from 1/8-27 NPT to 1/8-28 BSP. (Why does the Miata use a BRITISH pipe thread – did Mazda take the Triumph/MG imitation a bit too far?!) The easiest way to do this is to match the male threads on the adapter to those on the old pressure switch. Save the other adapters for some other project.

5. Open the J. C. Whitney oil pressure gauge kit. Locate the sender. Thread the sender into the brass adapter, using thread sealing compound on the male threads.

6. Apply thread sealing compound to the male threads of the adapter. Install the sender/adapter assembly into the hole in the engine block. Tighten the threads, using an open end wrench on the hex on the sender (this will ensure that both fittings are tightened to the same torque). Get it tight, but not too tight.

NOTE: Some mechanics would say that 1/8-27 and 1/8-28 are "close enough" and no adapter is necessary. If your mechanic says this, get another mechanic. You really don’t want to re-tap the hole in the block after you’ve messed up the threads.

7. Locate one of the crimp-on female "bullet" terminals in the parts bag that came with the J. C. Whitney gauge. Attach the terminal to the extension wire. Cut the terminal off the end of the Miata’s sender wire. Splice the extension wire to the Miata’s sending wire, using your favorite technique.

8. There are two terminals on the new sender. One is labeled "S," for "Sender"; the other is labeled "WL" for "Warning Light." (More on this later.) Using a mirror and a flashlight, identify the "S" terminal and attach the sender wire to the terminal.

9. Replace the manifold brace and torque the bolts to 28-38 lb.-ft.

10. Turn the ignition on and verify that the gauge reads lower than the first mark. Start the engine and verify that the gauge reads somewhere between the second and third marks. Check the sender assembly for oil leaks. Replace the lower engine cover. Test drive the car, allowing the oil to warm up, and verify that the gauge reading varies with engine speed and oil temperature.

11. Save the extra parts for future projects and the Mazda sender in case you sell the car.


The above procedure gives you a working analog oil pressure gauge, but does not provide the best resolution of normal variations in oil pressure. This can be improved by changing the electrical characteristics of the gauge circuit.

This procedure requires a reasonable degree of care and the ability to do electronic soldering. It’s not difficult but it’s also not for the faint-of-heart.

1. Remove the hood over the instrument cluster, by pulling it straight back. This was a bit scary, because it seemed like something was about to break, but eventually it came off without damage.

2. Remove the four screws retaining the instrument cluster. Move both control stalks all the way down. Carefully remove the instrument cluster, removing the three electrical connectors and not allowing them to touch anything metallic.

3. Carefully depress the plastic clips and remove the bezel and the lens from the instrument cluster.

4. Remove the three screws retaining the oil pressure gauge in the cluster. Carefully remove the gauge.

5. Examine the gauge and locate the 100-ohm resistor in the gauge assembly. The resistor is a blue cylinder with wire leads; the resistance is marked on the blue part.

6. Install the new 10-ohm resistor in parallel with the 100-ohm resistor. In other words, connect one lead of the new resistor to one lead of the existing resistor, and connect the other lead of the new resistor to the other lead of the existing resistor. Solder both connections.

7. Reassemble everything in the reverse order of disassembly.

8. Start the engine and test drive the car. The gauge should now read just short of the third mark at speed and somewhere above the second mark at idle (with hot oil and an engine in good condition).


I noted earlier that the new sender has two terminals. This gives you the opportunity to have the best of both worlds – a real oil pressure gauge and an idiot light. Just connect the "WL" terminal to a suitable 12-volt light bulb or LED and connect the bulb to any source of power that is switched with the ignition. Let me know if you find a good place to locate the light.

Alternatively, you could set it up with an audible loss-of-oil-pressure alarm instead of an idiot light. That would probably work better in bright sunlight with the top down, at the cost of another annoying buzzer when you start the car.

Do you need to do this? Like everything else, it depends. If I had an add-on oil cooler or a remote oil filter, I would, because a failure in these aftermarket systems can kill your oil pressure and your engine. If I raced a car with a highly-modified engine that would cost a lot to replace, I would. But since, right now, I’m running a stock Miata and I trust the stock lube system, I’ll probably stop with what I’ve done so far.


Neil Slavin is a self-employed engineering and management consultant. He has a red '99 Miata and has been doing unauthorized modifications to his cars for longer than he cares to admit.

Back to Garage

31 May, 1999