OK, here are some full instructions, which may deviate slightly from those on miata.net:
Installing a REAL oil pressure gauge in a Mk1 1.8 MX-5 (or post-1995 UK 1.6)
Until 1994, Mazda fitted an oil pressure gauge to the MX-5 which showed the actual oil pressure in the engine, measured by an analogue electrical sender unit next to the oil filter. The gauge in the dash was graduated at 30, 60 and 90 psi for UK and US cars and 2, 4 and 6 kg/sq cm (or bar) for Japanese and Australian cars. The advantage of the real gauge is that the driver gets some information about the condition of the engine and the oil. For instance, a worn oil pump or worn bearings will give a low oil pressure at hot idle, whereas a faulty oil pump pressure regulator or a blocked oil gallery will give an excessively high reading, as will oil of the wrong grade.
With the revised Mk1, Mazda fitted something that looks like an oil pressure gauge, marked “L” and “H” (without numbered graduations) but this is in effect a warning light. “L” indicates insufficient oil pressure (like the red warning lamp on most other cars) and the mid-point indicates “sufficient” pressure. Instead of a sender, there is a smaller and much cheaper binary oil pressure switch. The gauge is actually useless; it is so heavily damped that in the event of a catastrophic loss of oil pressure, such as a burst filter O-ring (I’ve been there), it can take over a minute to fall to “L”. Even if it reacted faster, most drivers would not notice until it was too late, unlike a bright warning lamp. This is still a problem with a “real” gauge (ideally a warning light would be fitted too), but because the gauge is always moving, the driver is more likely to make a habit of checking it.
The good news is that the “real” pre-1994 gauge and sender are freely available from Mazda, and fit straight on to a later car using the existing wiring (not the Mk2; that’s a whole different ball game). The bad news is that they are quite expensive; the gauge is about £55 and the sender over £70. Still, I thought it was worthwhile, and if you do too, here’s how to make the change. There is no need to go under the car at all, and it’s a relatively clean job depending on the state of your engine compartment. You can do this at any time; accessibility may be slightly easier if you have the filter off for an oil change, but it’s not necessary to drain the oil as the sender is mounted quite high up.
First obtain your gauge (part number NA01-55-4A1) and sender (B61P-18-501). The gauge is available in “UK” or “Australian” versions; I assume the Aussie version has metric calibration like those on early Eunos Roadsters. Anyway, get the UK version because the Aussie version is highly unlikely to be in stock anywhere in Europe.
When you have the parts, reach down (or up, if you really like lying on the floor with your head in the wheel arch) into the engine compartment and find the oil filter. To the left of it (as you face the filter), slightly higher and more recessed, is the oil pressure switch with a single wire leading to it. Squeeze the connector and pull the wire off. The reason to do this first is in case you need to drive the car with your new gauge and the old switch, or vice-versa; I don’t know if running the analogue gauge with the binary switch connected would damage the gauge (full battery voltage), but at £55 I don’t want to find out.
WARNING! My car doesn’t have an airbag, in common with most Mk1 UK cars. If yours does (perhaps it’s a Eunos Roadster) you will have to read the manual to find out how to disarm it before starting the following work involving the instrument panel. The steering wheel does not need to be removed, but you might accidentally short the connections which fire the bag and injure or even kill yourself. Normally the battery has to be disconnected a while before starting work, and this is a good idea on even a non-airbag car, just in case you blow a fuse or fry your PCM. I confess that I left the battery connected, because it took me ages to set up the stereo the way I wanted it, but do as I say, not as I do!
Next, make sure you have clean hands and then dismantle the instrument panel. Firstly remove the four screws which hold together the clamshell moulding around the steering column, and remove the top half, being careful not to break any pins or locking tabs. Note that one screw is shorter and not pointed. Next, remove the two screws which secure the bottom front of the black instrument shroud. This done, grip the shroud near the top and pull hard. It is secured by clips and takes a surprising amount of force to free. Then manouevre it away to expose the instrument panel itself. Undo the four screws, then reach behind (stand outside the car and look through the windscreen to see what you have to do) and undo the two electrical connectors, one at either side, while squeezing their locking tabs in. Now the worst bit - you have to squeeze a similar locking tab on the white speedometer drive connector. It’s on the plain white barrel, right next to the fluted tapered bit, and the tab is on the underneath to make it extra hard to reach. Squeeze the tab while pulling the panel and suddenly it will all come away. EDIT: I have now done this on a '96 Merlot, and the speedo drive was a slightly different connector. If anything, it was easier to undo.
There is one small electrical connector left, then you can take the panel away somewhere more comfortable. Firstly, unclip the glass (and black dial surround, which is integrated with it) by carefully releasing all the black clips round the edge. Now the three coppered screws which secure the old oil gauge should be obvious. Undo these and keep them safe, especially if the new gauge didn’t come with new screws (mine only had two!). Then ease the old gauge out from the front. The new gauge goes in the same way onto two white locating pins, being careful not to overtighten the screws. Only hold it by its edges - fingerprints on the dials would be really obvious in direct sunlight. Now you can put the dash back in the car in the opposite order, although it’s easier as all the electrical connectors and the speedo just snap on. When you’ve finished, check all the dash lights still work.
Now you need to swap over the sender. You’ll need a 24mm deep socket to clear the electrical terminal (about a fiver from Halfords), a 1/2" ratchet handle and a short extension bar. Fit the socket to the old switch first, followed by the extension bar and then the ratchet. There should be plenty of room under the manifold to swing the ratchet and undo the old sender. It will feel fairly tight all the way out until the last couple of threads, because there was sealant on it. When it feels loose, remove the ratchet and do the last bit by hand to avoid dropping it deep into the engine compartment. Very little oil, if any, will leak out. I found this was a 2 minute job, but some of the American contributors to miata.net reported that it took 2 hours. I can only surmise that LHD cars make the sender less accessible, or I was just lucky with the contents of my socket set.
Next, take your new sender - it’s much bigger - and put a smear of silicone sealant on the threads, but avoid the last three threads as you don’t want this stuff floating round in your engine. There’s no need to wait for it to dry. Reach in, find the tiny threaded hole (a small mirror helps a bit) and screw in the sender by hand as far as possible, being careful not to cross-thread it. Now you need a deep 30mm socket to tighten it fully. The manual suggests using an open-ended spanner on the hex nut immediately adjacent to the threads, but this is impossible unless the engine is out of the car with no oil filter, inlet manifold etc. Using your extension bar again, you should be able to get the torque wrench on the socket - it’s only 19Nm (13 ft lb) which is slightly less than a spark plug needs.
Finally, put the electrical connector back on, fire up the engine and you should have a REAL pressure reading. Expect over 60psi at idle with cold 0W40 synthetic oil (higher if you use a mineral oil such as 10W40), falling substantially as the engine warms up. The factory specification is 15-30psi at 1000rpm when hot. Mine shows just under 30psi, which suggests there’s plenty of life in the motor yet. Check for oil leaks after the first run.
For info, High Power 1.6 oil pressure should be…
Engine at normal operating temperature
At 1000rpm 28-43psi/2.0-3.0kg/cm2.
At 3000rpm 43-57psi/3.0-4.0kg/cm2.