Turning the NB oil pressure gauge into a REAL gauge. By nikkidanjo Full scale, quick response (no damping), behaves like the real gauges in the ’90-’94 cars! This How To contains two sets of modifications for the 1999-2005 Miata oil pressure gauges.  The easy mod involves removing the excessive damping from the oil pressure gauge.  You will get the same non-linear response as before but with quick needle response.  The more advanced mod involves disassembling and unwinding the thin wire inside the oil pressure gauge.  When completed the advanced mod will produce a fully functional oil pressure gauge.  The needle will move quickly and the tick marks will (if desired and trimmed correctly) correspond to 0, 30, 60, and 90 PSI (for a 0-100PSI sender). If you are reading this you likely know that the NB Miata doesn't have a real oil pressure gauge. It has a 7psi switch. Below 7psi the switch is closed and the gauge reads 0. Over 7psi the switch is open and the gauge reads ~2/3rds. Beyond that the gauge will never move. Not like Mazda gave it anything good to say. In an attempt to gain some actual functionality (needed or otherwise) many of us have tried replacing the original NB oil pressure switch with VDO or other brand oil pressure senders. Often people have used the VDO-360003 0-80psi, 10-180ohm sender. Some people have also used the VD0-360086 0-100psi, 10-180ohm sender.  I have included modification recommendations for both gauges. Now, Before I go any further I would like to apologies for being excessively wordy. With luck you didn't have to read every one of them.
The schematic to the right shows the factory oil pressure circuit. From the factory: R1 = 180 ohms R2 = 100 ohms Coil A: Consists of three sets of windings (1-3) and has a total resistance of 180 ohms. Coil B: This is the inner most winding in the motor. It has a resistance of 100 ohms. Oil Sender: This is the VDO or other brand oil pressure sender.
Using the conventions for winding conventions I describe in detail below, the factory gauge has four sets of windings.  Note:  Because I my digital camera seems to have trouble taking the close ups I really need for this write up I've included some quick CAD illustrations.  I used blue rather than white for the plastic part of the gauge because it shows better in the illustrations Coil A (three sets)  1.  XXX turns in the 10:30 direction. (The outermost set/the one you see when you look at the gauge) 2.  XXX turns in the 7:30 direction. 3.  XXX turns in the 10:30 direction Coil B (one set) 1.  XXX turns in the 7:30 direction.
Brian Sanborn has a very good write up about changing out the sender and doing a simple resistor to adjust the scale. http://webpages.charter.net/sanborn/Miata.html I'm including a link to his web page because he does an excellent job of illustrating the installation of the sender and has a discussion the resistor mod. The resistor mod/moved needle with a VDO sender are an improvement over the dummy gauge but it still has the following drawbacks.  1. Excessive damping: Yes, the needle does move but very slowing. A quick blip of the throttle shows virtually no needle movement.  Mazda wanted it to look like the gauge was really doing something.  It would be a bit odd if the gauge just sprung up the moment the engine started.  This was the part of the problem I was originally try to solve.  2. Non-linear output with lots of compression at the top end: While you could tell the difference between hot and cold oil at idle, it was almost impossible to tell the difference between 50 and 60 psi.  Correcting this problem wasn't my original intent but while I had everything apart...
The Cause of the Problems Damping: The gauge is damped with a small amount of viscous grease between part of the needle stator and the white plastic wire form.  Remove the grease and the needle moves as fast as the tach or speedo. The Response Curve: This one is a bit trickier. The way the coils are wound largely controls the motion of the needle. Resistors can be used to trim the movement but depending on the windings you can't do much to change the response curve. Mazda intended this gauge to show 0 with 0 ohm or resistance. They wanted it to show 2/3rds with infinite resistance. It's going to be hard to get trim resistors to allow it to show more when our sender only has a 10-180 ohm range.

Solutions

Version 1 (the easy version) Eliminating the excess damping:

I originally unwound the windings on my gauge because I wanted to know how it was damped with the hopes of reducing the damping.  I found the grease described above.  Unwinding the entire gauge to remove the grease isn't the easiest task.  Fortunately another Miata forum member found an easier solution that doesn’t involve unwinding.

SlicksterV suggests cleaning the gauge with Isopropyl alcohol. He posted the following about his method:

I decided to approach the problem in a different direction.

I removed the oil pressure gauge, pressure gauge needle, then the gauge face (by removing the 2 black screws) so the inner coils are visible. I am just too chicken to perform the extreme surgery to get to the grease so I decided to use 99% isopropyl alcohol & ultrasonic cleaner.

I filled the cleaner with alcohol so the gauge body is fully submerged. Caution, 99% alcohol is highly flammable so continue at your own risk & in a very well ventilated area. I performed this task in my bathroom with the exhaust fans on. My assumption was the alcohol will dissolve the grease & the ultrasonic waves will assist the alcohol to reach to the grease easier. Since my cleaner has no lid I covered the cleaner with a vinyl file folder & left the cleaner running for about an hour. I periodically check the level of alcohol (without a lid 100% alcohol evaporates very quickly) & move the gauge around.

I removed the gauge body from the cleaner & let the alcohol evaporated for about 30 minutes. I reassembled everything, let the car warmed up a bit & noticed the gauge responded much quicker than before. Too bad I didn't fully warmed up the engine due to rain (just washed & Zaino'ed my car yesterday) but I am very happy with the result.

For reference here is a picture of the gauge motor with the needle and gauge face removed.

(pic of gauge with face and needle next to it)

And the follow up:

I finally had a chance to go for a longer drive tonight with a fully warm engine testing out the oil pressure gauge "full" deflection & the damping is totally gone.

My conclusion is that alcohol & ultrasonic cleaner can be used to remove the damping grease.

Of course not everyone has access to an ultrasonic cleaner.  Other forum members tried soaking the part in other solvents for 24 hours or so and achieved similar results.  I have not personally tried this method so I can not personally recommend any particular solvents.

Note: Remove the gauge face and needle.  DO NOT put them in the solvent bath.

Solutions Version 2, Eliminating the excess damping and achieving a linear response curve. For those who want it all. For those who aren't afraid... or are to foolish to realize they should be... If you are good with soldering irons, don’t mind the risk of messing with the guts of the gauge and or have too much time on your hands but insufficient funds for a turbo this is the next step… time to change the coil windings! Warning: If you go forward with this I am assuming you have some knowledge and comfort with soldering, working with simple resistor circuits, multimeters etc. You should also be comfortable doing a bit of improvising to create something to help wind the wires off the coils and back on the coils. These aren’t 100% step by step. If this stuff scares you just do Version 1. I cannot stress enough the importance of labeling things (or even better taking pictures) as you disassemble the gauge. Also, the coils are wound from a near hair thin copper wire. It takes VERY little force to snap this wire. You can splice it if needed (ask me how I know) but who wants to do that.
Parts To complete this mod you will need: -85 or the original 180 ohm resistor* (aprox) ohm 1 watt (min) resistor. This can be made from several resistors. I used 2 1/4 watt 100ohm resistors in parallel with a 35 ohm 1/2 watt resistor in series. Net result was ~85ohm and ~1 watt power handling capabilities. -110 ohm resistor (or a combination capable of creating 110 ohms) or the original 100 ohm resistor*. I used a 10 ohm and 100 ohm resistor in series. -2200 or 500 ohm 1/4 watt resistor. I happened to have a 2200 ohm resistor so I used it. Again a combination of resistors is fine. -100psi or 80psi oil sender with 10-180 ohm range.  The 0-100psi sender can be used to give a 0-90 psi range (same as the factory gauge).  A 0-80psi sender can also be used.  The benefit to the 80psi sender is you can use the original resistor values but the "H" tick will indicate 80psi.  With some experimentation you can get a 0-90 scale even though it will never indicate over 80 psi.   A variable resistor capable of providing a load between 10 and 180 ohms, a 12V power supply (Makita drill battery in my case) is recommended but not required for testing. *The original resistors can be used with the 0-80PSI sender.  More on this when we talk about resistors later.
The construction of the windings and core. The gauge motor is basically made from just a few parts. You have the wire which is would around a two part white plastic former /core. With all the wire removed the two halves of the plastic core can be pulled apart. Inside you will find a black, round, magnetic disc. The grease in question acts on part of this disc and the plastic core. With all the wire removed removing the grease is a simple mater of wiping it away with a rolled napkin. The factory configuration consists of 4 sets of windings. When looking at the motor you can see layers of wires wraped around the plastic part Each time the wire changes its direction of wind that's a new set. Each set is split in half running on either side of the needle post. It is important to get our bearings when working with the gauge motor. Orient the motor so it faces you as if it were installed in the dash. The shaft will face towards you. The OIL terminal will be at top and the IGN terminal will be on the right. In this orientation you see the coils are wrapped at 45 degrees from vertical. The coils can be wrapped in two directions for each orientation. For instance, a coil can go from 7:30 to 1:30 or 1:30 to 7:30. It is very important to note the difference between 7:30 to 1:30 and 1:30 to 7:30. The difference can cause the needle to move backwards… ask me how I know.	(pic of the gauge motor with out resistors) (the windings and metal can aren’t shown in the illustration)
Lets get pulling things apart Remove the needle and gauge face from the gauge motor. Unsolder the resistors. With a pair of pliers, carefully remove the three terminals from the bottom of the gauge. They are a one way push in deal. It takes a bit of force to remove them but they will come out. Label them so you know where they go later on. Label their location on the metal can and the plastic core.
Next remove the metal can. The can is heat staked to the plastic core via some posts through the can bottom. Cut off the 4 mushroom shaped posts. Next push the core out of the can. I found it was easiest to push on the parts of the core where the terminals used to be.
Congrats, now you have just the windings and core of the gauge. It’s time to note a few parts. Pressed into the plastic you will see three wiring terminals, GND, OIL and IGN. GND and IGN both have one wire. The OIL terminal has two. As part of this job you will have to either cut the wires off these terminals or unsolder them. I chose to unsolder them. Start by bending the coil wire terminals up away from the coils.  This gets them out of the way for unwinding and allows you to get a soldering iron on them. Also... remember the wire is very thin and easy to break so BE CAREFUL with it!
To make winding easier I used to small electric fans like you might find in a computer power supply. Using some double sided adhesive I taped an empty solder spool to one and the gauge core to the other. A few little cut up foam pads helped mount up the awkwardly shaped core. Now I could wind the wire off or on the core simply by spinning the part with my hand. A short bit of wire to the gauge to make it easier to spin with one finger. I didn’t use this twin fan idea the first time I unwound and rewound the gauge. With the spool I just had the spool and core sitting on the table. It took me about 4 hours to unwind then rewind the gauge. With the fans acting as little turn tables I was able to unwind and wind the coils in about 45 minutes including stopping to solder and change direction.
Unsolder or cut the wire connected to the IGN terminal. I used a sharp solder tip to unwind the wire. Start winding it onto the spool. After unwinding coils 1-3, unsolder or cut the wires from the OIL terminal. Unwind coil 4. You can now open up the two plastic parts that support the coils. Inside you will see the needle stator and the damping grease. I just used a bit of napkin to wipe the grease away. The little black part in the second photo is a magnet or ferrite block that magnetically returns the needle to 0.	Location of Grease
Now it’s time to put it all back together with a new wind.
In the illustrations the arrow will always point a long the wire to the spool of wire that is waiting to be wound/ to the end of the wire. So for the first set of winds it points to the end of the wire that will solder to OIL.  For the later coils it points at the end of the wire that solders to IGN.  Use your fingers to guide the wire onto the core while you wind it.  Keep a little bit of tension on the wire so the coils aren't too loose.  Again, be careful, the wire is thin. Coil 4: Wrap 350 turns of wire around the cord in the same orientation and direction as the inner most turns of wire. For those who didn’t write down the orientation of that inner most bit, hold the gauge in front of you with the shaft facing you, with the oil terminal at 12:00 and the GND terminal at 3:00. Each time the wire comes around it should start (rise from the table) the 7:30 position and finish (set/goes back towards the table) at the 1:30 position. Put half of those 350 turns on one side of the needle shaft and the other half on the other side. As there are a lot of turns, I recommend counting in blocks of say 50 turns then putting a tick mark on paper. It also allows you to be a bit more absent minded when winding. Also, watch out for snagging the thin wire on the white plastic gauge former while winding things up. You want all the wire inside of the former, not hung up on the edges.	1:30 Direction of wind, 530 turns
Next coil: Change the direction of the wire so it rises at the 10:30 and sets at the 4:30. 530 turns, one half on each side of the needle shaft. Any time I change direction or solder the wire to a terminal I try to wrap it around one of the plastic posts the same way Mazda did when they originally wound the gauge. I don’t know if it’s needed but I feel better doing it that way. Part way through this you should run across what used to be the bit of wire that was soldered to the OIL terminal. If you cut the wire you will now need to join the two wire sections. If you didn’t cut the wire you still need to keep the solder covered wire away from the rests of the coils. I made one turn of wire around one of the plastic post, put the soldered section of wire in the hole in the white plastic post then made one more quick turn around the post to hold it in place. After than, continue wrapping.	Wrapping wire around the posts 4:30 Direction of wind, 530 turns Locations for the soldered joint in the wire
When you get to the end of the 530 turns, wrap a bit of the wire around the OIL terminal and solder it in place. I very carefully used a knife to scrape off a bit of the wire’s enamel so the solder could touch bare wire. I’m not sure if this is needed. Congratulations, you have created a new Coil B consisting of two sets of windings.
Lets get on to the new Coil A
Wrap 760 turns of wire in the opposite direction (rise at 4:30, set at 10:30). Again, split the winds between the two sides of the needle shaft.	10:30 direction of wind, 760 turns.
Next it’s time to change direction again. The rest of the wire will be used to create two windings that sit on top of each other and mostly cancel each other out. The first winding will go from 1:30 to 7:30. The second will go from 7:30 to 1:30. The exact amount of wire left (and the number of turns) is affected by exactly how it was wrapped around various posts and how tightly it was wrapped around the core (tighter means more turns). Because of this I can’t say wrap the last say 236 turns etc because we don’t know exactly how much wire is left. However, what we want is to have a few more turns going from 1:30 to 7:30 than in the other direction. To do this, wrap all the rest of the wire going from 1:30 to 7:30. Count the number of turns. It should be over 200, probably around 240. Divide the number of turns in half. Unwrap 10 less than one half the number of turns. Now wind that wire back going the exact opposite direction (7:30 to 1:30). When you get to the end of the wire, wrap it around the IGN terminal and solder in place. You should have about 20 more turns in the to 7:30 direction than in the to 1:30 direction. The second gauge I modified had only about 140 turns vs the ~240 for the first gauge.  I suspect the second gauge was not wound as tightly as the first.  Either way the range seemed fine (0-80 on the scale). Congrats you have rewired the gauge! Now it’s time to add the resistors and check it. If you haven't already done so, solder the coil wires to the terminals.  Fold the wire terminals back down. Put the metal can back on as well as the terminals that push into the bottom of the gauge.  Finally, attach the gauge face.  Don't attach the needle yet.	7:30 direction first Then 20 fewer turns in the opposite direction
Time for new resistors
We will change the values of R1 and R2 and add a new resistor (R trim) going from the OIL terminal to the GND terminal on the bottom of the gauge. I have included several possible resistor values.  Originally I had a set of recommended values that resulted in 0, 30, 60, 90 psi at each tick mark assuming a 100psi sender.  The gauge lined up perfectly with my calculated R oil valves.  However, those values assumed the sender has a linear output.  It assumes that 50 psi should result in a resistance of (180-10)*50% = 85 ohm.  Unfortunately what limited testing I could do with the sender and my experience with the sender in my car suggests that the VDO output is somewhat non-linear and reads a bit high (85 ohms at 40-45psi rather than at 50).   For the 0-80 psi sender the target tick mark values were 0, 30, 60, 80.  My victim car came close.  We saw 0, 28, 59, 80 assuming a linear output.  As the 0-100 sender seemed to read high I suspect the 0-80 senders will also read high.  R1: Use a combination of resistors to create a 85 ohm resistor to replace the original 180 ohm resistor. I used two 100 ohm resistors in parallel and a 35ohm resistor in series. It’s not pretty but it’s what I had on hand. The total combination of resistors should be able to handle 1 watt of power. A combination of two 1/2 watt resistors should be fine.  This resistor does not actually change the response curve of the gauge.  However, it does affect the start up time of the gauge (see note below).  R2: Use a combination of resistors to create a 110 ohm resistor between IGN and OIL (replaces the 100 ohm resistor). I used a 100 ohm and 10 ohm resistor. A 1/4 watt resistor is fine for this application.  This resistor changes the range of needle motion and the shape of the response curve.  A larger value makes a more linear response curve but reduces the total travel of the needle.  A smaller increases the total needle travel but makes the gauge less linear.  The gauge will show more motion at lower oil pressures.  I would suggest using at least a 1/2 watt resistor or a combination of resistors that equals 1/2 watt. For the 0-80 PSI sender use the original 100 ohm resistor. R trim: Use a combination of resistors to create a 2200 ohm resistor between OIL and GND. I used a single 2.2K resistor.  The 2.2K resistor resulted in a 0, 30, 60, 90 output assuming a linear output from the sender.  Based on some crude bench testing of the sender (resistance of sender compared to a pressure gauge) I think the sender resistance is on the high side.  To that end I replaced the 2.2K with a 500 ohm resistor.  The scale seemed closer to correct the choice is up to you.  If you used a 0-80 psi sender you can skip this resistor. I experimented with different values for R2 and R-trim before selecting the values above.  If you would like more low end sensitivity reduce R2 and increase R trim.  I looked at R2 values between 50 and 150 ohms and R trim values between 300 ohms and infinity (factory trim, no resistor).  If you are using a variable resistor for bench testing you can adjust the resistor until the needle points to the H mark.  Then measure the resistance of the variable resistor (remember to unplug it before measuring).  Now you now what resistance will give you the travel you want.  Figure out the resistance of your sender at say 90 PSI (I wanted H to mean 90).  For the 80 and 100 PSI senders that would be 201 and 163 ohms respectively.  Now figure out what resistor in parallel will give you the measured value.  In my case I measured about 152 when the needle indicated H.  (2200 ohms^-1 + 163 ohms^-1)^-1 = ~152 ohms. At 2200 ohms this resistor as virtually no effect on the 0 PSI position (2200 in parallel with 10 = 9.95)
Bench testing and needle installation.
By now you have rewound the coil and should have the correct resistors on the gauge.  All that's left is to position the needle and check our work.  We are going to simulate the gauge on the car. Very lightly put the needle on the shaft pointing at about the 9:00 position.  Based on my experience, when the gauge is not powered and the needle is about the hard stops it seems to want to settle at about the 9:00 position.  Connect ~12V to the IGN terminal. Connect GND to ground. I used a 12V cordless drill battery as the power supply. Finally connect a 10 ohm resistor across OIL and GRD (a variable resistor works nicely since it helps check the full gauge range). When you connect all this up you should see the needle move. When it stabilizes pull it off the shaft and reposition it so it points at the Low tick mark. If you are using a variable resistor you can simply dial up the resistance until the needle points at each of the tick marks. I got a very nice 15 turn 0-2K variable resistor from Radio Shack. Measure the resistance of the variable resistor and convert that back into oil pressure. I used the following formula to convert oil pressure to resistance R=10+(oil pressure/100)*170. Now you know what each tick mark means!  So even if you choose something other than 0, 30, 60, 90 you will still know what each tick mark actually means.  At this point you can also decide if you would like to increase or decrease the range of motion or change the response of the gauge via changes to R2 and R-trim. I know it’s a number of steps but you end up with an oil pressure gauge that works!
One more note: When I first changed my gauge I left R1 alone (180 ohms) and I didn’t mess with those last two sets of windings that go exactly opposite of each other thus canceling each other. Unlike R2 and R-trim, R1 doesn’t change the response curve of the needle. With that arrangement I noticed it took a while for the gauge to wake up when you started the car. In the morning it could take as long as 30 seconds (!) before the gauge started to respond. It would sit in the off position (below 0 PSI) then spring to life.  After it woke up it functioned perfectly.  That wasn’t acceptable to me. After a lot of testing and theorizing I figured this was some odd electrical interaction perhaps having to do with the gauge coils acting like inductors and upsetting who knows what. To fix it I wound the wires the way I instructed above and I reduced R1 to 85 ohms. The combination reduced the start up delay to about 5 seconds. That’s fast enough that I’m not worried about it. The second gauge I modified was set for a 0-80 psi sender.  It seemed to work correctly with the original resistors and 0, 28, 59, 80 psi tick marks.