I am posting this for eventual posting in the garage section. Please make comments and any corrections you would like, it belongs to you.
It's cold out, I will do pictures as soon as possible. I am somewhat foggy on the different NA transmissions, so any input is welcome. Here is a way to change 100% of the fluid in a Miata auto NB ( 99-2005 ) The NA will be about the same, You do not have a drain plug in a NA and have to drop the pan and I am not sure which line is the return, probably the same, but you'll know the one the fluid comes out of. I would suggest that you install a drain plug the first time you drop the pan, then it will be easy to do in the future.
The Miata comes with dino ATF in it. I would recommend that you change it to Synthetic. The reason is that heat is the # 1 killer of a automatic and synthetic ATF will take much more heat without failure or burning.
I choose Mobil 1 ATF, It's about $6.99 a qt at Autozone. You can find it cheaper if you look around.
Low prices on Repair Manual - Vehicle for your Mazda MX-5 Miata at Advance Auto Parts. Find aftermarket and OEM parts online or at a local store near you.
You'll need 8 qts. The Miata automatic is relatively easy to do. There are three things you have to decide. If you want to change the filter, change part of the fluid or all the fluid. The filter change requires you to drop the pan ( both models ), change the gasket and filter. When you take the pan off you'll get 4 qt's of the fluid, there is still 3.x in the torque converter.
( later ) Whether to change the filter is up to you. I never do unless there is a problem. Several have ran into problems when trying to do it and there not even any instructions I can find in the Miata service manual. There are three bolts that hold it on and they go through the control valve, two thread into the transmission body and the third has a nut on top of it and it must be hard to get back on. If you decide to not change the filter, then pull the plug ( or drop the pan ) and drain the 4 qt's, replace plug and washer or pan and gasket. If you drop the pan, you need to have a 30 to 200 in.
Lb torque wrench. It is very easy to strip the the pan bolts. The pan also needs to have equal pressure on the gasket. The specification is 35-43 in. ( different years are different best ck the specs of your year ) and that is not much.
To reinstall the pan there not any instructions that are Miata specific I can find. What I do is make sure the old gasket is cleaned off the pan and transmission. I use a dab or two of RVT to line up the gasket on the pan and hold it up in place and start putting the bolts back in. If the bolts have any old RTV on them I clean it off and use a new wipe of RTV on the threads.
While holding the pan in place thread them in finger tight, making sure the gasket stays aligned. I then put the socket on a handle driver and begin to tighten them hand tight from the center out in a crisscross pattern. I will then use the torque wrench in the same pattern. I tighten them two times as the gasket is compressing.
If that's all you want to do refill and the new ATF ( 4 qt's ) will mix with the old and you will have changed 60% of the old. ( I never do this ) To change 100% you have refill the pan with the 4 qt's of ATF. On the lower passenger side of the radiator there two hoses that go to the transmission cooler in the radiator. The lower hose is the return line ( left side radiator cooler). Detach the lower hose from the return pipe ( attached to the frame ) and slide in a short nipple 1/8' pipe or similar will work. Friction fit is ok, there not much pressure on it.
Attach a 6 ft piece of 3/8' hose. The amount of fluid you have to drain out will be have to be caught and measured. You're going to drain 3 qt's. Pour in a qt, start the car, it takes about 25 sec to get a qt at idle( shut off the car ). When you get a qt, you pour in another new qt ( you have 5 qt's in the pan now ) and then drain that. One more time you're done, that should have purged the full 7 qt refill.
Re-attach the line and your good to go. Start the car check for leaks and check the fluid level. Since all measurements are approximate and you spilled some like I do. The owners manual has instructions on how to do that. The last automatic I burnt up at 90,000 I kicked myself for not doing it at 60,000.
Now I do it every 30,000 miles ( overkill? Maybe ) It's $40.00 worth of oil. It cost me $1,400.00.
When you smell burnt transmission fluid you will remember that smell the rest of your life. The one thing that is nice with the Miata ( my 2004 ) is that it has a drain plug. Most US transmissions don't. The first time you change the oil you have to install one in the pan like the NA's. I look at like, they intended you to change it, or there would not be a plug. The other side of it is that with a US transmission you would change the filter when you drop the pan. The filter is a screen type and I have seen many, I have not seen one close to being plugged ( most are clean, nothing on them ) in a transmission that is still shifting and has not had a major clutch pack failure.
Here is a picture of the transmission cooler lines on the passenger side. The Blue Marked ( top )one it the one going to the cooler 'from' the transmission.
The Red marked one ( bottom ) is the return line. It is the red line that you want to disconnect from the steel line and that is where you would insert the 1/8 npt size nipple, or a piece of 3/8 ' size pipe and connect your 3/8' drain hose to.
It has no pressure so you don't need a clamp. When you idle the motor it will come out slow. ( 25 seconds for a quart ). I know for you purest the color of the lines seem backward ( red usually means hot and blue means cool ) so you would think the return blue, but it's not that way. Don't discount the filter change too much. If the unit has been neglected somewhat, you should change it.
The fluid can get varnished or shellacked which would reduce the flow thru the filter a bit. I am going back a 'few' years, so maybe the oil technology has changed somewhat, but tranny fluid can get a little 'gummy' with the right circumstances. If you have a load of miles on the car, or between fluid changes, or if it's sat for an extended amount of time (months and months) you should change it. Granted, the filter is mainly a 'catcher' for wear and tear but bad fluid can lower its flow efficiency. Being most things are based on internal line pressures, you don't want artificial limits on the flow.
It's not just shavings, etc. But also debris from the clutch packs that can clog a filter with time. Think of your filter on you home's HVAC unit or in the car's intake system. After it catches so many particulates, it loses efficiency and gives an imbalance to the pressure.
Checking the debris in the pan and/or filter is a good way to diagnose problems; by looking at whatever is stuck to it and determining the nature of the debris. The old rule of thumb was to change the filter every time but that was because you were dropping the pan anyway. With these fluid replacement systems, it's not really needed every time but it still should be changed from time to time. Cienega32, I have no issue with changing the filter as long as the changer understands how to do it, I sure don't. With at least 3 different transmissions and not having proper instructions on how to do it I hesitate to make the recommendation.
Mazda Mx5 Transmission
The bulk of all posted threads feel that most have nothing on them. Let's work on a write-up on how to do it. The forum member ' DavidK ' had a great description on how to get the 3 rd bolt started on the nut you cannot see or reach on top of the control valve. Here are some threads on the issue.