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PREFACE:
So you’re tired of leaking PS fluid all over track huh? Not wanting to keep dumping fluid & money into a 20+ year old power steering system? Well boy do I have the solution for you. (I think) (I am not responsible or liable for any self-inflected damage you may cause or, heaven forbid, any injuries to kittens if you screw up your steering column and crash)
This is somewhat of a guide/build thread of an EPS swap I completed in my '95 Miata recently. The goal of this guide is to provide more information and groundwork on EPS swaps for others looking to do the same thing on a budget. Any questions/criticisms/input are more than welcome. BE WARNED, this is a short 😉 read- a little over 4,000 words, with 80 photos.
Now with that out of the way, let’s get into it.
Long story short- turbo interfered with my ’95 pump feed hose, made a janky solution which became temporary until PS died at a track event. Pump works until high load turning to the right. Took car down for other maintenance and started thinking… I *could* have replaced the PS pump and made a new fitting, solving both issues… orrrr I could start a long, somewhat uncharted project which would sidetrack and delay getting the car back on the road…
Project it is. Here’s the results first-
-Full dynamic adjustment on the go of power steering assist level- from depowered rack to 1 finger turning.
-Deletion of almost all messy PS equipment!
-OEM+ fitment retaining original functionality
-~85% cheaper than Kmiata kit!
-Apparently weight savings????
Did some research and started with google (as you should always do, looking at you, newb MT members) to see if anyone has done this before and already made a nice spoon feed. NOPE! Unfortunately, there isn’t a 110% complete writeup for a DIY EPS swap into a stock Miata. However! There have been multiple people who have pulled off an EPS swap into racecars or, using different hardware than I used, and documented it before. Here’s a list of what I could find:
EPS thread from when kmiata first started selling their kit & another thread from someone else asking about the swap- “dietcoke” posted about his EPS swap with a Prius unit into an Exocet(?). Helpful and got me started on the right path but didn’t offer adjustment and had no reference for a stock Miata w/ interior.
General talk about EPS swap on other forums, mainly the differences between the Prius & GM units. Prius units don’t need an external controller but have a set level of assist (40mph) in failsafe mode and can’t be changed. GM units require an ECU to function but have the ability to adjust assist with eBay controllers. They also have caster/self-centering issues (apparently, will touch on this later…).
Finally, the most helpful post out of all- aMaff with a great writeup on his install of an EPS unit on his racecar. You’d be able to follow his post and come out with a working EPS swap, however there were a few key bits of info which I had to figure out for installing it in a stock Miata, such as ECU mounting, wiring, how to mount column to dash, fitment under dash, and how to mount/keep the turn signal stalk/ignition switch/etc. Highly recommend giving his post a read through regardless, as more photos/info are always helpful.
Completed this writeup last spring and never got around to posting it. Since then it looks like this guy has done the exact same process- except with a little more complexity of having to press out the lower bushing for the steering column. Oh well, 2 guides are better than 1!
Order all of the parts needed. The EPS unit itself can be found in junkyards or on eBay/other sites.
The following years/models have the correct unit. Make sure to only get ones with a metal ECU case, the ION has some with a black/plastic ECU case which will not work.
Saturn Vue – from 2002 to 2007
Chevrolet Equinox – from 2005 to 2007
Pontiac Torrent – from 2005 to 2007
Saturn Ion – from 2003 to 2006
I opted for the junkyard route, as I have multiple close by and only paid $45 for the entire unit.
Removal process is roughly as follows:
The airbag comes off with two little flip tabs accessed through two holes on each side. It’s a PITA and I used some big snips to cut some of the plastic. BE CAREFUL AND STAY OUT OF THE WAY OF THE AIRBAG. While there is no battery hooked up, I’d rather not take the 0.01% chance of having it go off while my face is 1in. away. The rest comes out pretty easy, unbolt the steering wheel, remove some plastic covers around the dash, etc. etc. The upper part of the column unbolts, and so does the lower. There’s a pretty large bolt which goes through the entire unit, but it’s not too bad to get out. MAKE SURE TO GET THE ECU with the unit. You can unplug or cut the 12v power cable, I opted to cut just in case. Leave the motor cables intact. No reason to cut those. I ended up ordering a new connector anyways, but it’s free at the junkyard. If you can’t figure out this part, I highly advise not attempting this swap, as this is the easy part.
Single photo I took in the junkyard
Step 2- Initial disassembly
Remove your steering column and all associated wiring/accessories. I took out my cluster for a little extra safety/visibility, but it’s not necessary.
Either from your column or the spare you plan on cutting up, remove the ignition switch assembly and turn signal stalk (and clock spring, etc. etc.). The two little screws on top look impossible but come right out if you cut a slot in them & use a flathead. The turn signal stalk unscrews with a few screws. Above is the EPS unit from the junkyard with my steering wheel temp. on it.
Photo of the slot & screws
Remove the C-clip at the top of the Miata steering column. Remove the lower half the Miata column. It is tight, but a few whacks with a hammer will coax it right off. You now should be left with the Miata steering column shaft, and the two outer casings. These will be modified later on. Repeat a similar process with the GM column. Remove the C-clip at the top of the column and undo the 3 10mm bolts at the bottom. Whack the top half off. The top half will not be used. The inner shaft and lower cover will be, hold on to those. It goes on the input shaft for the GM column, and there is a small retaining clip on the input shaft which can be discarded.
Picture of the GM unit with the upper and lower cover removed, along with the small black clip on the inner input shaft.
At this time, you can remove the motor with the two bolts on the side to make moving/carrying the assembly easier. Also, you can now remove the ECU. It has 4 wires for the hall sensors. These little connectors didn’t want to come out, so I had to cut & solder extensions on them. Details later on, but it is *EXTREMELY* important that you take photos, label, and remember which wire went to which. If you get these messed up, the unit will not work (I think, I haven’t tested it but I imagine it would be pretty screwed up.) The white stuff is thermal paste- the body of the ECU is thermally connected to the main unit to help with heat dissipation. I toyed with the idea of putting a heatsink/fan, as shown in the photos, but decided against it for space reasons. The Miata is much lighter and shouldn’t produce as much heat. So far, I haven’t had any issues, but this could be something to improve upon.
ECU photos.
Last edited by yet_another_na; 04-28-2023 at 07:47 PM.
Reason: photo was misplaced, and removed some spaces
Here’s where there aren’t exact steps. I went through various stages of R&D on this, going from using the Miata shaft to not using the Miata shaft. I ended up completely getting rid of the Miata shaft for the input side and reused the GM shaft. This saved some hassle with trying to get the right length, cutting and welding the input shaft, and honestly a whole bunch of work. The GM shaft actually has the same spline pattern as the Miata, so your steering hub will carry over. Also, it has the same size for the bearing in the upper half. It is not the same length, but because the upper part can slide/adjust length, this is not an issue. You will need to cut the outer Miata column cover high enough up to go past the two “pinch points” that squeezes the inside point, otherwise the GM shaft is just barely too big to fit. PLUG/COVER THE BEARING BEFORE CUTTING- you do not want metal shavings all inside your upper bearing. (ask me how I know 😊 See photos/video below.
Comparing the GM vs Miata upper shafts
A bunch of measurements showing the pinch. You want to cut after this pinch but before the mounts to allow the GM shaft to fit inside.
Fitment before cutting past the pinched part.
I unfortunately didn’t get any photos of the exact cut/length I made, but it was roughly 0.75” from the mounts. You want to leave enough meat to weld to, but far enough down to get past the squished section. I also recommend grinding down the C clip ever so slightly until it fully engages. Your mileage may vary, but I had to grind mine down just a touch to get the clip to fully engage. Also, you can stick the GM upper shaft into the column and use the C clip at this time. This will allow you to test fit different lengths and show off how it will all fit together.
A comparison of the Miata shaft, Miata shaft & cover, & GM shaft inside Miata cover.
Multiple test fit photos, the entire assembly is close, but doesn’t interfere with anything. This is the only positioning which seemed to work.
Now it’s time to cut the bottom upper half of the GM cover. I measured this to be between 60-65mm for proper fitment in mine and went with 62.5mm. This will affect the length of the rest of your parts as well. I advise taking your own measurements and confirming fitment and positioning.
Measuring, cutting, and cleaning up the bottom half.
Now for one of the toughest parts- getting the angle between the upper half and lower half correct. This locates the motor and needs to be pretty exact to avoid any fitment issues. I put the column in the car and made 3 marks-
one on the lowest angle (closest to the trans tunnel)
one in the ideal/middle
one on the highest angle (closest to the AC bits)
These were made before cutting the lower part, but should be done after for even better accuracy, hence why I am putting this step here. Scroll up to the test fit photos to see more photos of the marks.
Let's get those two shafts joined and welded together. Plug up the inside of the upper half to protect the bearing from splatter when welding, as done when cutting. I did this with 0 welding experience & a HF flux core, so, uh, take that what you will and ignore the boogered mess. Seems to be plenty strong though.
So close! But so far!
Now it’s time to work out the bottom half. This isn’t so bad either, since now you can mount the upper column in there and the 50000lb motor will stay in one place (and not fall right into your skull… ask me how I know). Getting the whole assembly in is a bit of a pain though. Anyways, let’s cut up the lower Miata shaft. I didn’t grab a measurement, but I cut it longer at first (see photos) and ended up trimming it down another 0.5” or so. Yours will vary, so just get everything in there and estimate. Leave enough at the bottom though to sit inside the coupler. Oh, and this will tap out of the bottom of the Miata bearing/mount if you hadn’t removed it already.
After cutting, you only want the small lil guy at the end.
Now cut most of the lower cover off, leaving the bearing and the mount. You might want to leave a little more, depending on preference, but I felt this was good enough. Make sure to protect that bearing again! Throw it together and get some good test fits in. The lower u-joint that connects to the rest of the steering shaft through the firewall has some move/play, so getting this 100% exact isn’t a huge deal. Now you can throw it all together and give it a test fit!
Finally, turning the steering actually turns the wheels!
The last big fab step is making sure everything fits, and then welding the coupler & steering shaft. I chose to take this to a shop to get TIG’d, as I was also doing the steering depower at the same time and didn’t want to risk critical steering components. Measure twice, weld once.
Wayyyy better than what I could do, and maybe over kill but I’d rather not risk it.
Well, you’ve made it this far. Time to finish the project off and get it wired up and working.
ECU mounting- this can vary on setup, but I found it to fit best right underneath the steering column cover. I designed & 3d printed a mount out of PETG to hook onto the little tab on the NA6 column (not found on NA8 for some reason). There wasn’t any other good place to mount it without modifying/extending the ECU harness to the motor, so there it went. Fitment is tight, but it’s been good for me so far. I included photos of measurements in the google drive at the bottom of this writeup with dimensions for the ECU & steering column tab. Here’s the photos of the mount:
Tight, but it makes it.
Now for wiring. I did the 4 signal wires for the angle sensors first outside of the car. I used a 4 pin Deutsch style connector which I had from another project in case I needed to remove the ECU for service later on. Any sort of connector would work, or even a hard wire connection.
Pretty happy with how it turned out. Cable stays out of the way as well.
Connecting power to the ECU from the Miata- the ECU needs only 4 wires- 12v, Ground, and then switched 12v & ground for the black box. The main power and ground need to be a pretty decent gauge -I chose 10 AWG- but feel free to make other decisions based on YOUR judgement. The connector kit in the spreadsheet has the correct sizes for hooking up a 12v feed directly off the alternator post. I ran my 12v from the alt. to the 50a fuse which I mounted near the back of the engine by the catch cans. Two screws did the trick, I reused an empty hole as well so only had to drill once.
The rest of the wire going to the cabin was wrapped as well.
And back under the dash! The 12v feed goes into the connector we ordered. Either look at the instructions included with the black box or look at the old cable to determine which wire goes into the plug. You don’t want to swap those and fry the steering ecu! The 12V ground can be tied into any ground connection you see fit. I chose to mount mine to a post already being used for a relay, along with the ground for the black box.
For the final bit- the black box wiring & adjustment ****! The black box comes with wiring instructions and it’s pretty straightforward. It needs 12v, ground, and potentiometer mounting. The ground goes right behind the relay in the same place as the main power ground, or at a ground of your choosing. I used a add-a-circuit off of the fuse box for switched 12v. You can take your pick, or even pull it off of the radio, but I figured this was the easiest and most reversible option. For the ****, I added a 2 pin connector in-between in it and the connection to the black box. The **** mounts right into a spare spot in the dash using a spare blank. You could mount it any place of your choosing though.
Some sort of black magic in here.
Wish I had taken more photos, but hopefully this is the jist of it.
Congrats! You’ve just installed a EPS system into your miata. If your back isn’t hurting by now you didn’t do it right. Double check all of your work, and make sure you torqued the u joints & steering column (seeing as that would be pretty disastrous if it came undone….) Here’s some final install photos:
Final notes:
The EPS system works great. Comes on with key on, goes off automatically. The assist is variable with the ****, and ranges from completely off to max-over-boosted-this-is-way-too-much, and everywhere in between. The system takes a few seconds to adjust to changes on the ****, as I believe it has some sort of dampening effect on inputs in the OEM install. Steering feel is good, feels like OEM power steering. If dialed to max it does feel a bit over boosted, but that’s the beauty of being able to adjust it. For the PS/AC belt, I just gutted the broken power steering pump and capped it off, so now it’s functioning as an idler/tensioner. A swap to a KIA tensioner or simply spacing out the AC with the stretch belt would yield some extra weight savings. I’m super happy with how it came out, especially given the $$ I spent on it.
These are the only three issues:
-The steering wheel lock does not work. This could be fixed by dremeling out the notch in the GM upper shaft to match the slot of the Miata one, and then I imagine it would work just fine. I didn’t have the foresight to plan for this among other things, but might be a good idea for a fresh install.
-The turn signal autocanceling does not work. This could be fixed by extending the two nubs/plastic ring so it engages with the hub again. I’ve gotten used to this and it really doesn’t bother me at all, but I’d like to fix it down the road. This could also be fixed by NOT using the GM upper shaft, and instead cutting it and welding it to the miata upper shaft. I chose not to do this because of cost & complexity as mentioned before. You really don’t notice the slight spacing outward, especially if you’re not looking for it.
-There is a slight wobble or slop in the steering when quickly turning left to right ~5 degrees. I believe this is due to the inertia of the motor reversing directions and lack of more solid mounting for the motor. Would be good to add a secondary bracket, but kmiata and others haven't seemed to need one so I haven't done one yet. It isn't noticeable when turning normally, both on the street or on track.
I will update the writeup if/when I fix these issues on my install, but for now, just something to be aware of.
Common questions:
Q: Will this fit my Miata?
A: Maybe. I cannot confirm nor guarantee fitment for anything other than my own car, but it should fit NA/NB’s just fine. DO YOUR OWN RESEARCH.
Q: How much will this cost?
A: Depends on what path you take. I ended up only paying $175 for the setup above, not including labor (expect for the welding) or tools I already had.
Q: How long will this take?
A: It took me about a month; however, I was doing a bunch of other maintenance and working on and off. (along with researching the entire design…) If you had all of the tools, parts, everything ready to go, and knew what you were doing, it should be possible in a weekend. Otherwise plan for a week or two.
Q: Can I still run depowered?
A: Yes. Just use the **** to turn the system off and it’s not there at all.
Q: What about self-centering issues??
A: I haven’t noticed any personally. My caster is ~5 degrees and I run 225/45/15s on 15x9 +35 in the front and it comes back to center each time. It doesn’t perfectly return to the DEAD MIDDLE as if held by a spring or anything, but it’ll return as a normal car and doesn’t hang in the last position. The issues online could be related to bad units, Prius units, different suspension setups, lower caster, poor alignments etc. etc. I also could have no idea what I’m doing and it might not be as good as I say it is, take it with a grain of salt.
Q: Power draw???
A: The max draw on the unit is roughly 60a according to GM. Bruno steering recommends a 50a fuse stating that it will be more than plenty, especially in a Miata. I have a stock ’95 alternator with AC, lights, radio, etc. etc. and have not noticed any voltage drop or battery related issues, despite what some claim. An upgrade to a FD alt. may not be a bad idea, depending on your electrical load.
Q: Reliability???
A: I’ve put roughly 3k mi on the car since installing it and haven’t had a single issue. These units are usually overbuilt in OEM form (being a critical component of the car) and will most likely last years. In the event that it fails, another trip to the junkyard, unbolt the lower and upper shaft, unplug the wires, and then pop in a fresh unit.
Future Improvements: -Fixing the steering slop
-Fixing the blinker auto cancel, possibly with 3D printed extensions for the plastic ring so it properly engages the hub.
-Fixing the non-working steering wheel lock.
-Implementing the assist with MS/RPiDash. I plan on piping the VSS signal from Megasquirt into the black box via a digipot and my raspberry pi dash. This would allow for custom assist curves such as normal, sport, etc. It would also allow for automatic adjustment & tapering off the assist at higher speeds, keeping the EPS active only when needed.
-Heatsink on the ECU might be a good idea just out of principle. I haven’t done any testing but might hook up a thermocouple to it in the future to see if it’s really needed.
Other notes:
The spreadsheet, photos, and STL for mount is available under this google drive link or on my website/portfolio, free of charge. The EPS swap is under the EPS Swap tab. Use of the information or parts for commercial products is prohibited and please let me know if you share/post it elsewhere! https://drive.google.com/drive/folde...rO?usp=sharing
Any other input or feedback is more than welcome, and happy to answer any questions!
TLDR- Get EPS from junkyard. Take out Miata steering. Cut Miata steering. Cut EPS. Weld in EPS. Wire in EPS. Put in EPS. Lie in a bed covered in fluffy kittens indefinitely while your back aches from working under the dash.
Worth noting you can request a 10K pot if ordering everything from Epower that gives you a bit more granularity in the assist range. I've been told the "normal" pots hover between "none" and "almost too much" with barely any movement. The 10K pot allows you to get that same range, but in about 200* of movement.
Good idea on moving the ECU- I didn't even think of extending the wiring harness to relocate it. That opens up way more mounting options as cramming the motor and ECU in that space was... challenging.
04-28-2023, 07:36 PM
DIY Electric Power Steering- complete writeup
2
See photos/video below.
