DIY Electric Power Steering for Miata NA

The Idea

Back in 2007 when I was looking for a donor car to start this project, I was specifically trying to find a Miata with manual steering.  I wanted to avoid dealing with the pump, fluid and plumbing associated with power steering.  Plus, the pump would steal power from my motor which I would rather use to power the car. For this reason the car has manual steering.  

Manual steering wasn't bad initually with the Lead Acid AGM batteries, or the 100ah Lithium cells, but with the recent battery upgrade to 340ah, the car's weight has increased making low speed steering (parallel parking especially) more of a nuisance. And I'm older now making a lot of things more of a nuisance...

I ran across an article about a kit that used a modern Electric Power Assist Steering (EPAS) unit adapted to a Miata and thought this might be my solution. 

As expected, there are a few things I didn't like about it -

1. Cost -  $1,080 is a bit much

2. It's a kit (as a DIYer that's a no go)

3. It uses a knob on the dashboard to adjust the amount of assistance. This seemed kludgy to me.

When researching power steering units I found the Prius unit has a failsafe mode that set the assist level to what's used at 43 mph.  That sounded good since I wanted moderate assistance because I assumed high assistance would pull a lot of power from the 12v system (kits include a 60a fuse!). Plus there was an add-on that used a GPS receiver to auto adjust the assistance to your calculated speed.  In my mind, this was the proper way to do variable assist - if I needed it.

Using the kit as my guide, I went about buying parts. It wasn't hard finding a Prius column to use and disassembly was pretty easy. A friend recommended I get a second Miata Column since this DIY project may take a while and I didn't want the car to be undrivable. Plus, I could sell the original column when done.   Good idea.

The Plan

My plan was to put the Prius EPAS as close to the mounting bracket as possible since this thing is heavy.  I'll cut off the lower part of the Miata column and connect it to the EPAS unit using an adapter. I'll do the same at the top end and, stick it under the dash and off we go.

Reality 

The Prius unit uses an 11/16" 36 spline shaft and the Miata's shaft is 3/4"  I bought an adapter for that before I started disassembly of the Miata's column.  Comes out the Miata uses what's called a "Double D" shaft (one that's flat on either side and round on top and bottom).  It is 3/4" from top to bottom, but I would have been better off buying an 11/16" 36 spline to double D adapter.  Next time.

I measured out the length of the column with the Prius EPAS roughly in place and cut the lower part of the shaft.  I had trouble welding it to the adapter squarely, but after a few tries I realized I needed to add some welds to the flat sides so it would fit into the round adapter solidly.  That got it. I'm sure it's not prefect, but it's not like the shaft is spinning at 10,000 rpm.  Turning it by hand it looks perfect and that's good enough. 

Since this side of the EPAS is the high torque side (where the assistance comes in), I wanted the connection to be extra solid. After welding it up and getting it straight, I drilled a hole through it and drove in a 3/16 expansion pin.  That should cover it in case my ugly weld decides to break u der load.

The other side of the EPAS was a bit more challenging.  The Prius shaft had a splined slip joint and the Miata shaft was close to 1" outside diameter and hollow. My plan was to cut the Miata shaft to length and weld it to a 2" section of the Prius splined shaft that mated to the EPAS shaft.  That way, no adapter was needed and the new shaft's length would be adjustable.  

It just didn't end up that way...

I cut the Miata shaft to length and ground down the cut end so that it was very square. I cut the 2" section off the Prius shaft and put the uncut end (which was square and flat - unlike the end I cut) onto the Miata shaft and got ready to weld.  Now the Miata shaft is wider by almost 1/4" so placement was tricky.  Once I got this right, I used a clamp to hold it together and welded it up.  And it was crooked. It was centered okay, but crooked.  I cut the weld on the short side and adjust it (with a hammer) until it was straight.  rewelded it and it stayed straight.  How 'bout that...

Unfortunately, the rewelding penetrated into the shaft so my slip joint idea was now gone.  I now had to shorten the splined shaft on the Prius EPAS. I should mention this shaft had a plastic coating of some sort - maybe Delrin? Just cutting the shaft with my angle grinder set off my garage smoke detector.  Probably some nasty fumes were released. Good idea to do this outside. 

Now, to review -  I had a cut end from the Prius column welded to the Miata column and a cut end of the spline shaft on the Prius EPAS. The two cut ends did not want to mate.   After some grinding filing, cussing, hammering, greasing and more cussing, they finally did. 

Now we have a complete Miata column with a Prius EPAS unit in the middle.  Time to put the outer housing over the shaft.  I tried to remove the Prius' housing from the EPAS, but the best I could do was cut it off right where it joined the EPAS.  I cut the Miata housing to length allowing room for an over lap with the remaining Prius housing.  The Miata housing is a little bigger than the Prius, but not "slips right over" bigger. I cut 4 slots in the Miata housing so I could expand it enough to get it over the Prius. My plan was to weld these two together, but since the EPAS motor needed to be at an angle to fit under the Miata's dash, I decided a temporary way of attaching these was better.  It was around 1.5" in diameter, so a muffler clamp would do the trick.  I bought one that was 1 5/8" just to be sure. 

I eyeballed the kits to check for how angled the EPAS motor was and made mine the same(ish). I figured if it was wrong, I could lossen the muffler clamp and adjust...

Installation

I removed the steering shaft from the car.  I jacked both ends of the car up to make this easier.  Then I put the cut off forward bracket over new shaft and slid it under the dash.  The whole unit is heavy so I used the stock Miata jack to lift it up into place.  It was a bit tricky to get the universal joint from the steering middle shaft onto the new unit, but after some fighting, it slipped on.  I then raised the unit up a bit and it got stuck. There is a relay held in place by a bracket and a single 10mm bolt. I removed it and pushed it out of the way. Now i had room to jack the unit into position and align the main bracket into place.  Before tightening the bracket, i needed to get the forward bracket bolted down. With the jack holding the unit up, there was no weight on the front bracket, so it was fairly easy to get it over the studs and bolt it down.  

There isn't a lot of room for the EPAS motor but with some twisting, angling and cussing (of course), it fit into place without having to adjust my muffler clamp.  I got the column bracket aligned, jacked into place and torqued down the two bolts holding it in.  With that done, I could pull out the jack and start on the wiring.

Wiring

The EPAS uses a little computer (ECU) to calculate the level of assistance needed based on a torque sensor on the input shaft.  The Prius power wiring from the motor to the ECU is meaty 8 gauge.  There is a connector for the torque sensor and another connector for the speed adjuster, CANBUS or GPS add-on. Since my plan was not to use any of these, I only needed the key switch connection to power on the unit.  

I couldn't find a connector for this but did find the pinout. I opened the unit and soldered in a wire with a connector to hook this up under the dash.  I did find the connector for the power into the unit, so ordered it. 

I used 8 ga in my wiring but I do think  it's overkill, especially with my moderate assist plan, but it wasn't a lot more expensive than 10 ga. 

I ran the 8 ga wire through the firewall and connected it to the fuse wire.  I crimped a ring connector to the other end of the wire and connected it to the stock Miata wire from the battery (what was connected to the starter). I connected it to the purchased connector and added a short negative lead with ring terminal to connect it to a bolt under the dash for a ground. 

I used a tap to connect a wire from the key switch to the power into the ECU.  I tie wrapped the ECU under the dash, tidied up all the loose wiring and reconnected the battery.  

It works!

With the key off it's a struggle to turn the wheels - maybe a bit more than before since I have the EPAS to turn too.

But with the key on, there is a slight click of a relay and steering is light and easy. 

A child could do it.  

What a change!

Parts List

Here is a list of the parts I used 

Prius Column from eBay $119.43

Miata Column from eBay. $52.59 

3' 8 gauge wire from Amazon $11.98

8 ga 60amp fuse from Amazon $6.79

Plug for ECU from ePowersteering $23.05

8 ga connectors from Autozone. $6.51

Steering Coupler from Kartek $29.75

- recommended double D model

Muffler clamp from O'Reilly $4.66

3/16" Expansion pin Lowes  $1.48

Total was $256.24 and I just listed the Miata column on ebay, which should bring the total closer to $210ish - which is less than a quarter of the kit's price.  

I don't know about you, but I would rather do it myself - I always learn something new, enjoy the challenge, save a little money and know how to fix it if it breaks - all good.

Mark 3

Lately I havent been driving the car much and I realize it's for a few reasons - 

1. I have a bad cell (#23) that limits the range. Everything will seem fine and then this one cell will drop to below 3 volts, shutting the car down. It's back to 20 mile range, which isn't horrible, but it's a nuisance. 

2. Sometime after the last clutch replacement, the throwout bearing failed. It makes a horrible noise when engaged and I'm thinking it could catastrophically fail, which would be a problem... 

3. The controller is running too hot. My passive cooling systemm using a transmission cooler without fans isn't cutting it. 

4. Public charging takes way too long. At home I can charge it overnight and all is well, but if I need to charge it out in the wild, the charger just can't produce enough current to charge the pack in an adequate time. 

5. Manual steering is getting to be a real hassle. It was workable when it was a commuter car - I parked in a parking lot, but now as a city car I have to deal with parallel parking - sometimes backwards uphill. Power steering would make the car much more useful. 

So while searching for replacement CALB cells I noticed the price of new cells has dropped significantly. I had thought about using a Tesla or other wrecked EV pack, but I was worried about doing something stupid with an unforgiving battery chemistry. I wanted to stick with LFP (or LiFePO) cells, since they seem more resilient to user error. Furthermore, I didn't want to rebuild the battery racks if I could help it and I am limited to 156v Nominal by the Zilla 1k I'm using. If I go above that I would need to change controller and likely the motor. 

And so...

Pulling the motor exposed what was left of the throwout bearing. It was a twisted mess and surprisingly worked at all. Didn't get a picture though...

I found I had room to fit EVE 280ah cells, but only 46. I had to modify the nose rack to accommodate 14 of these since they are quite a bit bigger. I had a little extra space in the racks with the CALB cells, that's all gone now.

 

With the added amperage of the new pack, the charging problem would be much worse. I rewired the J172 to use 10 gauge wiring and was able to get a new air cooled 6.6kw charger just before the new Chinese tariffs kicked in.

It was a tight fit but all good.
 

I found a PC cooling kit that would fit my front grill area nicely. It came with 3 fans with LEDs that glowed different colors. Not really my style, so they got painted black. 

Another thing that's been bugging me is the temperature gauge on the dash. It's way out of scale for the temperature of the controller. You can't tell if something is wrong when the needle barely moves. I figured i better do it now or it would never get done. I was able to get it re- calibrated by the manufacturer. Now it's actually useful. Glad I did it.

What's left to do?

Power Steering.  I bought an electric power steering unit and tried to fit it in under the hood below the brake booster, but there just wasn't enough room.  And fiddling with it was holding up the rest of the project. There is an under dash kit for the Miata that uses a Prius p/s unit.  It's around $1000, so I will probably make my own. We'll see how that goes.  

Late Updates

Mark 3 

Been awhile since I've posted so wanted to give an update on recent changes. There have been lots of changes since this project started, some minor and some major. I think it would help to lay them out like this: 

Mark 1 - Lead Acid (AGM) battery pack, Zilla 1K controller, Warp 9 motor, Zivan charger, 20 mile range

Mark 1.1 - revised motor mounts 

Mark 1.2 - electric heater added 

Mark 1.3 - changed to J1772 charging port 

Mark 1.4 - added LED Daytime Running Lights (DRL) 

Mark 2 - upgrade to Lithium battery pack (48 100ah CALB cells). Mini-BMS, 40-50 mile range. 

Mark 2.1 - replace Zivan with Thunderstruck 2500 charger with EVCC Charge Controller 

Mark 2.2 - replace Mini-BMS with Orion and remove EVCC 

Mark 2.3 - add OBD2 port to Orion's CAN with Bluetooth and Android tablet for display 

 

Mark 3 - update battery pack to 46 EVE 280ah cells, upgrade charger to 6.6kw, 100-120 mile range.

Refuel 2017 at Laguna Seca (Weather Tech) raceway.

 Since Monterey is a couple hours from where I live, I needed to tow EV Miata to the track. Dropped it off on Saturday for the event on Sunday. I saw they had installed a few 220 outlets around and thought I might need a converter cable. Made a trip to the local home Depot and wired up a converter in the hotel room. 

Race day came and I was surprised to see the track parking lot full of Teslas. Mostly model Ses but a few roadsters as well. Turns out a few were racing and the rest was just there for a meet up and parade lap.

The racers meeting was basic but they did point out where the power outlets were laid out and explained that if we were racing you have priority over any non-racer charging. 

We got 2 races in, but the rules were to not pass anyone unless they signaled you, so not much of a competition but still fun. It was great to drive the car flat out and it is a great course. There were some prototypes and a couple of conversions so I was able to pass a couple cars, but those Teslas were just too damn fast. 

After the first race I was below 40% SOC and the only free outlet wasn't working. I thought it was my converter so opened it up and rewired it, but no charge was happening. There were a group (maybe 10) Teslas charging and I wedged my car next to one of 'em and took his power. He was livid. I explained to him that I was racing but since they hadn't told the Tesla folks the same message. 

2nd race comes and I've only got about 50% charge.  Now I couldn't pass anyone. Oh well, still fun. 

Otmar at the Maker Faire

The Maker Faire is always a good time but while checking in I spotted a double VW Westfalia - two campers welded together to make one long bus.

I only heard of one of these creations and it belonged to Otmar - creator of the Zilla controller. Finding it had Oregon plates and a Biodiesel sticker confirmed my suspicion. 

No sign of him on Saturday, but on Sunday, my dad flagged me down to talk with a guy wearing a goofy purple hat - it was Otmar. He asked how I liked my Zilla (which I love) and we talked for 15 or 20 minutes about the state of the conversion market and the different projects he's working on. I asked him to sign my Zilla 

And I got a picture with him. It was an ethereal experience, which my smudged camera lens can attest to.

 

New Contactor

So the car has been in the road for 4+ years and running relatively trouble free. In late November, though, it suddenly shut down on my way to work. I was able to pull over and restart it, but it was troubling. The Zilla error codes pointed to motor over speed, contactor stuck and low 12v supply - the usual hodgepodge of errors I see when the SLI battery is low.

I recharged the SLI battery over night but on the way to work the motor shut down again. It did restart and once I was home again I checked my wiring to the controller, thinking a loose 12v, ground or tach lead could be causing the problem. Everything looked Kosher. 

The next day on the way to work it shut down. I coasted off the freeway and parked. I tried restarting it, but this time instead of the familiar contactor "thump" I heard a sizzling sound. I pulled the pack emergency disconnect, pulled my meter from the glove box and made some quick tests. I quickly found that the contactor points had welded closed - which was probably the sizzling sound I heard.

I was using a Kilovac  EV200 contactor and though rated for up to 2,000 amps and can handle 500 amps at 320volts continuous. EV Source points out that these ratings are based on a very large cable size -300 MCM, which translates to about 4.5/0 where I am using 2/0. I was told the connector bolts would get warm from the resistance and ultimately weld shut (which they apparently did), but they never got warm to the touch in my typical drive ad never discolored to indicate overheating. Evidently the bolt temperature doesn't relate well to the contact temperature inside...

This is how the original EV200 contactor looked in the High Voltage Enclosure:

So naturally I wanted to upgrade to a contactor that wouldn't die on me again.  I looked at Nanfeng and Albright but ultimately decided on a Gigavac. The Tech Support at Gigavac said my 350 amp continuous at 170 volts was within spec for the GX14 as long as I used copper buss bars to dissipate any heat.  

The GX14 is larger than the EV200 has a lower cable connection point than the EV200. After going through numerous mounting plans, I came up with one that  used buss bars to connect the cable from above to the input bolt below and to the Ferraz/Shawmut fuse. 

This is how the Gigavac GX14 looks in the redesigned high Voltage Enclosure:

You can see the copper bus bar on the right middle under the 2/0 cable. The contactor is the black circular unit in the middle. The output of the contactor goes to a short bus bar into the Ferraz/Shawmut fuse positioned vertically. It's bolted to the output cable to the Controller.

How to Retrofit an EV conversion to J1772 in 7 easy steps

Over the last 6 months or so, more and more  public charging stations have been installed around town so I finally decided it was time to join the J1772 club. For those of you not familiar with the term, J1772 is the designation for the plugs and inlets that are the official standard for EV charging in the US. With the standard connection on my car, I could take advantage of these charging spots - and many of them are free. 


My old charger port was a L6-20 which was cheap and fine for charging at home, but few public places had this style connector waiting for me... It had served me well over the last 4 years, but change is good.


Most of the local stations I ran across were from Coulumb, so I set up an account at ChargePoint and got a $5 card for using their chargers. How's that for thinking ahead?

Original L6-20 charging port

The whole process of installing the J1772 was easier than you'd think. Here are the steps I took:

Step 1 - buy a J1772 Inlet. The Inlet is the part that goes on the vehicle. I also needed to buy the plug end so that I could convert my home charging gear (a.k.a EVSE or Electric Vehicle Charging Equipment)  to J1772 also. There are a few places that sell these parts - and they aren't cheap, btw. I bought mine from TucsonEV . I had a few questions which they promptly answered and they were quick to replace a pin which wasn't correct from the factory - no complaints.


Step 2 buy resistor(s) and diode. The J1772 standard expects to see signalling between the charger and the outlet in order to power up the connection. Here's a link to a description of the electronics for the signalling involved.  Your charger is probably like mine in that it doesn't have a way to provide these signals. One could wire up a switch to manually provide the signalling, or buy a circuit board to to fake it out, or you could do like me and hard wire it so the EVSE thinks the charger is always ready and turns on the juice. This is a low voltage, low current application, so the parts are cheap and should be readily available. You need a diode that will handle 20ma and 856 to 908 ohms worth of resistors. I couldn't find a 900 ohm resistor so I used a 560 and 330 in series. These parts came to about $6.00 at 'the shack."
  

Parts needed -J1772  Inlet, diode and resistor(s)

Step3 remove old inlet and test fit the new I pulled out the old inlet and disconnected the wiring from it. Then I dropped in the new J1772 unit in it's place. The screw holes for the old inlet didn't match the J1772, but to my surprise, the holes for the original Miata gas filler neck did! This made the swap real easy. If you aren't this lucky, you'll need to drill the mounting holes for your new inlet.


Step 4 Soldering the power pins Disassemble your inlet and figure out where the wiring needs to be placed for proper assembly. Mine had a pin retainer that needed to be over the wires before the pins were soldered on. I added a short wire to the ground so that I could easily connect the resistors and diode later. The power pins took a lot of heat to get up to temperature - more than my soldering gun would put out. I ended up using a butane plumbers torch at a low setting to warm up the pins. I tinned both the wire and the pin and then mated them together one at a time. 



Step 5 solder the pilot pin connection The pilot pin is the small one on the lower left. The one on the right is the proximity pin, which I understand isn't used. Connect your resistor(s) in series (end to end) with the diode. The stripe on the diode should go toward the ground wire. Solder one end of the component is soldered to the pin and the other end to the ground wire we put in earlier. It doesn't matter which order the parts go in as long as the diode is properly oriented.  Once you solder it, shrink tube it or tape it up and insert the pin in place. 


Step 6 Install the inlet assembly  Time to put the whole thing back together. Make sure the wires don't bind and a properly held in place by strain relief nut on the back.

Inlet installed - I later rotated it 90 degrees to allow the cord in my garage to connect easier

Inlet with cover and strain relief installed from inside the turnk

Step 7 test it out Drive out to your local charging station and plug in. Mine was at a Walgreens Drug store. Though the lot was full, the EV spot was empty, so I pulled in, plugged in and waved my little ChargePoint card in front of the charger. I heard a click and then the sound of the  charger in my car starting up - sweet.

Free parking and a free charge - Life Is Good!