The Idea
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
Steering Coupler from Kartek $29.75
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.
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