Build Your Own Paddle Shift Kit

AbstractPlease remember this page is informational only. It is stated on the main page. BMW was one of the first car makers to introduce Steptronic like transmissions into their cars. However, they are one of the few that do not have steering wheel controls for changing gears.

For Example, Lexus and Audi both have buttons on their steering wheels to shift gears. However, reviews have been pretty poor regarding their button design. When the E46 M3 SMGII came out, everyone was surprised on how well paddle shifting worked. Since the paddles are much larger than the buttons, they can be used more effectively, especially down shifting while turning the wheel at the same time.

Previous Work

So naturally, fanatics have been wondering if the SMGII wheel could be retrofitted on to a Steptronic transmission car. Rumor had it that someone in Japan was able to complete the retrofit with a custom circuit but had flaws in the design. To my knowledge, he was the first one to come up with the retrofit idea.

Then a guy named Vincent Seah developed a kit called the VS Paddle-Shift Retrofit. I contacted Vincent about the specifics of his kit and how he managed to complete the project. He responded that he would not give out any information or hints and that the only way to get any information would be to buy his kit. His DIY kit was $385, which I felt was a bit too expensive, so I decided to give the retrofit project a shot myself.

My work

I proceeded with many questions about paddle functionality on SMGII M3 steering wheel in the online forums, but never received a response due to protection of Vincent’s kit. So I started from scratch by looking through my ETK CD. I found the part number for the SMGII M3 steering wheel and noticed a local fanatic was selling the exact part in the forums. I drove to his house and paid $340 cash for the new steering wheel. I inspected the wheel and all the cabling in the unit. Using a voltmeter with a continuity tester, I concluded that two separate wires were allocated for the paddles only. If you press either paddle it will ground the respective wire. If you press both paddles, both wires will be grounded. The paddles operated independently of each other.

Now that I knew how the SMGII steering wheel worked, it was time to take apart the shifter assembly.

Again, I brought questions on the shifter to the forums and similarly, no responses I remove the trim along the shifter and disconnected a bunch of cables. I removed the LED module from the trim and heavily inspected it.

Here is a close up of the LED display.

Notice how there are two rows of LEDs. The right row of LEDs are for lighting up the translucent icons on the shifter.

At first I thought the shifting was all handled by the circuitry on the reverse side of this module. But then I realized that  it was just a bunch of capacitors and resistors regulating the voltage to the LEDs on the front. This module was simply an display module and does not alter the input. I forget the specifics, but I recall that one of the red or white connectors is the power supply for the LEDs and the other being a continuous connection from the ribbon cable.

So I looked else where for the Steptronic signals. I figured out that the P, R, N, and D modes were controlled by a manual line.

With all the cables disconnected in the shifter area, the car would still run under P, R, N, and D modes. This suggests that these modes are only electronically related when reported to the LED module.

Notice how once the shifter is engaged into manual mode it disconnects itself from the gold color ring. This way upshifts and downshifts do move the manual cable.

Next, I figured out that the M1, M2, M3, M4, and M5 modes are all controlled by electronics. Without out the four cables joined together by the red and black clip, the transmission will never electronically go into manual mode.

The next step is to find out what happens when you go from D => S D mode. My understanding is when a state is selected, the shifter grounds that particular line. Since P, R, N, D are controlled manually, all these cables have to do is output the mode to the LED display; therefore, it goes directly to the LED display. The Steptronic cables differ since two events are necessary: telling the car to shift gears and displaying what gear is currently active. Therefore, the Steptronic cables need to be routed to both the car and the LED display. By process of elimination and testing, the gray, purple, blue, and green wires of the Steptronic cable are Steptronic On, upshift, downshift, and ground. The colors of the cable may vary from car to car. I suggest you probe them to be 100% sure.

If you move the shifter to the left (D => S D) Steptronic On becomes grounded. From there if you upshift, the upshift lead is grounded and vice versa for the downshift. At this point in time, I figured that you can connect the leads directly to the SMGII wheel since it also grounds on contact. However, you run the risk of upshifting and downshifting when not in manual mode. You can also press both upshift and downshift at the same time. I am not sure what happens in these two cases.

Using this newly gather information, I concluded that there are four states for the paddles: upshift, downshift, both pressed, and none pressed. The solution to this problem is designing some sort of logic gate array so that you can only press one button at a time and only when Steptronic On is engaged. I also noted that the connector on the shifter isn’t standard but seems like a regular 4 pin audio cable from a CDROM drive would work perfectly. I also asked the forums if other people with Steptronic transmission could test if their cars operated the same way mine did, no response as usual. I asked this because I know that BMW had switched around their upshift and downshift design. My 2000 323i has upshift away from the driver and downshift towards the driver.

Eagerly wanting to test out my wire theory, I grabbed some spare cables and put the shifter in D mode. I grounded the Steptronic on line with a bare cable to the frame of the car. Sure enough it moved into the S D mode. Then I tried grounding the upshift and downshift leads. It worked out perfectly. I also noticed that the electronic system can actually upshift and downshift pretty quickly. Moving up and down M1-M5 was a breeze. The actual transmission making the gear change is slower though. At that time I did not try something that might have broken the system, like upshifting in neutral or pressing both buttons at the same time. I was afraid to damage something. I did test legal state transitions such as M3 => D => S D => M2.

The next step was to create a logic table and diagram for this specific application. A colleague named Chris Cheng helped me tremendously since I forgot how to do gates.


0 = low or GND
1 = high or off

S + – | upshift downshift
0 0 0 | 1 1
0 0 1 | 0 1
0 1 0 | 1 0
0 1 1 | 1 1
1 0 0 | 1 1
1 0 1 | 1 1
1 1 0 | 1 1
1 1 1 | 1 1

S = 0 means shifter is to the M position, Steptronic is on.
S = 1 means shifter is in the D position.

So for the bottom 4 states when the shifter is in D mode, the circuit should not do anything, therefore upshift and downshift are 1 (high or off).

For the first case when the shifter is in M mode, and no buttons are pressed, again nothing happens thus 1 1.

For the the 4th case when the shifter is in M mode and both buttons are pressed, it should do nothing hence 1 1.

Finally, when the shifter is in M mode and + is pressed and – is not, upshift should be 0 and downshift should be 1. Vice versa for the downshift.


upshift = + or not(-) or s
downshift = not(+) or – or s

You can pick and choose your preferred gates you want to use. Just make sure the latencies do not ever allow for GND when s = 1. Basically, no shifting when not in M mode.

Another alternative is to build a custom PIX chip to do the logic. Since it will be a one chip solution, there will be less wiring and neater.

I ended up building my circuits completely out of three input NOR gates since that was all I had lying around. The 2 pin connector on the left goes to the steering wheel. The 4 pin leads on the top right go to power and the shifter.

Here is the back of the circuit. Since I only used 3 input NOR gates, there was excessive wiring.


I attached my newly built circuit to the shifter, wheel, and power. I started up the car and fiddled with the paddles. Nothing happened, which is correct because it is in P mode. I moved the shifter to S D mode and fiddled again. Sure enough, it upshifts and downshifts perfectly. So I tried to break it, I pressed both paddles at the same time, then + really fast then – really fast and it was just fine. Then I realized I never tested out the paddles without the circuit, I’ve only heard rumors from rumors that pressing both paddles screws things.

I removed the complete circuit from my car and just tapped the middle two wires directly into the steering wheel. To my surprise it worked. I realized that I wasted a bunch of time building a circuit for no reason.

I thought about the project some more and it makes sense. BMW doesn’t make the transmission (I think GM does), but does make the electronics controlling the engine. Anyway the point is the transmission and the electronics are separate, so these two systems need to have input protection. It would suck if some how you fed the transmission + and – at the same time that it would stop working. So therefore my theory is that BMW must has already provided input protection from the shifter. Since the SMGII wheel is retrofitted to the shifter, it is also protected.

Purchase – Support Steering Column Switch P/N: 61 31 8 379 091

Parts – BMW SMGII M3 Steering Wheel, Copper wire, electrical materials.

Installation – You can build a custom circuit like what I have done above for no reason, or you can simply make your own custom wire that bridges the two leads from the steering wheel to two middle pins in the shifter. The steering column switch just allows the two pins from passing through the steering joint. For the steering wheel change, I followed the DIY instructions on E46Fanatics. Basically, once you have the SMGII wheel and the column switch you only need about $2 in electrical materials.

Conclusions – It works great, I love the thickness of the wheel and the added benefits of paddle shifting. Below are some extra pictures I took throughout the process.

Both comments and pings are currently closed.

Comments are closed.