e30 m3 tach converter

[Rev Engineer]

I've finished development of my e30 m3 tach converter. It is a small microprocessor based board that takes in the 6-cyl tach signal and down converts it to a 4-cyl signal. I've been using it for a couple weeks now and it has been working great. I'm willing to provide details related to the schematic and software to anyone who's interested or has enough ambition to make one themselves - it's really not very complicated. To anyone else, I'm offering to sell these for $40 shipped. As an added "feature" I have the tach sweep from 0 to redline and back to 0 with the car is turned on. A downside to this is that the SI lights go out quickly when the car is turned on. I can modify the software to skip this or I might be able to have a switch to enable or disable this feature.

My next idea is to adopt this board to a speed signal converter which could allow us to bring the speedo signal back in-line after changing tire diameter. Any interest in a device like this?

[NickP]

Any possibility in applying something similar to adjust the fuel consumption / rate.

[LINUS]

So since I'm doing the same thing you have done (6 cyl. to an e30 M3) - I know just swapping coding plugs does 80% of the battle, but the tach is still ~12% off (reads low) - does your module take care of this?

If so, I'm in.

What's the reason for a tach sweep? If there's a good reason, I'm fine with it too. How long do the SI led's stay on now vs. before?

--Ben

[Rev Engineer]

It should be possible to adjust the mpg rate. The best approach might be to record actual mileage and then compare that to the obc. There should be some percentage error which I could dial into the software to shorten or lengthen the fuel rate signal. I could also add a potentiometer to the board which could allow the user to dial in a percentage. I haven’t looked at the fuel rate signal so I can’t be 100% sure this approach would work. This idea is on the back burner as I’m more interested in correcting the speedo right now. After I accomplish that I might take a look at the fuel signal.

This converter takes care of the 325i coding problem and you would need to go back to the e30 m3 coding plug. The rpm signal is accurate, but I’ve found that there is some small error in the tach itself which I can’t easily adjust out. My tach happens to be off a few hundred rpm’s between ~2500 and ~4500. Above 5k it’s really accurate. I think all e30 tachs may be slightly different.

As for the tach sweep, I just did it for fun. The SI lights only show for a few seconds because my tach sweep tricks the SI board into thinking the car just started and hence it turns them off. It’s a gimmick and I can easily disable it. I had another idea to let the tach hit each 500rpm increment so people could find out how accurate their tachs are. If there is interest in this I might spend some time this weekend to see if I can add this as a “test” mode.

I’ll post some pictures tonight or later this weekend.

[Jason89i]

It should be possible to adjust the mpg rate. The best approach might be to record actual mileage and then compare that to the obc. There should be some percentage error which I could dial into the software to shorten or lengthen the fuel rate signal. I could also add a potentiometer to the board which could allow the user to dial in a percentage. I haven’t looked at the fuel rate signal so I can’t be 100% sure this approach would work. This idea is on the back burner as I’m more interested in correcting the speedo right now. After I accomplish that I might take a look at the fuel signal.

great work. like to see some techno savvy individuals out here.

mpg rate gets a pulsed / length signal from the injectors. with the 3.2 and 24# injectors.....obc says 84mpg = 24.5 mpg actual.

to calculate actual speed vs. speedo readout is quite simple. set cruse on the intersate (lets say at 70) then use your obc timer to find exactly how long (in seconds) it takes to go 1 mile (use the milemarkers for exact reference.) cliffnotes for the math challenged.......take 3600 divided by the seconds (lets say 54.9s) and viola.....your going exactly 65.93mph. to get a more accurate reading take 7200 divided by the time (109.8s) it takes you to go exactly 2 miles.

[nmlss2006]

I'd be in for one of these, if it were available.

[Van Westervelt]

That is awesome. Quick question. On the tach, you changed the signal to go from a 6 cyl m20 to a 4 cyl m3 plug to the m3 tach. How would these work on m50/s50 swapped cars? There is a big issue there and if this solves that problem, then where do I pay!?

Pics or a video would be awesome too!:up: Nice work my friend.

[Rev Engineer]

Thanks for all of the interest guys! I got a handful of them built this weekend and I'm pretty close to being able to ship some. I made some software and hardware modifications and want to test it out for a few days in my own car before I ship anything.

I've added a pair of dip switches so there are now 3 modes. One is normal with tach sweep upon turning the key to ignition on. Another is normal operation with no tach sweep (allows SI lights to remain on until the car is started). And a third mode is a test mode which hits every 500 rpm increment on the tach. This will help people figure out how accurate their tachs are.

I've posted some pictures of the board and video of the tach sweep and test mode here:

http://www.cityofdust.com/RPMConverter/

A few more notes: the pcb board was made for an entirely different device. I've cut up some traces and modified them for our application. I do not have any housing or enclosure for the board. I suggest just wrapping it up in electrical tape and tucking in behind the dash or just let it sit in the glove box. There are four wires that come off of it that you'll need to connect to. I've described these on the little webpage I just put up.

Just to be clear, this board converts the s50/m50/s52 6-cyl tach signal into a 4-cyl tach signal. This board mostly benefits us e30 m3 people who want an accurate tach, but could also help the 318i folks who can't find a 6cyl coding plug. You keep your original coding plug that is intended for a 4-cyl tach.

[LINUS]

Just in time - my motor should get in my car quite soon, and this will save me a lot of hassle. Sign me up!
PM me your Paypal addy, and thanks in advance!

---Ben

[equate975]

Good work, im definatly intersted. Tell me when you start shipping them out.

What color is your e30 m3? Some buddys of mine told me they saw "a white car just like yours " (yours being me :) ) but I dont believe them

[Rev Engineer]

I've got a white e30 m3. If he was up around the west metro, it was probably me. I've been racking up the miles over the past two weeks. The car is just so much fun! and......quick!

I think I've got enough units built for everyone who's been interested. If you want one and haven’t seen a PM from me, send me a PM and I'll give you my paypal address. I'm bumping the price to $40 shipped usps standard mail and $45 for usps priority as I've put more effort into these than I anticipated. But, I hope people find the extra features worth it. You can now disable the startup tach sweep and also test the accuracy of your tach at every 500rpm increment. I’ve also wrapped the boards up in electrical tape to cover the electronics and make it a little more robust. You’ll just need to splice it into the wiring and go.

[equate975]

Would you want to give out the source code for these? I would have more fun with it then :)

Also, they said they saw your car in rochester, not up in the cities :/

[325Projectz]

Would you want to give out the source code for these? I would have more fun with it then :)

I would like that aswell...

[Rev Engineer]

I might consider releasing the source at a later date. But, it won't do you any good unless you have an avr compiler. I'd be happy to explain how it works, tho... Other than the test modes and startup code, it's only about 15 lines of interrupt code - no main loop. A 16 bit timer controls everything. Input capture is used to get the pulse-width and calculate the adjusted pulse period. Two output capture interrupts divide out the calculated pulse and toggle i/o to get high and low pulses. It could actually be simplified to use a single hardware based output capture as I've learned that the instrument cluster doesn't care about the pulse duty-cycle. As it works now, the code replicates the pulse duty cycle comming in - which is why I used two output compare events.

[nmlss2006]

Count me in for one of those. Actually, for several, I spoke to a few people and we're all interested. Now, if you could also get the correct MPG in the OBC, we'd be even happier :). Please keep me posted.