Variable reluctance VSS
 

Hi, All. I just finished building my MPGuino and it came out and works great! I will post some pictures soon. The problem I am having Is that my '94 Ford Ranger has a variable reluctance VSS . This, as near as I can understand, produces an AC sine wave with frequency proportional to speed. The upshot is my 'guino doesn't start to register until I hit about 12MPH and it's a bit dodgy until 20MPH. Above that speed it's rock steady and right on. I gather variable reluctance sensors are not as accurate as hall effect and and other pulse type sensors especially at low speeds. Do you think the low frequency signal could be processed (Rectifier?) to produce a better digital signal? I would prefer not to have to retro another sensor on the drive shaft, but if I must then I will Modd away! I have played with pulses/mile and vss delay to no avail. 16000 pulses/mile seems right on though it's double what the VSS is supposed to put out. Thanks for your ideas!

Look on thirdgen.org for information on the GM DRAC module(all 90-94 GM RWD and 4WD). It converts the AC signal to a standard pulse per mile signal. It should be pretty easy to hook up but I haven't dug into the details of the module much other than to fix mine when it got burned up.

DRAC is a good keyword, here is a thread with pics and pinouts:
Adding a VSS to a Jerico Trans - Third Generation F-Body Message Boards

Thanks for your help guys! I'll head over to the junk yard tomorrow and see what I can find. It looks like many GM vehicles have those modules, so it shouldn't be to hard. In the mean time I'm gonna get the injector end dialed in. I'll fill up the tank at my local station tomorrow and see how close my initial settings are. Thanks for the great project and support!

Another solution
 

I went to visit my friend who has an electronics based business and he helped me design and construct an on-board signal processor for the MPGuino. We used one side of a dual Opamp IC to create a comparator for the VSS input. We first took the O'scope out for a drive and saw that at low speeds the signal from the VSS was as little as 250mv. We then designed a simple comparator with a hysteresis loop for clean transitions state to state. Unfortunately the only op amps my friend had were surface mount so I got to do some soldering under the microscope (usually reading glasses are enough :D). Initial in vehicle testing showed the need for a zener voltage clamp on the input. that plus some fine tuning of the hysteresis resistor value resulted in perfect VSS functionality! Speed is very smooth all the way down to .5 mph. Instant MPG is steadier now too. I have no doubt that the DRAC module could be made to work, but this little board glued right on to my Arduino and works great! I will publish the schematic as soon as I get some freeware for drawing them. Thanks again for your help.

Sweet :) I'd love to see the pictures.

Pictures and schematic
 

All right! Here are the long awaited and much anticipated photos of Mossman's MPGuino! Here she is with the mark I yellow plexi faceplate. The great location in the dash. The new board mounted on the back. And a new better looking faceplate. Finally the schematic of the signal processing circuit. Hope this helps others out there! And I hope that my injector calibration is right 'cause I like the numbers I'm seeing! I guess I'll know in a week or so.

The processor board
 

Oops I didn't upload this one. If you make one of these it would be much easier if you did not use a surface mount IC. Good luck!

How is it working out? I just built my meter and am planning on installing it this weekend into my '98 ranger... the more info, the better!

It works great!
 

I never even turn the radio on anymore, driving is more like a video game than ever and I'm always trying to beat the high score! My first calibration tank suffered from a power inturruption to the 'guino. I am now 1/4 into the next tank and looking good. I have adjusted the VSS pulses per mile to 17200. that seems really close to the odo now. I should check that on a measured road some day... anyway I also adjusted the resistance in the hysteresis loop to 500k. After I finally installed the wire splices (replacing the alligator clips) on the PCM the 'guino started showing what apeared to be injector signal leaking over onto the VSS circut. decreasing the value of that resistor (R2 above) solved the problem. Of course too low a resistor and you cut off your low speeds (coasting up to a light). I love my MPGuino it's been a great project and I hope to customize it more in the future. And I want to build one for my fiance (maybe the kit this time?) but she wants outside temp on it. Looks to me like that should be doable and entertaining. I wonder if can figure out the code end...