Page 1 of 2 12 LastLast
Results 1 to 10 of 18

Thread: measure speed without OBD/CAN

  1. #1
    Newbie
    Join Date
    Aug 2008
    Posts
    21

    Question measure speed without OBD/CAN

    I found this site (Linky) where a guy was able to measure the rpm's of his car by reading the pulses of the signal going to the distributor. This method seems to be pretty universal between all vehilce all that needs to be added is a voltage attenuator to make it universal.

    So my question, is there a way to read the speed of the vehicle in a similar fashion without it be vehicle specific???

  2. #2
    SuperMod - OBDII GPS Logger forum
    Auto Apps:loading...

    Join Date
    Mar 2009
    Location
    Los Angeles
    Posts
    929
    You mean, as academic interest? Or is your primary goal really to measure vehicle speed? Because if your primary goal is to measure vehicle speed... get a GPS antenna.

    Gary (-;
    OBDGPSLogger, for logging OBDII and/or GPS data
    OBDSim, an OBDII/ELM327 software simulator
    mp3car forums: obdgpslogger, obdsim

  3. #3
    Newbie
    Join Date
    Aug 2008
    Posts
    21
    yea a gps would work until you get in the city (I live in one) or under bridges. I could build and adaptive gps that uses an accelerometer when there is no signal.

    But i would like to get the speed from the vehicle incase the OBD fails.

  4. #4
    SuperMod - OBDII GPS Logger forum
    Auto Apps:loading...

    Join Date
    Mar 2009
    Location
    Los Angeles
    Posts
    929
    I'd go so far as to suggest that if your OBD fails, and you can't get a GPS signal, you have deeper problems than whether or not you're going to get a ticket.

    As an academic exercise, though, it's an enticing question. The obvious path is hardware attached to your wheels, but there's forces at work there that hobbiests probably want to avoid.

    Accellerometers turn out to really suck most of the time - and as the name implies only measure forces applied [F=m dv/dt] and not instantaneous velocity. IIRC if you run the math, you actually end up dropping the mass from the equation at some point, but the inaccuracy turns out to be pretty large.

    Gary (-;
    OBDGPSLogger, for logging OBDII and/or GPS data
    OBDSim, an OBDII/ELM327 software simulator
    mp3car forums: obdgpslogger, obdsim

  5. #5
    Newbie
    Join Date
    Aug 2008
    Posts
    21
    yea i know if both fail I have a big problem but mostly this was out of curiosity.

    For on eof my EE class i built an enhanced GPS and it worked pretty well when I mounted it on a push cart and tried it around campus. was a lot more accurate when the two devices were working together and the accelerometer was able to hold the gpa heading when we disconnected the gps

  6. #6
    North of the land of Hey Huns
    Auto Apps:loading...

    Join Date
    Jun 2004
    Location
    Westminster, MD
    Posts
    2,144
    Relative positioning (accelerometers) in conjunction with absolute positioning (gps) is a good way to keep track of where you are at any instantaneous point in time. The longer you are without gps, the more inaccurate it would get... but for going under bridges and in between skyscrapers, it might work decently.

    On another interesting note, if your OBD fail, you're probably going to quickly go towards 0mph cause that likely means the ecu has died

    As another option, most vehicles use a pulse (similar to the tachometer) to measure speed. The way it works is there is a magnet attached to either the output shaft of the transmission, or somewhere thereabouts. A sensors reads this, and sends pulses back to the ECU which converts it to a speed value. You could tap into this line, and get a frequency to voltage converter, or read the frequency directly.
    "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
    RevFE
    My Shop

  7. #7
    Newbie
    Join Date
    Aug 2008
    Posts
    21
    yea that would work. But some cars are different with this. Don't some send out 4 pulses per mph while others might send out one.

    I have a VW Golf 2010

  8. #8
    North of the land of Hey Huns
    Auto Apps:loading...

    Join Date
    Jun 2004
    Location
    Westminster, MD
    Posts
    2,144
    I think most of them send out one pulse per revolution of the drive-shaft. From there you have to calculate it out of course.
    "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
    RevFE
    My Shop

  9. #9
    Constant Bitrate
    Join Date
    May 2008
    Posts
    223
    Well, my car (2007 Hyundai Accent) outputs a vehicle speed signal on pin 6 of the OBD connector. Not sure what format the signal is in (PWM, variable frequency, etc) but it's the same signal used to drive the spedo in the instrument cluster.

    (digging...) Looks like most cars have this signal available somewhere, some cars with nav options have it on the radio harness. One of the sources showed 4 pulses per wheel revolution so you would have to know the wheel diameter to calculate speed.

  10. #10
    Newbie
    Join Date
    Aug 2008
    Posts
    21
    Yea i figured it would be about 4 pulses for wheel position accuracy. I could just use a voltage clamp and feed the signal into a frequency to voltage computer and measure the voltage with a microcontroller

Page 1 of 2 12 LastLast

Similar Threads

  1. Low Speed GMLAN Interface
    By Hebe in forum Engine Management, OBD-II, Engine Diagnostics, etc.
    Replies: 55
    Last Post: 06-19-2014, 09:13 AM
  2. Ultimate Interface FREE - Skinnable , Speed Zones,Laptop SDC, Vehicle Security
    By ThunderStick in forum Software & Software Development
    Replies: 20
    Last Post: 04-26-2010, 10:19 AM
  3. SpeedZone w/GPS Position Alarm - GPS POSITION TRIGGER - 28SEP09
    By ThunderStick in forum RR Released Plugins
    Replies: 129
    Last Post: 10-07-2009, 05:39 AM
  4. Replies: 44
    Last Post: 03-19-2009, 08:55 AM
  5. Got my first speeding ticket...grrr
    By studmonk3y in forum Off Topic
    Replies: 49
    Last Post: 05-02-2006, 12:36 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •