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'[EE] On-Board Diagnostics: open question day'
|I received an off-line response to the "[AD] microOBD 200 article" post:
"I find the concept of publishing an article on a project to be interesting.
The problem is that I never got my hands on an on board diagnostic system.
It is all greek and latin to me."
Wikipedia has an article on OBD:
You can think of OBD as a gateway to the vehicle's computer system. Most
cars have a network of electronic control modules (ECUs), each one of which
performs a specific function such as controlling the engine, transmission,
ABS, airbags, power windows, locks, seats, et cetera.
With the introduction of OBD-2 in 1994, the physical, data, and application
layers were standardized and made available to third party manufacturers. A
lot of this information is now available on the internet, and there is a
number of OBD to UART converters on the market.
The most common use of OBD is diagnostics, any decent scan tool nowadays can
pull trouble codes and display vehicle parameters (some cars support
hundreds of parameters). Another use involves controlling actuators, for
example you could send a message to start the vehicle, or control the volume
of the radio, or even control the throttle position.
While some of the information may be had by intercepting messages that
modules send to each other (for instance, instrument dashboard could be
periodically requesting fuel level status), most of the time you have to
specifically request it. For example, if you wanted to know the RPM, this is
what a typical exchange would look like:
01 0C // Mode 1, parameter ID 0C = RPM
41 0C 01 23 // Response to Mode 1, PID 0C
The last two bytes are the RPM, encoded as 1/4 RPM per bit. You can find
more generic PIDs here:
I would be happy to try to answer any OBD related questions that you may
|I recently got a ScanTool OBDII to USB/Bluetooth tool, I can't call it a
scanner because it is much more than a basic scanner. Connected to a
Nettop (small laptop) computer, the amount of information is huge. For
my 2002 Mercury Cougar, there are703 supported PID's (data items). From
very basic like RPM and Coolant temperature to voltages and other
parameters of both sensors and commands to action items. I have been
using it to track down a slightly out of tolerance sensor (I think) that
is causing pinging at idle with a slight load (air conditioner and/ or
in gear). With the help of the vehicle's shop manual including the
emissions book it has been a learning experience.
The setup provides readings to a few tenths of seconds, that are easily
converted to a CSV file and imported to Excel for review. Other computer
screens look like a car dashboard with speed, RPM, and other items
displayed. To see these numbers by the fraction of a second is necessary
to catch an intermittent or short term event, in my case it seems to
change in less than 2 minutes after a cold start. Some of the neat
things including 0-60 mph and drag race scenes to determine your
vehicles performance. :)
On 7/14/2010 7:27 PM, Vitaliy wrote:
Carl Denk wrote:
>I recently got a ScanTool OBDII to USB/Bluetooth tool, I can't call it a
> scanner because it is much more than a basic scanner. Connected to a
> Nettop (small laptop) computer, the amount of information is huge. For
> my 2002 Mercury Cougar, there are703 supported PID's (data items). From
> very basic like RPM and Coolant temperature to voltages and other
> parameters of both sensors and commands to action items.
What kind of refresh rates are you getting on your Cougar?
Also, I think it's worth pointing out that the majority of the 703
parameters are GM-specific PIDs, which are not supported by generic software
(you need ScanXL w/ the GM add-on to view them).
This is with SCANXL with the Ford add-on. With 36 selected PID's, I got
79 frames of data in 3 minutes = 2.27 seconds per refresh. This is using
the USB cable. I'm still playing with the Bluetooth, I think the issue
is with the computer and pairing. At this moment just happy with the
USB. There was an update today that raises the speed from 38,400 to
115K, I don't know how that will effect the speed, but I'm happy with
has been. It's a big step from my older scanner that just got a few PID's.
On 7/14/2010 8:57 PM, Vitaliy wrote:
|Carl Denk wrote:
> This is with SCANXL with the Ford add-on.
Sorry, I don't know why I wrote "GM".
> With 36 selected PID's, I got
> 79 frames of data in 3 minutes = 2.27 seconds per refresh. This is using
> the USB cable. I'm still playing with the Bluetooth, I think the issue
> is with the computer and pairing. At this moment just happy with the
> USB. There was an update today that raises the speed from 38,400 to
> 115K, I don't know how that will effect the speed, but I'm happy with
> has been. It's a big step from my older scanner that just got a few PID's.
You won't see much difference over USB, but over Bluetooth the difference
will be substantial (several hundred %). It's not the baud rate increase per
se, it's the fact that most messages will come in quickly enough not be
spread over multiple Bluetooth "bursts".
The instructions for switching old BT modules to the new baud rate, also
include a command that tells the module to optimize for latency (since we're
sending small messages back and forth, instead of large chunks of data):
Latency optimization and the baud rate switch must be done simultaneously
for best results.
Which site provides a good tutorial for bluetooth? I have yet
to have seen one yet!
--- On Thu, 7/15/10, Vitaliy <maksimov.org> wrote: piclist
I haven't found it yet. One of the work arounds I found with XP home,
and built in Bluetooth on the Gateway Nettop. Where it shows all the
devices after searching, needed to delete all devices, but was unable to
delete the last one, which was the one with pairing issues. Had to pair
another device (Nokia phone), then was able delete the troublesome one.
Then search, found, and pairing went OK. The device has a fixed pairing
number, which doesn't seem to be the issue. Plan to spend some time on
the issue today and sort out what's happening, but seems to be, the
Nettop gets confused with the pairing number.
On 7/15/2010 1:56 AM, John Chung wrote:
The documentation has been good. The speed up instructions were right
there with the download, with reference to the Bluetooth instructions
(which I have the hardcopy that came with the unit also). The Bluetooth
instructions are aimed more at XP PRo, and the Nettop came with XP Home,
but have been able to get there. Just haven't had the time/priority to
sort out all the Bluetooth quirks. :) Hopefully today will have the
opportunity to set things up in our Ford Bronco (Full size SUV) where I
can spread out the various items and instructions.
On 7/14/2010 9:46 PM, Vitaliy wrote:
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