interfacing uC w/vehicle odometer

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I'm interested in using a microcontroller (PIC probably) to interface to a
car/truck odometer to track usage (read only of course ;-).

Haven't had much luck w/Google/DejaNews finding relevant info, so I was
wondering if anyone here had any experience they'd be kind enough to share,
particularly what type of interface circuit to use, best place/way to get the
signal off the vehicle, and what types of signals are available.


Re: interfacing uC w/vehicle odometer

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It depends.  Different vehicles have different sensors and signals.  Modern
vehicles usually have a digital Vehicle Speed Sensor
(VSS) signal that can be found various places in the vehicle, including the
engine control module harness, an instrument cluster
harness, and somewhere near the sensor (usually near the transmission).  The
characteristics of this signal vary.  The first step to
understanding a particular vehicle is to get the service manual and understand
the relevant part of the schematics, and try to
figure out what kind of sensor and signal it uses.  I have seen optical,
variable reluctance, and hall-effect sensors.  After that,
I put the target vehicle on stands in the garage and put it in gear and observe
the signal at various speeds with a scope.  I have
seen open-drain, open-source & active signals of various voltages, DC and AC.
The number of pulses per rotation is also variable,
and can be quite high for vehicles with ABS.  On some cars, this signal counts
rotations of the transmission output shaft, on others
it is usually one of the undriven wheels.  I then start experimenting with the
interface circuit to get a useable digital signal
without interfering with the vehicle computers.  Depending on the signal
characteristics, you may need to pull up, pull down,
isolate with a diode, and/or add a buffer amp.  If you use this signal, you need
to make sure that you are not interfering with it
because it can confuse the engine management and/or ABS systems.  If the car has
cruise control, this can be an easy way to test the
signal for interference.  

My experience comes primarly from working with rally computers, which are
essentially precision odometers.  Most off-the-shelf rally
computers will only work with a particular type of signal, so we often leave the
original vehicle sensor alone and add one or two
sensors (known as "probes") just for the rally computer.  This allows us to
avoid interfacing with and interfering with the engine
management system, to use a sensor that is directly compatible with the
particular computer and to have redundant sensors for
instant switch-over in case of failure.  Depending on your application, you may
find it easier to simply add your own sensor and
connect that to your computer.  I would recommend googling on "rally computer
probe".  Here are some companies that I have dealt
with that sell odometer sensors to the rally market: /

Good luck.

Chuck Cox
SynchroSystems Motorsport Computers
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Re: interfacing uC w/vehicle odometer
or you could use a different approach and get the data from a gps in the

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Re: interfacing uC w/vehicle odometer
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Aren't the U.S. Interstate mile markers reasonably accurate?

The distance-to-go signs in California only have a casual
relationship to reality.  They seem to include a +/- 5 mi.
randomization factor for amusment of CalTrans personnel.

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