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No more tach! Use OBDII to get Engine and Vehicle Speed...

Siemens Legend Siemens Legend
Siemens Legend

Who wants to spend time setting up a tachometer? You have to mount it on a greasy engine, run cables, etc.


Instead of doing all that, just measure engine rpm or vehicle speed directly thru the OBDII connector of the vehicle! One single connection to the underdash OBD2 connector is all it takes!


It can even be done without a vehicle specific CAN bus DBC file.  Due to emissions regulations, certain signals like engine and vehicle speed are legislated to be present on all newer vehicles, and can be read over the vehicle bus directly.


It is easy with Simcenter Testlab Signature 15A and higher, and a Simcenter SCADAS with CAN Bus interface.  Note that  Simcenter Testlab was formerly called LMS Test.Lab. Here are the steps:


1.  Connect to the SCADAS (9 pin) to the OBDII connection on your car, usually located under the dash:


obdii.jpgOBDII port is located under dashPicture1.jpgA 9-pin to OBDII cable connects Scadas to vehicle

2.  In Simcenter Testlab Signature, make sure "Vehicle Bus" add-in is ON under "Tools -> Add-ins".  The add-in requires 22 tokens.


3.  In "Channel Setup" worksheet, select "CAN Settings" from the pulldown menu in the upper right.


Can Settings.pngChoose "CAN Settings" from the pulldown menu in the upper right of the LMS Test.Lab Signature Channel Setup menu.

4.  Set Baud rate to 500000, Select "Can Device", set "Can ACK" to "Active"


5.  In "DB Path", select the generic OBDII files provided from the directory: C:\Program Files (x86)\LMS Test.Lab {rev}\central\Application Resources\CAN or  C:\Program Files (x86)\Simcenter\Testlab {rev}\central\Application Resources\CAN.  There are two different files - one with longer descriptive names, one with very short names.  Both files contain the same signals, which include Engine and Vehicle speed.


obdii_can_tl.pngSelect generic OBDII file from DB Path

6.  In the CAN Overview, select the Engine_RPM and Vehicle_Speed channels (using the checkbox).


engine_rpm.pngIn "CAN Overview", select the Engine_RPM and Vehicle_Speed via the checkbox

7.  In the CAN Overview, also set the 'update rate' to "20 Hz" (the fastest request rate for OBDII) and the Interpolation to "Linear".  Interpolation smooths out the rpm, otherwise it will have "stair steps" due to the digital nature of the OBD II bus.  It may be necessary to use the slider bar at the bottom of the menu and scroll to the right to find the 'update rate' field.



Picture2.pngGraph of OBDII RPM vs Time Data: Interpolation "Linear" in Green (smooth curve), "Sample and Hold" in red (as from OBDII)


8.  Press the "Apply" button, and acknowledge the OBDII message. Go to the "Measure" worksheet to take data.


Apply.pngPress "Apply" button after making OBD2 selections

How does it compare? 


Can the OBDII really replace a tachometer and get accurate results?




Here is a comparison of a physical tachometer installed on an engine with a 1 pulse per revolution signal (Red curve) with the engine rpm acquired via the OBDII with 20 updates a second and linear interpolation (Green curve).


Questions?  Call or download the Simcenter SCADAS Brochure.


More Simcenter Testlab Acquisition Tips:


Simcenter Testlab Displays



Is it possible to use engine RPM from CAN to process order analysis?

Siemens Phenom Siemens Phenom
Siemens Phenom

Hello Dreamer, yes any of the CANBus or OBD-II channels can be used to track against for Order Analysis including engine RPM and Vehicle speed.  The channels are placed in the static group and rather than tracking on time or tacho, select to track on static channels and then you can track on any channel in the static group.  You can then calculate fixed sampled orders, overall level, etc. and place the waterfalls and colormap displays to track on engine RPM or vehicle speed.


Since there is no raw tacho, or crossing of the tacho moments, they cannot be used for Order Tracking or Angle Domain processing by default but fixed sampling methods work fine.