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

Siemens Genius Siemens Genius
Siemens Genius

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 LMS Test.Lab 15A and higher, and a LMS Scadas with Canbus interface.  Here are the steps:

 

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

 

OBDII port is located under dashOBDII port is located under dashA 9-pin to OBDII cable connects Scadas to vehicleA 9-pin to OBDII cable connects Scadas to vehicle

2.  In LMS Test.Lab 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.

 

Choose "CAN Settings" from the pulldown menu in the upper right of the LMS Test.Lab Signature Channel Setup menu.Choose "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 in LMS Test.Lab from the directory: C:\Program Files (x86)\LMS Test.Lab {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.

 

Select generic OBDII file from DB PathSelect generic OBDII file from DB Path

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

 

In "CAN Overview", select the Engine_RPM and Vehicle_Speed via the checkboxIn "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.

 

 

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

 

Press "Apply" button after making OBD2 selectionsPress "Apply" button after making OBD2 selections

How does it compare? 

 

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

 

obd2_vs_tach.png

 

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).

 

More LMS Test.Lab Acquisition Tips:

 

LMS Test.Lab Display Tips:

 

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