DAVIS DriveRight Installation - VSS (8156, 8160) - Manual (Page 14)

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If your speed is not registering correctly, consult the following check list to help
you identify the cause of the problem.
❏ Make sure the display is reading in the units (km/hr or mph) you expect.
Hold down the Set/Clr button in the Current Screen to change
the units.
❏ Make sure the unit has been calibrated correctly.
Check the calibration section of the DriveRight User’s Guide.
❏ Check the switch settings on the VSS in-line circuit and reset the settings
if necessary.
See “Configure the VSS Signal Settings” on page 10.
❏ Double check your power and ground connections using a multimeter.
I‚‡r) Remember that BAT will not be visible on the display if the
DriveRight is connected to the harness cable and receiving +12
VDC from the vehicle.
❏ Make sure the VSS wire is correct.
If the green test LED of the VSS in-line circuit does not flash at
low speeds, you may not have the correct wire (switch 4 must
be on to enable the test circuit). The following other methods
can be used to see if you have the correct wire. In order of
preference:
• Use a portable oscilloscope and verify the square wave or sine wave
VSS signal. Watch the signal frequency change with speed.
• Use a multimeter to measure the frequency of the VSS pulse. Usu-
ally, the PPM (pulses per mile) of the VSS signal is known. At 40 km/
hr (25 mph) the following frequencies should be:
2000 PPM: 14 hz
4000 PPM: 28 hz
5000 PPM: 35 hz
8000 PPM: 56 hz
• A voltmeter can be used on square wave signals to verify the 2 DC
levels when the vehicle is moved. Note, in most cases the vehicle
must be running and driven slowly to detect this change.
❏ Check with your local vehicle dealership to make sure your vehicle is
equipped with a complete VSS signal.
07395.062 Rev E Body D121.fm Page 12 Monday, December 1, 2003 10:52 AM