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Picture
Do I need a VSS (vehicle speed signal) ?

In most LSx conversion projects there is already an existing provision for a VSS.  However, with some unique conversions this may not be the case.  Older manual transmissions (including a T56 from a Dodge Viper) and older non-electronic transmissions may not have a VSS.  In four wheel drive applications you may be using an LSx based transmission, but the accompanying transfer case may not have a VSS.

With a Gen 3 (24x) engine you could tune “around” the missing VSS input - but you will likely be paying a tuner to do this work for you.  What you may not realize, is that without a VSS input the computer (PCM) will operate as if the vehicle was permanently stationary - never using many available algorithms - like the High Octane Spark Table or tables used to maintain a steady Idle RPM during coastdown transitions.


One possible VSS add-on option - from Dakota Digital
Dakota Digital GPS-50-2 GPS Speed Sender  (around $200 USD)
Dakota Digital GPS-50-2 External Antenna  (around $20 USD)

The GPS Speed Sender, as the name implies, uses satellite based GPS signals to calculate vehicle speed.  Additionally, the unit has an internal accelerometer which also calculates vehicle speed.  These two speed calculations are constantly cross compared to arrive at a precise vehicle speed.  This dual system means that if the satellite signals are interrupted (i.e. driving through a tunnel, or an underground parking structure) an accurate vehicle speed is still being calculated (via the internal accelerometer).

The optional External Antenna allows mounting the main GPS Speed Sender unit in a remote (or hidden) location.  The External Antenna may be discreetly mounted on the dash of the vehicle (often near the base of the windshield) to capture GPS satellite signals.


Example Setup - LS1 style PCM  (using a Dakota Digital GPS Speed Sender)

VSS High (often a Purple) wire:
  • 512Kb PCM C2 (Red) pin 21
  • 1Mb PCM C2 (Green) pin 21

Connect the above wire to the "Speed 2" terminal of the GPS Speed Sender.  The VSS Low (often a Green) wire is not used (does not need to be connected to anything).

PCM Speedometer output circuit:
  • 512Kb PCM C2 (Red) pin 50
  • 1Mb PCM C2 (Green) pin 50

Connect the above wire to your electric Speedometer.

Set the GPS Speed Sender output to "Normal" (i.e. pulse) not to "Bus" (i.e. CAN Bus or Serial Data).  Set the pulse rate to 128K (the highest pulse setting).

*** the PCM must be programmed such that 128,000 (128K) is the number of input (VSS) pulses the PCM will receive per mile travelled ***

In test mode the GPS Speed Sender will output a 60 MPH (96 Km/h) signal.  Confirm with a scanner that the PCM is correctly calculating this pulsed input - displaying as either 60 MPH or 96 Km/h.

The engine does not need to be running when receiving the test input from the GPS Speed Sender, but the key does need to be in the "ON" or "RUN" position.  During this test mode the PCM will output a Speedometer signal - which will allow you to test your electric Speedometer.

By default the PCM outputs a 4000 (4K) pulse per mile (PPM) signal to the electric Speedometer.  Some aftermarket Speedometers are preprogrammed to various PPM settings - 8K, 16K, 24K, etc.

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