If you've connected the tach signal, and the rpm has lots of large fluctuations:

  1. Make sure the shift light has a good, clean ground connection
    1. Consider testing connecting ground to the engine block somewhere to see if there is any improvement
  2. Make sure any ignition module wires (if your vehicle has them) have good, solid connectors
  3. Make sure all ignition system grounds are clean and well-connected
  4. If connecting to the "-" coil terminal, check that this connection is clean and tight
  5. Make sure the shift light wires are not routed near spark plug wires, or other wires that carry high current (power wires to any ignition modules, or electric fans/fuel pumps)
    1. Consider testing the shift light by temporarily not routing the wire in any other wiring harness, just to see if any difference is noticed
  6. If your vehicle has distributorless ignition or coil-on-plug (or coil-near-plug), you may need a tach adapter such as the MSD #8918. This adapter is easy to use as it just clips onto the outside of a coil power wire, no cutting needed.
  7. If your vehicle has OBD-II, especially if it is newer than 2006, you may consider using a wireless OBD-II plug in module. See the Trac-Pac shift light website for details.
  8. If none of the above is helpful, contact us.