I found that I had quite a lot of error in my 1996 Cobra's factory tachometer. From my measurements I found that it was reading 700 RPM above actual at 6,500 RPM. I could not find any procedure to calibrate the Tach in my Haynes manual or in the forums on the Net, so I decided to do it myself. The following procedures will illustrate how to adjust your stock Tachometer.

Materials Needed:

• Manual, for Instrument Cluster removal instructions.
• OBD II Scanner, for real-time RPM readout from the PCM.
• Hand tools to remove Instrument Cluster.
• 5/16 or 3/8 drill bit and drill motor to make an access hole.
• 2.5 mm allen wrench to make the adjustment.

1. Remove the Instrument Cluster from the vehicle and take it to a suitable workplace.  Here is a picture of the Cluster with the hole already in it.  This will give you an idea of where you will be working.	Click to Enlarge
2. Mark the location of the access hole you are about to drill.  Hole should be centered 30/64" from the screw boss and 44/64" back from the front lip, as shown here.	Click to Enlarge Click to Enlarge
3. CAUTION!!    When you drill the hole, DO NOT let the bit penetrate more that 1/4" through the plastic!  Use slow drill speed and light pressure to maintain control of the drill.  A drill stop could also be used if you have one. Drill your access hole.

4. Take the Instrument Cluster back to the car, connect it to the wiring harness and set it in place.  Connect your Scanner and start the car.  Bring your engine RPM's up to 3000 as indicated by the Scanner.  Hold RPM's steady.  Look at your Tach, is the Tach indicating hi or lo?  How many RPM's error is indicated?  Shut off the car and get the Instrument Cluster back out where you can get to the adjustment hole.

Use the 2.5 mm allen wrench and turn the adjuster as follows:

• If the tach indicates lower than the Scanner, turn the wrench clockwise.
• If the tach indicates higher than the Scanner, turn the wrench counter clockwise.

You may have to test, then adjust a few times before you get it corrected.  This may mean connecting and unconnecting the instrument cluster, and starting and stopping the scanner and the car.

5. When you have the tachometer adjusted where you want it, reinstall your instrument cluster per your manual.