By Benjamin Ratterree

1. Check whether the conversion constant in the speedometer is correct with the following procedure:

  • Press and hold the RESET button on the front of the speedometer while turning the key to RUN.
  • Release the RESET button. The pointer will prove out and some codes will appear on the odometer display. If an "E" is on the left side of the display, the module is programmed for English display mode and should have English graphics (mph). A lower case "o" signifies an overseas (metric) graphics (kph). Verify the graphics and display modes agree. (The number appearing after the type is the microprocessor revision level and is not used in this test.)
  • Rress and release the RESET button again. The conversion constant and the word CAL will be shown on the odometer display.
  • Check the conversion constant shown on the odometer display with the chart below and verify the correct constant is entered.
  • Turn off the key to exit this test.

  • 2. Reprogram speed conversion constant using the following procedure:

  • Locate the dealer mode programming enable single-terminal connector. The connector is located at the bottom of the instrument panel below the center of the glove box. On all vehicles, the dealer mode enable wire is Circuit #567 (LB/Y).
  • Insert metal clip in service wire panel connector. Clip other end to vehicle ground while the key is in the OFF position.
  • Turn key to RUN (not crank) position while holding down the trip odometer RESET button on speedometer. Release RESET button.
  • Speedometer will prove out (sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display is the dealer mode programming lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a conversion constant change is required, a service instrument cluster must be ordered. Refer to this section for ordering information.
  • Press and release RESET button again and PSOM conversion constant (without the decimal point) followed by the word CAL.
  • Press and release the odometer SELECT button as many times as necessary to change conversion constant to the correct value.
  • NOTE:
    Pressing and releasing the RESET button to lock in the new Conversion Constant reduces the number of times the PSOM can be reprogrammed by one count. The module can only be reprogrammed six times. If you change your mind and want to go back to the old conversion constant, turn the ignition switch off before you lock in the new constant.

  • When the correct new value is displayed in odometer window, press and release the RESET button to lock in the new conversion constant.
  • Turn the key off and remove the ground wire. Verify proper speedometer operation.
  • To find conversion constants for tires with 15 inch rims that are not listed in chart, above multiply the revolutions per mile (at 45mph) of your tire times .0135. For 16 inch rims multiply by .015. The revs/mile can be found at your tire manufacturers web site. There are slight variations in revs/mile per tire manufacturers, but using one like the popular BFG A/T (see below) will get you very close.
  • Example: BFG AT 33×12.50R15LT/C
    Revs/mile= 636
    636 rev/mile X .0135 = 8.59
    8.59 would be your conversion constant

    Here is a listing of popular oversize BFG All-Terrain tires with conversion constants:

    Tire Size Revs/mile (at 45mph) Conversion Constant
    30×9.50R15LT/C 705 9.52
    31×10.50R15LT/C 677 9.14
    32×11.50R15LT/C 656 8.86
    33×12.50R15LT/C 636 8.59
    35×12.50R15LT/C 599 8.09
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