1987 Plymouth Horizon AutomaticFACTORY OEM SERVICE PROCEDURE

Code 28

  1. Turn the ignition off. Remove speed sensor from transmission. Disconnect speed sensor connector. Connect ohmmeter between terminals of the speed sensor connector. Slowly rotate speed sensor one turn. Ohmmeter should show approximately 8 pulses for every turn of the speed sensor. If ohmmeter reading is okay, proceed to step 2). If ohmmeter reading is not okay, replace the speed sensor.
  2. Disconnect speed sensor connector. Connect the voltmeter positive lead to the White/Orange wire of the speed sensor connector. Connect the voltmeter negative lead to the Black/Blue wire of the speed sensor harness connector. Turn ignition switch to the "RUN" position.
  3. Voltmeter should read at least 3.5 volts. If voltage is okay, inspect transaxle drive gear to speed sensor drive gear for proper contact. If zero volts, move voltmeter negative lead to a good engine ground. If voltmeter now reads correct voltage, repair ground wire for an open circuit. If voltmeter still reads zero, proceed to step 4)
  4. Turn ignition switch to the "OFF" position. Disconnect the computer 14-pin connector. Connect an ohmmeter between cavity No. 2 of the 14-pin connector and the White/Orange wire of the speed sensor harness connector. If ohmmeter shows continuity, ensure the terminal in connector cavity No. 2 is not spread apart causing a poor connection. If connection is okay, replace the computer. If no continuity, repair wire for an open circuit.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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