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Home >> GMC >> 1992 >> Forward Control 7.4 N >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - Transmission >> Component Check Charts >> Code 68, Overdrive Ratio Error >> Notes
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Code 68, Overdrive Ratio Error: Notes

The PCM/TCM monitors the difference in engine RPM and input shaft RPM. With transmission in Drive, scan tester reading should show engine speed closely matching input speed.

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. Code 68 will set under the following conditions: engine speed is 200 RPM greater than input speed, 4th gear is indicated, TCC is enabled, or if all conditions are met for 2 seconds.
  2. This test monitors signal sent to the PCM/TCM from transmission.
  3. This test confirms engine RPM signal to PCM/TCM.
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