Code 68, Overdrive Ratio Error - LT. Truck: Notes

Fig 1: Code 68, Overdrive Ratio Circuit Diagram (Lt. Truck)

Courtesy of GENERAL MOTORS CORP.

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 number on diagnostic chart.

Code 68 with 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.
This test confirms engine RPM signal to PCM/TCM.