Code 85, Undefined Gear Ratio: Notes

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While in each gear, PCM/TCM calculates actual gear ratio from input and output speed readings, also referred to as vehicle speed, then compares these to what gear ratio should be, taking into consideration selected gear range. This monitor includes reverse gear, but does not include overdrive gear.

Fig 1: CODE 85, Schematic, Undefined Gear Ratio

Courtesy of GENERAL MOTORS CORP.

NOTE: Test numbers refer to numbers on diagnostic chart.

This test verifies the proper function of the input sensor.
This test verifies the proper function of the output sensor (vehicle speed sensor).