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Home >> GMC >> 1991 >> Value Van 7.4 N >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Automatic Transmission - Diagnosis - 4L80-E >> Diagnostic Charts >> Code 85 - Undefined Ratio >> Notes

Code 85 - Undefined Ratio: Notes

Fig 1: Code 85 Circuit Diagram
G92I14124Courtesy of GENERAL MOTORS CORP.

PCM/TCM compares input and output speed sensor values of transmission in each gear to acceptable values in PCM/TCM memory. If PCM/TCM senses sensor values are not within acceptable range, Code 85 will be set. This process takes place in all gears except Overdrive.

NOTE: Test numbers refer to numbers on diagnostic chart.
  1. This step checks operation of input sensor.
  2. This step checks operation of output sensor.
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