DTC 85, Undefined Ratio Error (4L80-E): Notes

PCM calculates ratio based on transmission input speed and output speed sensor readings. PCM compares known transmission ratio to calculated ratio, for particular gear range selected. DTC 85 will set when:

DTCs 21, 22, 24, 28, 71, 72 and 87 are not set.
TP is greater than 25 percent.
Not in Park/Neutral or 4th gear.
Engine speed is greater than 300 RPM.
Vehicle speed is greater than 7 MPH.
All conditions are met for 2 seconds.

PCM will default to line pressure set to maximum and inhibits TCC operation.

NOTE: Test numbers refer to numbers on diagnostic chart.

An out-of-range transmission range pressure switch could falsely indicate the actual transmission range.
This test checks calculated ratio to determine if ratio is within parameters.