Diagnostic Aids

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The PCM/TCM relies on the Power Switch Module (PSM) to indicate what gear range has been selected. PSM must be functioning properly or Code 85 may be set. Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

Gear	Less Than	More Than
1st	2.38	2.63
2nd	1.43	1.58
3rd	.95	1.05
Reverse	1.97	2.17

Fig 1: CODE 85, Schematic, Undefined Ratio

Courtesy of GENERAL MOTORS CORP.

Fig 2: CODE 85, Flow Chart, Undefined Ratio

Courtesy of GENERAL MOTORS CORP.