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Home >> GMC >> 1992 >> Forward Control 5.7 K, Standard >> Repair and Diagnosis (Single Page) >> Transmission >> Automatic Trans >> Automatic Transmission - Diagnosis - 4L80-E >> Diagnostic Charts >> Code 28 - Pressure Switch Manifold (PSM) >> Diagnostic Aids

Diagnostic Aids

Code 28 will set if an illegal switch combination is detected. See PSM COMBINATION CHART  table. Check transmission pass-through connector connections.

PSM COMBINATION CHART

Gear Shift Position (1) Range Signal Circuit(s)
Park "A" & "C"
Reverse "C"
Neutral "A" & "C"
4th "A"
3rd "A" & "B"
2nd "A", "B" & "C"
1st "B" & "C"
Illegal "B"
Illegal None
(1) See mini-schematic to determine range signal circuits.
Fig 1: Code 28 Troubleshooting Diagnostic Flow Chart
G92D14103Courtesy of GENERAL MOTORS CORP.