Diagnostic Aids

DTC 28 will set if PCM/TCM detects one of 2 "illegal" PSM assembly combinations. See PSM COMBINATION CHART for range combinations. Check all wiring connectors for proper terminal tension and location.

PSM COMBINATION

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: DTC 28 Schematic - 1994 Diesel

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC 28 Schematic - 1994 Gasoline

Courtesy of GENERAL MOTORS CORP.

Fig 3: DTC 28 Flow Chart - 1994

Courtesy of GENERAL MOTORS CORP.