With 4L80E Automatic Transmission

DTC 28:

CIRCUIT DESCRIPTION:
The fluid pressure switch assembly consists of five pressure switches combined into one unit and mounted on the valve body. The PCM/TCM supplies battery voltage on three separate wires. By grounding one or more of these circuits through various combinations of the pressure switches, the PCM/TCM detects what gear range has been selected by the vehicle operator.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This test checks the indicated fluid pressure switch assembly to the manual valve position actually selected.

2. This test checks for correct voltage from the PCM/TCM to the transmission pass-thru connector.

3. This test checks a short to ground in any one of the three fluid pressure switch assembly circuits.

Valid Combination Chart:

DIAGNOSTIC AIDS:
DTC 28 will set if the PCM/TCM detects one of two illegal" fluid pressure switch assembly combinations.

Refer to accompanying chart for various combinations.

Check pass-thru connector for good contact.