28 Check Shifter Signal

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THEORY OF OPERATION:
The T1, T3, T41, and T42 sense circuits communicate the shift lever position to the TCM. Each circuit is terminated at the transmission with a switch. Each switch can be either open or closed, depending on the shift lever position. The TCM can decode this information and determine the shift lever position. Each shift lever position has a certain combination of switches which will be open and closed, this is called a PRNDL code. Since there are 4 switches, there are 16 possible combinations of open and closed switches (codes). Most vehicles have only 7 gear positions and there are three recognized between gear codes, this results in 6 codes which should never occur, these are called invalid codes.

LIMP-IN:
NO

WHEN MONITORED:
Continuously with the key on.

SET CONDITIONS:
Immediately following invalid PRNDL code for more than one second.

TRANSMISSION EFFECTS AND POSSIBLE CAUSES:

Scenario 1)
All PRNDL lights stay illuminated indefinitely in Park following a Key start. Wrong part number TCM for application.
- Wrong part number TCM for application.
- T1, T3, T41, T42 (C1 -C4) **circuits are either open, shorted to ground, or shorted to 12 volts.
- CCD bus failure (open or shorted resulting in no communication to BCM or Cluster).
- TRS, or PRNDL switch connector not plugged in.
- Defective TRS, PIN, or PRNDL switch
- Defective TCM.
- Defective BCM.

Scenario 2)
"P" is indicated following a key start, but all PRNDL lights illuminate in "N" following a shift from "R" to "N". If PRNDL lights illuminate in "N" and shifter is moved directly into "3" or "L' position without pausing in "OD", then the "OD" position shift schedule and electronic display will indicate "OD" until the shifter is shifted into the "OD" position and held for at least 3 seconds.
- Worn Manual Level (Rooster Comb). Check for heavy wearing by TRS or switch contacts.
- Defective TRS, P/N, or PRNDL switch.
- Defective TCM.
- Intermittent T1, T3, T41, T42 **circuits. Check for corrosion, terminal push-outs or spread terminals at 60-way and/or TRS, P/N, or PRNDL switch 10-way connector.
- Defective BCM

Scenario 3)
If the invalid code happened while operating in the "3" or "L' position, then the "3" or "L' shift schedule and electronic display will be frozen (regardless of whether "OD", "3" or "L" is selected) until the shifter is moved to the "N" position (all PRNDL lights will illuminate) and then back to the "OD" position and held there for at least 3 seconds in order to resume the normal "OD" shift schedule and electronic display.
- Intermittent T1, T3, T41 or T42** circuits. Check for corrosion, terminal push-outs or spread terminals at 60-way and/or TRS, P/N, or PRNDL switch 10-way connector.
- Defective TRS, P/N, or PRNDL switch.
- Defective TCM.
- Defective BCM.

NOTE: These same symptoms may occur without the fault code 28 being set. It is possible that the invalid code did not last for longer than 0.1 second.