1, 2 and 3 Digit Codes: 28

Name of Code:
28 Check Shifter Signal

Set Condition:
Invalid PRNDL data sensed by the TCM with the ignition key in the "run position".

When Monitored:
When the TCM is powered-up the TCM always looks at the T1, T3, T41, and T42 PRNDL circuits to determine if the gear selector lever is being moved to anticipate a shift. An invalid code that lasts for more that .1 second will set the fault code. Also, an invalid code that is sensed for less than 0.1 second but happens 3 times in a given key start will set the fault code 28. If only one or two invalid codes are sensed by the TCM that lasts for less than 0.1 second, the electronic display and transmission shift schedule will operate the same as if the fault code was set.

Theory of Operation:
If all electrical and mechanical systems are functioning properly, the gear selector lever can be swept from Park to Manual Low and indicate valid in-gear codes (such as Neutral, Reverse, Overdrive, Third and Low) as well as valid temporary codes (codes that are in-between valid in-gear codes). All in-gear codes and temporary codes are considered as valid codes and do not cause a problem. However, if an invalid code is sensed by the TCM either during shifter movement (electrical intermittency on T1, T3, T41, or T42 circuits or a worn Manual Valve Lever (Rooster Comb) or while the shifter is stationary (electrical intermittency on T1, T3, T51, or T42 circuits), the TCM will set fault code 28.

Transmission Effects:
Scenario 1) All PRNDL lights stay illuminated indefinitely in Park following a Key start.

Scenario 2) - "P" is indicated following a key start but all PRNDL lights illuminate in "N" following a shift from "R" to "N". If shifted into "Overdrive position" and held for at least 3 seconds and PRNDL data becomes valid, the "3" and "L" position will illuminate if selected. However 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.

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 "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.

It is imperative that the scenario described above be understood fully since it will greatly help the technician diagnosis the problem. Also these same symptoms may occur without the fault code 28 getting set. The problem is that the invalid code that was sensed by the TCM only occurred once or twice during the given ignition key start and did not last for longer than 0.1 second.

Possible Causes:
Scenario 1)
- Wrong Part Number TCM for application
- TRS, MVLPS, P/N or PRNDL switch connector not plugged in
- T1, T3, T41, or T42 (C1 - 04) **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)
- Defective TRS, MVLPS, P/N, or PRNDL switch
- Defective TCM
- Defective BCM

Scenario 2)
- Worn Manual Lever (Rooster Comb). Check for heavy wearing by TRS or MVLPS switch contacts
- Intermittent T1, T3, T41, or T42** circuits. Check for corrosion, terminal push-outs or spread terminals at 60-way and/or TRS, MVLPS, P/N, or PRNDL switch 10-way connector
- Defective TRS, MVLPS, P/N, or PRNDL switch
- Defective TCM
- Defective BCM

Scenario 3)
- Intermittent T1, T3, T41 or T42** circuits. Check for corrosion, terminal push-outs or spread terminals at 60-way and/or TRS, MVLPS, P/N, or PRNDL switch 10-way connector
- Defective TRS, MVLPS, P/N, or PRNDL switch
- Defective TCM
- Defective BCM

** T1 = Manual Valve Lever Position Switch O4 Sense or PRNDL Switch T1 Sense circuit. T3 = Manual Valve Lever Position Switch O3 Sense or Park/Neutral Position Switch Sense circuit.

T41 = Manual Valve Lever Position Switch C1 Sense or Park/Neutral Position Switch Sense circuit.

T42 = Manual Valve Lever Position Switch C2 Sense or PRNDL T42 Sense circuit.