DTC 28 TFP Manual Valve Position Switch Fault L57 MFI
Circuit Description
The automatic transmission fluid pressure (TFP) manual valve position switch consists of five normally--open pressure switches. The transmission control module (TCM) supplies battery voltage to each range signal. By grounding one or more of these circuits through various combinations of the pressure switches, the TCM detects what manual valve position has been selected and compares the actual voltage combination of the switches to a TFP Val. Position Sw. combination chart stored in memory.
The TFP manual valve position switch cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. With the ignition ON and the engine OFF, D2 is indicated. When the transmission 20-way connector is disconnected, the ground potential for the three range signals to the TCM is removed, and with ignition ON, D2 is indicated.
If the TCM detects an illegal state of the TFP manual valve position switch circuit (range signals A and C are both ON), by deciphering the TFP manual valve position switch inputs, then DTC 28 sets.
Conditions for Setting the DTC
The TCM detects an illegal TFP manual valve position switch combination, (both A and C range signals are ON) for greater than 2 seconds.
Action Taken When the DTC Sets
| • | The TCM does not illuminate the malfunction indicator lamp (MIL). |
| • | The TCM commands maximum line pressure. |
| • | The TCM assumes D4 for the PRNDL shift pattern. |
| • | The TCM inhibits TCC. |
| • | The TCM inhibits 4th gear if in hot mode. |
| • | The TCM stores DTC 28 in TCM history. |
Conditions for Clearing the DTC
| • | A scan tool clears the DTC from the TCM history. |
| • | The TCM clears the DTC from the TCM history if the vehicle completes 40 consecutive key cycles without a diagnostic fault occurring. |
| • | The TCM cancels the DTC default actions when the fault no longer exists, and the ignition switch is OFF long enough in order to power down the TCM. |
Diagnostic Aids
| • | Refer to the accompanying table for the normal range signals and the illegal combinations. |
| • | Inspect the wiring at the TCM, the transmission connector and all other circuit connecting points for the following conditions: |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Reduced terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. |
| • | DTC 28 can be falsely set during a fluid fill procedure. After refilling the fluid, cycle key down then start and run the vehicle for 20 seconds. Key down and allow the TCM to power down, and then restart the vehicle. |
| • | DTC 28 can be set falsely by low pump pressure or a stuck pressure regulator. |
| • | First, diagnose and clear any engine DTCs that are present. Then inspect for any transmission DTCs that may have reset. |
| • | DTC 28 may be inhibited from passing if the state of the battery is low, or there is a low cranking voltage. To pass DTC 28, turn the ignition switch to the ON position with the engine OFF. Check that all the range signal switches (A, B, C) read OFF, OFF, OFF. Start the vehicle, and move the PRNDL through each selected transmission range. |
| • | Refer to the Range Signal table. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests the indicated range signal to the manual valve that is actually selected.
-
This step tests the voltage from the TCM to the transmission 20-way connector.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||
2 |
Were the inspections performed? | -- | Go to Transmission Fluid Check | |||||||
Important:: Before clearing the DTCs, use the Scan Tool in order to record the Failure Records. Using the Clear Info function erases the Failure Records from the TCM. Does each selected transmission range match the Scan Tool TFP Switch A/B/C display? (Refer to the Range Signal table.) | -- | Go to Diagnostic Aids | ||||||||
Do all three circuits display B+? | -- | |||||||||
5 | Inspect the circuits that did not indicate B+ in Step 4, for an open or short to ground condition. Refer to General Electrical Diagnosis in Wiring Systems. Was a condition found? | -- | ||||||||
6 | To verify that circuits 1224 (PNK), 1225 (DK BLU) and 1226 (RED) are not shorted together, use a fused jumper to ground on each circuit , while monitoring the Scan Tool TFP Switch display. When a range signal circuit is grounded, are the other range signal circuits affected? | -- | Go to Transmission Fluid Pressure Manual Valve Position Switch Resistance Check | |||||||
7 | Replace the TCM. Refer to Powertrain Control Module Replacement/Programming in Engine Controls. Re-use the PROM (Mem-cal.) Is the replacement complete? | -- | -- | |||||||
8 | Repair the affected wiring as needed. Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | -- | |||||||
9 | Perform the following procedure in order to verify the repair:
Important:: Before performing this test be sure to apply vehicle parking brake.
Refer to the Range Signal table. Does each selected transmission range match the Scan Tool TFP Switch A/B/C display? | -- | System OK |
