Diagnostic Strategy

Diagnostic Strategy
Troubleshooting an electronically controlled automatic transmission is simplified by using the proven method of diagnosis. One of the most important things to remember is that there is a definite procedure to follow.

NOTE: Do not take any short cuts or assume that critical checks or adjustments have already been made.

Follow the procedures as written to avoid missing critical components or steps.

To properly diagnose a concern have the following available:
- TSBs.
- Wiring Diagram.

These provide the information required when diagnosing transmission concerns.

Use the Diagnostic Flow Chart as a guide and follow the steps as indicated. Diagnostic Flow Chart

Preliminary Inspection
- Know and understand the customer's concern.
- Verify the concern by operating the vehicle.

Vehicle Speed (VSS) Sensor
The source of Vehicle Speed (VSS) is model dependent. Possible sources of vehicle speed input are Anti-Lock Brake Sensor (ABS), a gear-driven Vehicle Speed Sensor (VSS), or an Output Shaft Speed Sensor (OSS). The VSS signal is either an A/C signal whose frequency changes with speed, or an SCP data message depending on the source. Some applications will have both. The vehicle speed signal is an input to various vehicle subsystems such as the Powertrain Control Module (PCM), instrument cluster (speedometer and odometer), speed control system, etc. The vehicle speed source must be operational to enter Output State Control (OSC) mode for diagnostics.

Regardless of the type of vehicle speed system, the PCM always uses the OSS for transmission and engine control.
- Check the fluid levels and condition.
- Check for non-factory add-on items.
- Check shift linkages for proper adjustment.
- Check TSBs regarding the concern.

Diagnostics
- Carry out on-board diagnostic procedures Key On Engine Off (KOEO) and Key On Engine Running (KOER).
- Record all diagnostic trouble codes (DTCs).
- Repair all non-transmission codes first.
- Repair all transmission codes second.
- Erase all continuous codes and attempt to repeat them.
- Repair all continuous codes.
- If only pass codes are obtained, refer to Diagnosis By Symptom for further information and diagnosis. 4R70W

Follow the diagnostic sequence to diagnose and repair the concern the first time.