Diagnostic Strategy

Diagnostic Strategy

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

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.

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

To diagnose a concern correctly, the technician should have the following publications available:
- Technical Service Bulletins (TSBs).
- Wiring Diagram.

These publications provide the information required when diagnosing transmission concerns.

Barometric Pressure (BARO) Sensor-7.3L DI Diesel Only
The Barometric Pressure Sensor (BARO) sensor operates similarly to the Manifold Absolute Pressure Sensor (MAP sensor). It measures barometric pressure instead of intake manifold pressure. The Powertrain Control Module (PCM) uses the signal from the BARO sensor to determine the altitude at which the vehicle is operating. The Powertrain Control Module (PCM) then adjusts the transmission shift schedule and EPC pressure for the altitude.

Manifold Absolute Pressure (MAP) Sensor-Gasoline Engines
On gasoline engines, the Manifold Absolute Pressure (MAP) sensor senses atmospheric pressure to produce an electrical signal. The frequency of this signal varies with intake manifold pressure. The Powertrain Control Module (PCM) monitors this signal to determine altitude. The PCM then adjusts the transmission shift schedule and EPC pressure for altitude. On diesel engines, the MAP sensor measures boost pressure. The PCM monitors this signal and adjusts EPC pressure.

Using the Diagnostic Flowchart as a guide, follow the steps as indicated.

Preliminary Inspection
- Know and understand the customer's concern.
- Check the fluid level and condition.
- Verify the concern by operating the vehicle.
- Check for non-factory add-on items.
- Check shift linkages for correct adjustment.
- Check TSBs for the concern.

Diagnostics
- Vehicles that have power take-off capability will not have on board diagnostic capability when the power take off is in operation.
- 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 which reappear.
- If only pass codes are obtained, refer to Diagnosis By Symptom for further information and diagnosis. 4R100

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