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 available:
- Technical Service Bulletins (TSBs).
- Wiring Diagram.
These provide the information required when diagnosing transmission concerns.

1 Can be purchased as a separate item.

Air Conditioning (A/C) Clutch
An electromagnetic clutch is energized when the clutch cycling pressure switch closes. The switch is located on the suction accumulator/drier. The closing of the switch completes the circuit to the clutch and draws it into engagement with the compressor driveshaft. When the A/C clutch is engaged, Electronic Pressure Control (EPC) is adjusted by the Powertrain Control Module (PCM) to compensate for additional load on the engine.

Manifold Absolute Pressure (MAP) Sensor
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
- 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, proceed to Diagnosis By Symptom for further information and diagnosis. 4R100
Follow the diagnostic sequence to diagnose and repair the concern the first time.