Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Engine Control System Self-Diagnostics - Diesel: Introduction
The Powertrain Control Module (PCM) controls Torque Converter Clutch (TCC) engagement, transmission, fuel control and fuel injection timing. The PCM also monitors a number of transmission functions and uses the data to control shift solenoids, TCC, and pressure control solenoid to regulate upshift pattern, downshift pattern and line pressure (shift quality).
Most engine/transmission control problems are not computer related, but result from mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, carefully perform visual and mechanical inspections covered in appropriate BASIC DIAGNOSTIC PROCEDURES article. Failure to perform these inspections can result in lost diagnostic time.
If no faults were found while performing visual and mechanical inspections in appropriate BASIC DIAGNOSTIC PROCEDURES article, proceed with diagnosis of self-diagnostic system. See SELF-DIAGNOSTIC SYSTEM . If no Diagnostic Trouble Codes (DTC) are set and driveability problems exist, proceed to appropriate TROUBLE SHOOTING - NO CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). If only intermittent DTCs are set, see INTERMITTENTS in appropriate TROUBLE SHOOTING - NO CODES article.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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