What does it mean?
P1638 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Generator F-Terminal Circuit .
Symptoms
- Battery warning lamp and MIL (check engine lamp) on
- Charging voltage too high or too low, so the battery either goes flat or is overcharged
- Dash and headlights dimming or flaring as engine speed changes
- Battery not holding charge and hard starting in the morning
- Idle speed not compensating when heavy electrical loads switch on
- A smell of hot electrolyte in the overcharging case
Likely causes
- Open, shorted or corroded wiring on the alternator field or monitor terminal circuit
- Voltage regulator failed and holding the field signal at a fixed state
- Alternator plug loose, heat damaged or full of green corrosion
- Drive belt slipping, glazed or a failed one-way clutch pulley
- Poor alternator ground or a weak engine-to-body ground strap
- Battery in poor condition, forcing the regulator to work at its limits
- Module input circuit fault, the least likely cause
How to fix it
- Measure charging voltage at the battery at idle and at raised rpm, with and without electrical loads
- Read the charging system live data and compare the commanded field value with the actual reported value
- Inspect the alternator connector for melted plastic, corrosion and loose or backed-out terminals
- Test continuity of the field or monitor wire from the alternator to the module and check for shorts to ground and battery positive
- Voltage-drop test the alternator output cable and the ground path under load
- Check belt condition, tension and the operation of the pulley clutch where fitted
- Replace the regulator or the alternator once the wiring is proven, then retest charging under a heavy load