What does it mean?
P1635 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Generator Battery Terminal Open Circuit Malfunction .
Symptoms
- Battery warning lamp and MIL (check engine lamp) on together
- Battery going flat, hard starting or a no-start after a short journey
- Headlights dim, wipers slow and dash lights dropping at idle
- Electrical accessories cutting out under load
- Burning smell or a discoloured, heat-damaged terminal at the alternator
- Voltage at the battery staying near 12 V or falling with the engine running
Likely causes
- Loose, corroded or burnt alternator B+ output terminal and its nut
- Main charging cable open, or its fusible link or high current fuse blown
- Corroded ring terminal at the battery post or at the fuse box end of the charging cable
- Alternator internally failed, with an open diode pack or a broken internal output connection
- Charging cable damaged inside the insulation, with strands broken after chafing or heat
- Poor engine-to-body ground so the charging circuit cannot complete
- Cable disturbed and not properly retorqued after a previous repair
How to fix it
- Measure battery voltage with the engine off, then running; a running value no higher than the resting value confirms no charging
- Check the main charging fuse or fusible link first, as it is the most common single point of failure
- Inspect the alternator output terminal, retorque it and look for heat discolouration or a melted insulator
- Voltage-drop test from the alternator output stud to the battery positive post with electrical loads switched on
- Check the alternator ground path and the engine-to-body ground strap the same way
- Inspect the drive belt tension, condition and the pulley or one-way clutch where fitted
- Replace the cable, terminal or alternator as the tests identify, then verify charging voltage under a heavy electrical load