P1637 - FAULT CODE - OBD2

POST UPDATED IN February, 2023

P1637 Code failures by brands

P1637 BUICK, CADILLAC, CHEVROLET, GMC: GENERATOR L-TERMINAL CIRCUIT

Description of DTC code P1637

The generator L-terminal circuit is discrete, i.e. this circuit has no junctions and only one source and one destination in the PCM (Powertrain Control Module). This controller applies the ignition voltage to the generator L-terminal circuit. A small amount of voltage flows from this circuit through the generator windings to create a magnetic field that starts the generator process. When the generator is at operating speed and producing voltage, the solid-state switch for the generator’s L-terminal circuit opens. The PCM senses that the initial starting current flow has stopped.

The PCM expects to detect low voltage on the L-terminal circuit before the generator turns to operate speed. Conversely, expects the circuit to be at ignition voltage potential when the generator is running. If the PCM detects a fault (short or open circuit), the diagnostic code P1637 OBDII will be set.

Symptoms of fault code P1637

  • Check Engine lamp illuminated.
  • You will not be able to start the engine.
  • Problem with gears.
  • The engine is unstable.
  • Car voltage drops.

Causes of OBD2 P1637

DTC P1637 OBD2 is set for one of the following reasons:

  • The generator (alternator) wiring may have been damaged.
  • The generator circuit may be faulty.
  • The alternator may be in poor condition.
  • The PCM may have programming errors.

Possible solutions of the DTC code P1637

If you have no knowledge of mechanics, it is recommended that you go urgently to the nearest garage in your area. A professional in the field will be able to detect and solve this code without any problems. On the other hand, if you have the necessary knowledge to repair this code, you can follow these possible solutions to troubleshoot the P1637 OBDII fault code:

  • Consult the Technical Service Bulletins.
  • Verify that the generator wiring and connectors are not damaged. Make the appropriate repairs if you discover burns or wear on any of these.
  • Check that the alternator connections are in optimum condition for proper operation. Repair if you discover improperly tightened connections that could cause an open or short circuit.
  • Check the alternator and verify that it is not damaged. If inconsistencies are found on the alternator, the PCM may require reprogramming.
  • You can also use an OBD2 meter that will be helpful in removing this code.

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