Q: Alternator and battery issues. Car has trouble starting.

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I replaced my alternator about 2 weeks ago. The alternator cannot keep the battery charged and drains it down. Initially, the battery light was coming on after driving for about a 1/4-mile and would stay on until the car was shut off. Recently, the battery light started coming on immediately upon startup and the car started dying. I replaced the battery, but soon thereafter the battery light came on again. While driving on the freeway, the motor went into LIMP MODE when I drove over 45 MPH. Once I got home, I turned the car off and tried starting again. At first it would not start, but I eventually got it to turn over.

From all the detailed description you provided me, I have been able to narrow down the problem with your Jeep to a failure with the PCM (Powertrain Control Module). It is the computer that controls the engine, as well as regulates the voltage externally for the alternator. Most alternators have an internal voltage regulator. If the voltage regulator fails, it will cause the PCM to not function properly and in turn cause the problems you were experiencing, such as the vehicle going into LIMP mode. I would next have a certified mechanic come out and verify this repair. A mechanic can confirm and repair the PCM, however your Liberty will need to visit a dealership to have the PCM reprogrammed.

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