* Alternator Diode Failure: This is the most likely culprit. A failed diode in the alternator allows current to flow backward, causing a voltage spike. When the engine is off, the problem disappears because the alternator isn't spinning.
* Voltage Regulator Problem (within the alternator): The voltage regulator controls the alternator's output. A malfunctioning regulator could overcharge the battery, leading to the voltage spike. This is closely tied to the diode issue; a bad regulator can *cause* diodes to fail.
* Wiring Issues: A short circuit in the wiring related to the alternator or the voltage gauge itself could cause erratic readings. This could be a frayed wire, a corroded connection, or a problem with the gauge's ground.
* Battery Issues (less likely): While less probable given the description, a severely degraded battery *might* exhibit unpredictable voltage behavior, especially under load. However, this is less likely to cause a full-scale spike.
* Faulty Voltage Gauge: It's possible, though less likely, that the gauge itself is faulty and providing inaccurate readings.
What to do:
1. Have the Alternator Tested: This is the most crucial step. Take your Jeep to an auto parts store; most will test alternators for free. They'll check the output voltage and assess the diodes. If the alternator is faulty, it needs replacement.
2. Inspect Wiring: If the alternator tests okay, carefully examine the wiring harness leading to and from the alternator, as well as the wiring connected to the voltage gauge. Look for any signs of damage, corrosion, or loose connections.
3. Battery Check (less urgent): Have your battery tested for voltage and load testing (to check its ability to hold a charge under load).
Don't delay: Driving with a faulty charging system is risky. Continued overcharging can damage your battery and other electrical components. Get it diagnosed and repaired promptly.
