Jeep Not Charging? Don’t Blame the Alternator Just Yet...

Battery light on? Jeep randomly going flat? Voltage all over the place?
If you’re experiencing charging issues, your first instinct might be to replace the alternator—but that’s not always the fix, especially on many Jeep models where the alternator is PCM-controlled.

From the mid-2000s onwards, Jeeps began using a system where the Powertrain Control Module (PCM) regulates the alternator output by controlling the field circuit. This means the alternator relies on the PCM to tell it when and how much to charge. So if the voltage regulator inside the PCM fails, the alternator won’t function properly—even if it’s brand new.

🛠️ How to Test Before Replacing the Alternator

Before spending hundreds on a new alternator, here’s how you can test if the alternator itself is fine:

1. Check for codes: A basic OBD2 scan can reveal voltage regulator or charging system faults. But a lack of codes doesn’t rule out PCM issues.
2. Measure battery voltage: With the engine running, check voltage across the battery terminals. It should be between 13.5V and 14.5V. Anything consistently lower (like 12V or less) usually means it's not charging.
3. Bypass the PCM control: On many Jeep alternators (like the Denso units), you can apply 12V to the field control pin (usually labeled “F” or “IG”) while grounding the case. If the alternator starts charging normally, it’s working fine—and the PCM isn’t sending the correct signal.
4. Inspect wiring: Make sure the alternator field wires from the PCM are intact and not corroded or broken. But if wiring checks out and the alternator charges when manually excited, it’s pointing straight at a faulty PCM voltage regulator.

🔄 Unfortunately, we’ve seen customers replace one or even two alternators without solving the problem—only to find out the PCM was the issue the whole time.

💡 Save yourself time, stress, and money.

At ECUHack, we can test your Jeep’s PCM on the bench and confirm if the internal voltage regulator is functioning as it should. If it's faulty, we offer replacement PCMs that are fully tested, programmed, and ready to plug in.

Message us today if your Jeep’s showing charging issues—we’ll help you get it sorted properly the first time.