FJCruiser Undervoltage Issue

Hey, I had almost the same issue with my FJCruiser a while back. You've probably checked the usual suspects, but what's the battery voltage reading when the engine is off, then while cranking? Have you load-tested the battery? A voltage drop below 9.6 volts during cranking usually indicates a problem with the battery's ability to deliver current. Also, check the battery terminals and ground connections for corrosion, as even a little bit can cause a voltage drop.

Thanks for the feedback. I measured a voltage drop to 9.1V during cranking. Terminals look clean. What was the specific cause in your case?

9.1V is below spec. In my case, it turned out to be a failing voltage regulator within the alternator. While the alternator was still charging, the voltage regulation wasn't functioning correctly, which led to inconsistent charging and eventually, premature battery wear. But definitely take it to a trusted workshop to perform a complete load test and diagnose the charging system properly before throwing parts at it. My repair bill came to about 300€

Appreciate the detailed explanation! I took the vehicle to the workshop. The battery was indeed heavily discharged due to the voltage regulator. Replaced both, with a final cost of 290€. Thanks again for your insights.