Diagnosing Mk4 VW Alternator on the Troublesome Jetta

[@rickpontificates3406]

Friend of mine has a 2009 Jetta that's "extra". She drives for about 10 mins and the engine shuts off with a CURRENT CHARGE FAIL, but it always starts right back up. Alternator was checked "good" at the store. Is there maybe a signal wire on the regulator that sends a "OK" or "FAIL" signal to the CPU that I can put a scope on, or does the CPU monitor the regulator output as an analog voltage?

[@CliffjumperCars]

I've heard the Mk5's have the same issue as the Mk3 and Mk4 with the battery ground cable and alternator positive cable getting internal corrosion and introducing excessive impedance to the system, resulting in decreased voltage and amperage. I'd check those for resistance as a first step. Beyond that it's a CANBUS electrical system, so it's quite possible diminished voltage is tripping faults in the ECU or other control modules.

[@rickpontificates3406]

@ thanks for your reply. I'm gonna look at this car tomorrow. I've already replaced multiple coil packs in this car more than once so it's no stranger to electrical problems. VWs are awful cars! In fact, one of my coworkers bought a 2023 Jetta a year ago and recently, he was driving and all the instrument lights came on and the engine died. His car was towed to the dealer and it's getting a new engine.