VW T5 Alternator Problems: Warning Signs & Diagnosis

I had nearly identical issues with my VW T6 2.0 TSI from 2011, and yes, it turned out to be a failing alternator. Given my background doing basic maintenance and repairs, I first suspected a charging system problem when my battery warning light came on. The first sign was dimming headlights when idling, then the electrical problems got progressively worse. My mechanic ran a voltage drop test that confirmed the alternator was only putting out 11.2V instead of the normal 13.5-14.4V range. The battery was getting drained because the charging system wasnt keeping up. The full alternator replacement, including parts and labor, cost me 960€. The job took about 4 hours since they had to remove several components to access it properly. Since the repair, all electrical issues disappeared and the car runs perfectly. To properly diagnose your case, could you share: Current battery voltage readings if you have them, Age of your current battery, Whether the serpentine belt has been replaced recently, If you notice the issues more during cold starts or when the engine is warm A proper voltage test while the engine is running would definitely confirm if your alternator is failing. Given the symptoms match mine exactly, I strongly suspect you're dealing with a classic alternator replacement case.

Last service was at 105000km and here is what I found based on the previous discussion: After checking voltages with a multimeter, I got readings around 11.8V while running, which confirms the suspected alternator issue mentioned earlier. The serpentine belt was replaced 20000km ago and looks fine, so that is not the culprit. The battery is only 1 year old. The issues are definitely more noticeable during cold starts, and all electrical systems strain heavily when using multiple accessories. The dimming headlights and battery warning light perfectly match the alternator problems described in the previous reply. The voltage readings are clearly showing the alternator is not charging properly. This aligns with the classic symptoms of alternator failure, the car won't start reliably and the car electronics are struggling. Will schedule the replacement soon before it leaves me stranded.

Thanks for sharing those voltage readings, they definitely confirm what we suspected with the alternator issue. Getting only 11.8V while running is way too low for proper charging system function, which explains all those symptoms youre experiencing. When I had my T6 fixed, the mechanic mentioned that readings below 13V while running almost always indicate a failing alternator. Your 11.8V is even lower than my 11.2V was when mine failed. Good thing you caught this now, my alternator completely died shortly after seeing similar readings. The fact that your serpentine belt is relatively new and the battery is only a year old really helps narrow down the alternator as the culprit. On these VW engines, when the alternator starts failing, it typically shows exactly these symptoms: car won't start reliably, weak electrical systems, and battery warnings. Based on my experience, I would not wait too long to get this fixed. When my alternator finally gave out, all electrical systems shut down while driving and the car electronics went completely dead. Get it replaced soon to avoid being stranded like I was. Make sure to get the charging system thoroughly tested after replacement, my mechanic did this and found it restored proper voltage to 14.2V at idle.

11.8V while running is definitely concerning, especially combined with all these classic alternator symptoms. After reading about the experienced voltage drop and similar issues, I'm convinced replacing the alternator is the right call. The mechanic should be able to do the full repair in about 4 hours based on the previous reply. The fact that my battery is only 1 year old and the serpentine belt was replaced recently helps confirm the charging system issue is isolated to the alternator itself. Having the voltage readings really helped diagnose this, much better than guessing or replacing parts unnecessarily. All the symptoms match perfectly with a failing alternator: dimming headlights, electrical issues with accessories, and those problematic cold starts. The car electronics struggling under load is exactly what happens with insufficient charging voltage. I'll get it scheduled for replacement this week before the alternator fails completely. No point risking getting stranded when the diagnosis is this clear. The voltage readings are well below the 13.5-14.4V range needed for proper charging system operation. Will make sure they test the charging system thoroughly after installing the new alternator to confirm it resolves these voltage drop issues. At around 960€ for parts and labor, it's not cheap but seems worth it to fix such a critical component properly.