VW Touareg Starting Issues Linked to Terminal 30 Power

Had nearly identical symptoms with a 2018 Tiguan. As someone who regularly does basic maintenance but leaves electrical work to professionals, this one was tricky. First thought was also the battery showing good voltage, but the workshop found an electrical fault in the charging circuit. Despite normal voltage readings, load testing revealed the battery was actually failing under starting demands. The starter motor would get insufficient power, leading to those weak cranks. A full diagnosis revealed: Battery tested fine at rest but failed under load, No short circuit issues in the wiring, Circuit breaker and ignition switch tested normal, Alternator output was within spec The workshop did a thorough voltage drop test across the entire starting circuit. Even though the battery showed 12.6V, it couldn't deliver enough amperage when needed. Final fix: New battery plus cleaning all terminal connections. Total cost was 290€ including diagnosis. To better assist with your case, could you share: Battery age, Any warning lights on dash, Whether alternator was tested, If problems occur more in cold weather, Recent electrical upgrades or accessory installations

Thanks for checking. My car has 64980km and had its last service 3 months ago. After reading your analysis, I did some basic checks today. The voltage drop test across battery terminals shows 0.7V during cranking, which seems high. Found slight corrosion at the main ground connection to chassis. The wiring harness near the fuse box shows signs of heat damage, possibly from engine bay temperatures. This could explain the intermittent nature of the problem, as connections might expand/contract with temperature changes. Would cleaning ground points and inspecting the wiring harness for potential short circuit spots be a good first step before replacing parts? The voltage drop reading concerns me most, as it points to high resistance somewhere in the circuit.

Thanks for those additional details. Looking at the voltage drop you measured, it definitely mirrors what I found on my Tiguan before getting it fixed. That 0.7V drop across the terminals is a classic sign of connection issues. My voltage drop was around 0.65V and the corrosion at the ground point was actually worse than it looked on the surface. The fuse box connections were also affected by similar heat damage you described. Here is what worked in my case: Cleaned all ground connections thoroughly with electrical contact cleaner, Replaced corroded battery terminals, Had the workshop inspect the entire starter motor circuit for voltage drop, They found a deteriorating wire in the main harness near the fuse box causing intermittent electrical fault conditions The wiring inspection is crucial, my harness damage wasnt obvious until they traced the whole circuit. It was causing resistance spikes during high current draw from the starter motor. Would strongly recommend having the complete circuit checked. The combination of corroded grounds and potential harness damage points to a compound issue, just like in my case. While cleaning connections will help, the heat-damaged section near the fuse box likely needs professional attention to prevent future starting problems.

Your diagnosis steps sound spot-on. The voltage drop measurement across the battery terminals during cranking has revealed a potential starting point for the electrical fault investigation. A 0.7V drop is definitely excessive, the starting circuit should show much lower resistance. The corrosion at the ground connection combined with the heat-damaged wiring near the fuse box creates a perfect scenario for voltage drop issues. My next steps would be: Clean all ground connections with proper electrical contact cleaner, Check resistance at each connection point in the starter circuit, Trace the wiring harness carefully, paying special attention to areas showing heat damage, Test voltage drop between the fuse box and starter motor during cranking The intermittent nature matches perfectly with heat-cycle related connection problems. Before replacing any major components like the starter motor or ignition switch, fixing these basic connection issues could solve the problem. Will start with cleaning the grounds and checking voltage drops at each major connection point tomorrow. Will update once I measure the circuit with clean connections and can isolate exactly where that 0.7V is being lost in the system.