Slow Starter Motor Points to Terminal 30 Power Issues

I had a very similar issue with my Audi A4 1.8T (2002). With some experience working on cars, I found that slow cranking was indeed related to the charging system. After checking the obvious battery connections like you did, I discovered the battery was heavily discharged. The starter motor was struggling due to insufficient power. My workshop performed a full diagnostic and found that while the wiring harness was intact, the battery had reached the end of its life. They tested: Battery capacity and voltage, Alternator output, Circuit breaker function, Main power distribution points, Starter motor draw The complete diagnosis and battery replacement cost me 290 Euro, but it solved the starting issues completely. Before suggesting specific solutions, could you share: How old is your current battery? Does the battery warning light come on while driving? Have you noticed any electrical issues like dim lights? Did you recently install any aftermarket accessories? This information would help determine if your issue matches what I experienced or points to a different problem in the ignition switch or elsewhere in the circuit.

Last service at 154412 KM Thanks for the detailed response. After reading through the suggestions, I tested the voltage drop across the terminals with a multimeter and found significant resistance in the power supply line. The reading shows 0.8V drop between the battery and starter, which seems excessive. I also noticed the circuit breaker gets quite warm during starting attempts. The battery is only 18 months old and holds 12.6V when fully charged, so Im confident its not the primary issue. No warning lights appear during operation. The starter was replaced 2 years ago, but the original ignition switch and wiring harness are still in place. Would testing the voltage at different points along the Terminal 30 line help isolate where the resistance is highest? Looking to avoid throwing parts at the problem without proper diagnosis.

Thanks for those additional details about your A3. Your voltage drop reading of 0.8V definitely points to a resistance issue in the power supply circuit, I experienced similar symptoms before finding the root cause in my A4. The warm circuit breaker is particularly concerning, as this can indicate excessive current draw or a failing component. When I tackled my starting problems, I used a voltage drop test method: 1. Tested voltage at battery terminals (12.6V like yours) 2. Measured at the main power distribution block 3. Checked the circuit breaker connections 4. Tested at the starter solenoid The highest voltage drop occurred at the circuit breaker connections. After cleaning and retightening these connections, the starting performance improved significantly. The electrical fault was caused by oxidation at these connection points, creating resistance in the charging system. Since your starter motor is relatively new, I suggest focusing on: Cleaning and securing all ground connections, Inspecting the circuit breaker mounting points, Testing voltage drop across each major connection point These steps helped diagnose my starting issues without unnecessary parts replacement. The repair cost for cleaning and rebuilding connections was around 150 Euro.

Thanks for the thorough technical details. Your advice about voltage drop testing makes perfect sense. I did some additional checks following your method and discovered the main culprit, severe corrosion at the fuse box connection where the Terminal 30 line connects to the main power distribution block. After carefully cleaning these connections and applying dielectric grease, the voltage drop decreased to 0.3V and the starter motor now spins much faster. The circuit breaker also runs cooler during starting. The ignition switch connections looked fine when I checked them. Im still planning to have the entire charging system professionally tested to ensure no underlying electrical fault remains, but the immediate improvement is encouraging. Appreciate the systematic troubleshooting approach instead of just replacing parts out of nowhere. One question remains, should I consider upgrading the wire gauge on the Terminal 30 circuit while I have everything apart? The original wiring looks a bit undersized compared to newer models.