* Weak Battery: This is the most likely culprit. Even with a new alternator, a weak or failing battery may not have enough initial power to fully engage the starter and associated systems. The voltage drops causing the light, then after restarting (and charging) the battery has enough juice to run fine for a while. Get your battery tested at an auto parts store – they'll do it for free. A load test is crucial; a simple voltage check isn't enough.
* Poor Connections: Check all battery terminals and alternator connections for corrosion, looseness, or damage. Clean them thoroughly and make sure they're tightly secured. Loose connections can cause voltage drops, triggering the light.
* Alternator Wiring Issues: Although you have a new alternator, there might be a problem with the wiring to or from it. A broken or damaged wire can intermittently disrupt charging. This is less likely given the light only shows at startup, but it's worth looking at.
* Starter Motor Issues (Less Likely): While the light's behavior doesn't directly point to the starter, a very high resistance in the starter's windings can cause a voltage drop that triggers the light briefly at start-up. This is less probable than the battery/connection issues. If the engine cranks very slowly or struggles, then starter problems become more likely.
* Faulty Voltage Regulator (within the Alternator): While unlikely since you *think* you have a new alternator, it's possible the replacement alternator itself is faulty or its voltage regulator is not functioning perfectly. Even new parts can be defective.
Troubleshooting Steps:
1. Battery Test: This is the *first* step. Have the battery load tested.
2. Connection Inspection: Thoroughly clean and tighten all battery and alternator connections.
3. Visual Inspection of Wiring: Look carefully at the alternator wiring for any damage or loose connections.
4. Check the Alternator Output: With the car running, have someone test the alternator's output voltage at the battery terminals. It should be around 13.5-14.5 volts. A lower voltage indicates a charging problem, even with a new alternator.
5. If all else fails: Consider having the car's electrical system professionally diagnosed.
It's important to address this issue promptly. Driving with a consistently weak battery or charging problem can lead to further problems, leaving you stranded. A battery test is the cheapest and easiest place to start.
