2005 Type X. Battery light goes on then off periodically while driving. The Diehard battery w/18 months, checked at parts store found to be in good state, then tested w/ engine running found to be in good charging state. Any thoughts appreciated.

 

Two possibilities come to mind. I have experienced both. First, clean and polish the battery and cable terminals bright and tight. Second with a helper watching and the engine running move the cable ends around and check for the light coming on. The cable ends are known for inner corrosion.

 

jrdelohery, I want you to try something for me. Let the car sit for a few hours. YOu want the motor down at ambient temp. Now, start up the car and turn on everything electric (dash fan on high, A/C on lowest temp, seat heaters on, headlights on, rear defrost on). Let the car idle for say 5 minutes. You can leave the car running or turn it off, up to you. But, immediately open up the hood of your car and remove the cover off of the battery. Now, cup your hand over (DO NOT TOUCH!!!!!) the battery posts. Do they feel about the same temp as the rest of the parts under the hood of the car or is one of them hot enough to cook an egg on it? The one that is really hot is your problem. May be the positive cable, may be the negative cable. But, this cable will need to be replaced. Takes about a half hour to replace once you get a new cable. If you have a second car, then you can remove the bad cable, take it to your local auto parts store and they can match up one they have in stock to what you have in your hands. Replace and you should be golden.

 

Problem solved per your suggestion. Thank you.

 

Battery light back on, drove to mechanic (light still on). Amps or such should be 14+ range? He saw a drop to 13 surge to 15, feels it's time for alternator replacement (122k miles of service) and must agree.

 

Replace the ground (negative) cable or add a new one to it and the battery light will disappear. I had the same problem.....

 

jrdelohery, the voltage in this car will jump around some. What you are seeing I would not call it abnormal. I have seen very few alternators fail in the X-Type that were true alternator failures. Most people replace their alternators only to find that they still have the same issue. I bet you will find that Jag82 and myself will prove that you have a battery cable that is failing and that is causing your issues. Do as I recommended and see what it reveals. It is free and only takes a little bit of your time.