This is regarding my 2006 GS300 I purchased in December 2008.

June 2009, battery died while cleaning my interior with ML stereo on, engine off. Lexus replaced the battery under warranty.

01/10/11, try to start up the car for work, no response, I was able to jump it and drive it to the stealership (~10 mile drive). They call back saying they tested the battery, it failed, they tested the alternator, it also failed. They described the failure as: bad diodes, quoting the alternator replacement to be $840. yikes. They replace the battery free of charge under their battery warranty. Car starts strong and normal at this point.

I wanted a second opinion, drove the car to my local mechanic, who took a few readings from the battery terminals, his recommendation being the alternator seems to be working fine, the battery charge is normal. He states if the alternator was bad, the drive from the dealership to the shop (~10 miles) would affect the battery charge.

The alternator in our lovely 3gs's are expensive (~$500 parts price). Unfortunately, I am out of the Lexus Warranty, and did not purchase an extension.

I am not sure where to go from here, car still starts fine, no signs of a bad alternator (which I've had plenty of experience with), no warning lights on the dash, the only odd thing I find is a 1.5 year old battery dying overnight. Obviously, I do not want to live with the fear of car dying at any point, or any given morning, at the same time do not want to dish out $840. I'd like to add that I am also very meticulous about not leaving interior lights on, leaving doors open, or any kind of electronics without the motor running since my June 2009 incident.
Any recommendations from similar situations?

 

If the alternator were bad, you should get the battery icon on the dashboard right?

It's difficult to test what the problem is yourself. With a multimeter, if you are getting around 14.5 volts at the battery posts while the car is running the alternator should be good from my understanding. You can try testing it at idle before a drive, and again when you return after everything is hot and see if perhaps something happens during a drive that makes the alternator stop working properly.

I would guess that a bad alternator would have left you with another dud battery far sooner than the interval you mentioned unless you've been charging it manually. You'd run out of battery power really fast if the alternator wasn't charging it.

 

I appreciate your input BinaryJay, will try what you recommend and post results, I have not been charging it manually in anyway, I agree that I would run out of battery power alot quicker than 1.5 years. I'm hoping the stealership is justifying their faulty battery going bad by trying to blame my alternator, of course I'm not putting my money on my hopes.

 

should be easy to tell. with your car running, put your headlights on and test the voltage for the alternator. it should be around 14 volts. you can test the battery the same way except with the car and headlights off. check the voltage and it should be around 12 when off.

to me it sounds like a bad battery. if you want to replace your alternator you can find them any where from $125 to $200 on ebay and the same on car-part

 

Automotive batteries should test at well over 12V, I believe that most consider 12.9 volts to be a fully charged battery. One that is testing 12.6 volts is already probably a quarter discharged or more.

 

yea when the car is running it should.

 

No, the battery itself. When the car is running you're no longer testing the voltage of the battery, you're testing the output voltage of the alternator which should be around 14.5.

 

By the way, it appears that the alternator part is shared between the GS and the IS, at least for V6s, so you can go ahead and purchase one from a wrecked IS250/350 which are far more plentiful.

Since I'm keeping this car for a long time after it's out of warranty, and have had an alternator fail on me in the past, after I started looking into this I couldn't help myself and I purchased one I found from a donor 09 IS250 with only 18,000 km on it from an auto recycling yard in Ontario for $100 just to have around as a backup. That's like 10 times less than what an OE part purchased through a local dealership would run and should be good as new. Cheap insurance. Wouldn't surprise me if the same part ends up getting used in the next gen as well.

Our alternators are listed as rated 150A for those who are just curious.

 

Thanks,great tip