I had a bad alternator a few months ago. Went and bought a new one at Checker's. Replaced the original 17yr old battery as well since it had a swell starting on the casing.

Now 3 months later, the car won't keep a charge more than 20 minutes with the accessories on. I put a multimeter on the new battery. It's at roughly 12.6-12.8 volts with the car off. Starting the car and it reads around 14.4 volts. Sounds good so far right? Turn on the A/C and headlights and it starts at 14.4 volts, but drops by 0.01 volts every 3-5 seconds. So after a minute it's at 14.15 volts and steadily, but slowly dropping. Which in fact explains why I can drive about 10 minutes at night before all the dash lights light up and the A/C nearly shuts down. Yes, it won't start after that. It needs to be jump started and even then only drives 3 minutes before going dead at a stop while in drive.

When I took the alternator back to Checker's for testing, of course it tests at an average of 14.6 volts. They test it for 3 seconds on the machine. The clerk hands it back to me saying it's good.

Is it possible for the alternator to put out sufficient voltage to test, but the amperage is too low to run everything, leading to the voltage drop at the battery over a short period of time? Is the regulator possibly bad?

It's a 1990 Lexus LS400.

Any ideas? Exchange the alternator again?

 

Hmm... maybe its a short somewhere? Do you have an alarm, cd player, amp, or anything like that that could be drawing a great deal of power?

But even if so, thats a REALLY short time for the car to die after driving it (only 10 minutes!?!) I had a 2005 Altima with 3000watts of stereo and my car, even with a bad alternator would run for a few hours.

Maybe your battery REALLY dead? I'm stumped man... haha

 

My car starts at 14.4 but idles at 13.5-13.8 cause the RPM drops after ur engine warms up. Make sure ur idleing at at least 600RPM or else the alt wont be putting out any real power and ur just draining ur battery.

 

The battery is 2 days old now. I took the 3 month old one in and the salesperson didn't even test it, just brought out a new one and sent me on my way.

The car doesn't have anything aftermarket. It's 100% stock. The factory radio is broken though. 3 speakers don't work. Only the front tweeters, subwoofer, and rear left speaker. Other than that, the car is in pretty good shape with ~136,000 miles.

It holds charge during the day idling in the garage with no accessories on. But turn on the A/C and headlights and all the dash lights come on in about 10 minutes. It'll die out idling in the garage like that. And then the starter won't even crank, just little ticking sounds from the solenoid.

 

It idles at around 700RPM. And the first time the dash lights all lit up, I was driving on the freeway! Cruising for 10 minutes at ~2,500 RPM, they all came on and it was the same as 3 months ago when the old alternator was completely dead. I did make it home this last time though. But after pulling in the garage, I turned the car off, and tried to restart it. Nothing, only the ticking noises. The car was dead after 20 minutes on the freeway!

 

Are u sure everything has a solid connection? My alternator wasn't putting out power and I thought it was dying. Turn out the connection on the alternator was loose and would randomly get disconnected.

 

Also take it to someplace like Autozone the machine could test to see if it actually puts out enough amps for ur car. Not just to test if its 14.4vs

 

I'll do that now. Another 30 minutes to remove it from the car. But if they can test for that, it's worth a shot!

Any other ideas in the meantime?

 

My sentiments exactly...me thinks that your alternator is not putting out enough amps, so when you load down your system, its just running a majority of the amps off the battery till its drained.

 

CHeck the alternator wiring the power wire might be corroded and the heat causes more resistance causing it to reduce power to the car.

 

Just finished removing it from the car. The 3 pronged connection was in solid. Very clean too. The single positive connection has zero corrosion. Shiny and was on tight. No problems there. I even used the multimeter on that power wire. It showed the same voltage as at the battery, so no problem with that particular wire as far as I can tell.

I hope Autozone is open on Labor Day...

 

They are I was just there getting my gxsr1000 battery tested

 

I was talking about the full length of the battery and alternator cables not just the ends. I would look at the terminals and grounds leaving the battery as well.

 

Did you get the correct alternator the lexus uses a 100 amp alternator the problem is coming from the alternator if you can't run everything and having the battery go dead. Make sure the power steering isn't leaking oil into the alternator this will cause big problems. The next time you have the alternator tested ask the tech to put a load on the unit and run it about 10 minutes this should show up the problem. And make sure it's an 100 amp unit.

 

Yes. Thats exactly what's happened. It happened to my 92 lexus. Power steering leaked into alternator. Fried the alternator. It took a couple alternator before we figured it out.