Hello all. Looking for some help for my brother. He has a 2007 TL. He thought the alternator was bad, replaced it. The alternator was only charging around 12V and then over period of about 5 to 10 minutes, dropped down to the point where the fans stopped working and the car stalled. He then thought it was the battery, and replaced the battery with a brand new one. Got a battery, and it was still only charging while idling about 12.5 V. The fan would kick on, it will drop down to 12.2 but then back up to 12.4/12.5. I know the normal charging range should be between 13 and 14 while idling. What could be causing such a low charging amount? Is there something else he should look for? He just took the car for a 10 minute ride, came back, let it sit, and then came back to check it and it was down to 11.9, and then 12.10. Why is it charging so low? Could it be the alternator is just a junk one? He did get it from AutoZone, but between a brand new alternator and a brand new battery, I’m kind of stuck on what else could be causing such a low charging voltage. Also his light was coming on, and then shutting off intermittently. Thanks in advance for your help.

 

Check the ground cables.

 

That was going to be my next suggestion. Would they cause a reduction in charging power like that?

 

Just kind of scratching my head with this. The charging system essentially consist of the battery in the alternator. Both are brand new. And it’s still not charging at four capacity. And furthermore, the charging output has gone down ever so slightly since the two new things were installed. That almost leads me to believe that the battery is just slowly dying, and the alternator is not working??

 

Check the voltage directly at the alternator. Use the big stud as + and alternator body as ground. If meter still shows below 12V then alternator is not working.

If still below 12V then take a look at this:

You want to make sure that all fuses listed here are good. There are 4 of them. Use test light to test them. Some of them get powered only when key is in ON position.
If all fuses good, unplug the alternator and check if you have battery voltage at Black/Yellow wire (at that plastic connector). Check if you have battery voltage at the big alternator stud (at the main wire).

If all checks out, then I would assume alternator is bad. There is a chance ECM can go bad, but unlikely.

Checking grounds and positive battery cable is not a bad idea either. I doubt there is something wrong with main ground or motor ground if it can pass enough current to start the engine, but maybe something is not right with positive cable?