I have a 2007 IS250, and about a year and a half ago, my battery light came on. I thought my alternator had gone bad, so I replaced both the alternator and the battery just to be safe, but that didn't make the battery light go away. I then changed my serpentine belt, thinking it might be old and running loose, but that didn't fix the issue either. I checked every single fuse and relay related to the car's charging system, with no luck. I even removed all grounding wires, cleaned them at their connection points, and replaced the battery terminals. I got so frustrated that I eventually gave up. It's been almost two years since the light came on, and the car is driving fine, but that light is driving me crazy. I'm wondering if anyone knows what could be causing the light to stay on for this long without affecting the car's performance. Thanks

 

OEM alternator? Have you verified the ground cables that go from the battery to the chassis and the battery to the engine block are in good condition with no voltage drop?

 

yes, it’s OEM, and yes, I’ve have checked all the ground wires and cleaned them.

 

Are there any codes coming up with a scan? Oh and I saw that you checked the cable ends but did you do a voltage drop test? Using a voltmeter put the positive lead on the battery +, then use the negative voltmeter lead to check the battery - and then the ends of both of those cables. I've seen the terminals look decent but the inside of the negative cables be filled with green crusties and you might see 2-4 volts difference between the negative terminal and the ground cable ends.

It's just something I check no matter what vehicle.

GL!

 

I just Checked for code and there is no code. also I check the Voltage from the battery terminal and from the the positive connector of the alternator, it reads 13.786v while the engine is idled, and 12.94 when the engine is off. this is the weirdest issue I have ever ran into.

 

When I'm talking about voltage drop maybe an example will help. Take a gander at this video:

 

Any aftermarket stuff in the vehicle? If not, if you haven't owned it since it was new, have you checked under your dash for a dealer-installed GPS tracker possibly? This would likely go hand in hand with Protholl's suggestion for a voltage drop test. Or have you checked for any rodent damage to your wiring?

 

Wow I didn't think about rodent damage because I live in the south. Good call.

 

So, looking at the schematic, the alternator has two wires that go to the engine ECU labeled L and M at the alternator, while the ECU they are RL and ALT. These leave the ecu on CANH and CANL which go to the Gateway ECU and leave on the MPX+ and MPX- which connect to the combination meter.

The kicker is MPX+ goes through the A/C amplifier ECU and the MPX- goes through the TPMS ECU.

I'd suggest you connect this car to Techstream and look at eachaof the modules I've listed. It will do a system check of each one.

Do any of the listed modules have issues in your vehicle? There are a lot of junctions along these paths but I suspect if any failed, other systems would have faults.

 

#2IS troubleshooting
#2IS electical
#2IS charging system

 

OP - do verify the 10A MPX-B fuse in your car is good. It's up by the battery.

@natenb21