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Ok new here to the forum . I have a 2009 sl 550. The red battery light has been on since i got the car. It sat for a while. Replaced the rear battery. Still on. Front battery is good reading mid 12vs engine off rear batter mid 14s with engine on as well at the front battery . Check the 100amp fuse Uber the subwoofer and that was good. I have a little code reader but only o2 sensors are popping up out of no where but nothing else. My buddy checked everything with a nice snap on computer and saw “picture below” . I heard that the ground cable has a sensor but none of mine do. I’m out of ideas lol. Helpppp thanks
On your 2009 car, the alternator probably is a little different to earlier cars . (If I have your VIN I can check exactly what alternator is fitted)
The voltage and current output is controlled by the main engine ECU (ME-SFI) and not by the normal inbuilt regulator inside earlier alternators.
This allows the alternator to run optimally depending on engine load.
The ME unit communicates with the alternator with a serial interface (LIN)
I enclose a document that might be relevant to your car.
so what do you think would be the root of the problem? I looked into the lin system . But most people with the red battery light had an existing code that played a part in it . That code that is in the picture was in the beginning but with my code reader i don’t see that code.
The document i enclosed was not the correct one for your car, but seems identical function. Here is correct one.
An alternator code could cause a red battery light, but throttle sensor seems totally unrelated.
If the rear battery voltage rises to about 14.5V when engine is running, then that suggests alternator is running fine.
Do front and rear have identical voltages when engine running. They should not be the same. K57.
If you can get it on DAS/STAR SDS system, you can get more information on codes, and it will give guided diagnostics to help understand/fix the problem.
The document i enclosed was not the correct one for your car, but seems identical function. Here is correct one.
An alternator code could cause a red battery light, but throttle sensor seems totally unrelated.
If the rear battery voltage rises to about 14.5V when engine is running, then that suggests alternator is running fine.
Do front and rear have identical voltages when engine running. They should not be the same. K57.
If you can get it on DAS/STAR SDS system, you can get more information on codes, and it will give guided diagnostics to help understand/fix the problem.
rear battery is 13.94 running
front battery is 14.5 running
Strange that there is a definite error in the system (the red code) but that scanner can't see it. I'd try one of the new hand-held scanners. With both batteries charging, it sounds like if you can clear the code then all will be well. Red light is common issue after the battery dies.
For probably less than the cost of one dealer trip you can buy yourself an Autel, Launch Creader Elite or Foxwell. I know the Foxwell will read and clear the code.
Alternatively an independent workshop would be a better bet than the dealer.
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. (If I have your VIN I can check exactly what alternator is fitted)
