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Q: battery alternator light still on even though i have a new battery and alternator

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I have a mercedes 230 kompressor year 2005. i just replaced the alternator and battery-both brand new and both are checked and work fine. But when i start my car the battery/alternator light still comes on. We have checked the voltage of the battery and its good but when we check the voltage in the alternator it seems like the alternator is not charging. We have checked all the fuses and they are good. We checked all the connectors and they are tight. We hooked up the car to the computer to see what codes it shows and nothing comes up. What could this problem be? No one can tell me what we are missing? Please we are desperate. Some people say the computer...some say the big black cable leading out of the alternator which we can not find the name for. We have been without the car for months trying to get this solved . Would appreciate any help at all. Thank you for your time.

My car has 75230 miles.
My car has an automatic transmission.

If you check the big black cable from the alternator to the battery and there is no voltage with the engine off, then that cable needs to be replaced. On the transmission bell housing, that cable has a 175 amp fuse in the harness that may get shorted on the transmission bracket which would blow the fuse. This keeps the alternator from charging the battery.

The new cable part number is: A203 440 88 32 Positive starter/generator cable.

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