M5 Expert (>4000)
Join Date: Oct 2010
Location: Cobourg Ont
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I wrote all below, then checked it to see if my memory was tricking me and it seems it was not, but I did find an interesting set of notes on a car with a single failed diode in the alt. Then a second failed and he had the car towed to me. I replaced the alt, and it was a beast again. That is the opposite of what I said below, don't know what to tell you, the notes are similar to your car. I have no recollection of the car at all.
It has been a while, I hope my memory holds up. Basically a car has two electrical systems or maybe better described as two sides. That is how the idiot light works, it shows the balance between the two systems. The OBC basically reports the ALT voltage 90% of the time, not really the battery voltage. Yes they are connected, but.... Sorry all I can really say is but.
Without a 5 page essay explaining how things work, take my best guess as you definitely seem to have a bad connection or a shorted battery, something wrong on the battery side of the two systems.
Step one clean your terminals on the battery. Disconnect it and charge it, then read the volts after you remove the surface charge. Easiest would be to connect it again turn the lights on for 30 seconds or so, then disconnect and read the volts. Perfect battery would be 12.9 good battery 12.8, usable battery 12.6 - 12.7. 12.5 might need another charge or charged at a higher rate but likely is not really very good any more.
That is your battery test if it tests good then you need to test voltage at different locations to help find where a connection is not so good. You can't use a volt meter alone because it does not draw enough, you need something that draws power and a volt meter together.
But maybe you just need a short cut, I have lots of them. If you have never noticed any issue starting the car and it turns over strong, we can likely rule out the battery and all the wiring to the starter. But there is a connection in the right side trunk compartment when looking at the battery. That is not ruled out so loosen the red and look at it clean it bolt it back up.
Next you have to get to the starter, even though we have power to the starter there is a second wire that sits on the same bolt, that might be where the problem is. It is not uncommon place for corrosion.Then after that spot you need to check the booster terminal that is where the ALT side of the power joins to the battery side of the power.
Last edited by Sailor24; 6th July 2019 at 06:24 AM.