I have an '89 240DL, B23F. After improper diagnostics by the shop, I have replaced the alternator to no avail to solving my charging problem. There was nothing wrong with my alternator to begin with.

My battery is a bit over 2 years old. I have checked cell to cell voltage and they range around 2 volt each. With the state of the battery at that time, I feel that this value was acceptable.

With the electrical system fully intact, i.e., the battery connected, the car will charge the battery to a full 13.7 volt if no other load is running (lights, blower, etc.). Once activating any load with the car running, the battery will rapidly discharge (voltage reading from the alternator dropped to less than 10 volt over a 5 minute duration). Upon de-activating the load, the system will charge again to 13.7 volt within a reasonable amount of time. I let the car sit over night and the battery still has more than adequate power to start the car.

If I start the car and disconnect the battery, the alternator takes over and allows for the car to continue to run. Upon activating a load (or even several loads) the output of the alternator remains above 13 volt.

All grounds appear to be good and I don't suspect a short.

I realize the logical culprit would be the battery but I don't get it due to the tests I have ran on the battery. It charges fine and holds a charge.

Additional info:

Start the car fine with no loads on - battery VERY strong.
Check alternator voltage at 13.+ volts - battery connected
disconnect battery - Alternator reads 13.56 volts
Turn on lights - alternator reads 13.3+ volts
re-introduce the battery - alternator reads 12.5+ volts
disconnect battery - alternator reads 9,6 volts and will not recover.
turn off lights - alternator recovers to 13.56 volts

Also, Today I took the car back to the "pros" to test the battery mainly but also to get feedback on the entire charging system. These are the same folks that mis-diagnosed my trouble in the first place. They say my battery is good and their equipment showed "no voltage to the battery indicating the alternator is not functioning. I put my fluke DMM on their testing equipment terminals and read 13.86 volt while connected to my battery.

Any thoughts?

regards

Rev