I have a '81 300SD and the battery won't hold a charge. I had the battery tested twice and it is fine and I disconnected the positive cable and the car kept running just fine eliminating the alternator. The cables and posts are in good condition - no corrosion. Anything else it could be? I can drive the car for a day or two until the battery goes dead. If the battery and alternator are fine I can't think what else it would be.

Thanks for any help!

Patrick

 

Ounce your Diesel Engine has started the Electrical system has nothing to do with the continued runing of the Engine. In fact it is Vacuum that shuts off your Engine.

Sol disconnecting the Battery Cable proved nothing. Also in the Factory manual it tells you that disconnecting a Battery Cable while the Engine is running can ruin the Voltage Regulator in the Alternator.

When you first turn you Ignition Key all of your warning lights should light up; including the Battery Charging Light. If it does not light up the Bulb is burned out. If the Bulb is burned out your Alternator will not charge because it needs the resistance of that Bulb to excite the Alternator.

If after you start your Engine and it is running if your Battery Charging light is still on your Alternator is not charging.
There could be several reasons for the above, the Brushes on the Voltage Regulator could be worn out, voltage regulator could be no good or there could be some internal issue with the Alternator, a bad Wire connector at the Alternato' or even poor or bad Battery Cables/Engine Ground Strap.

Since it sounds like you do not have a Volt/Ohm meter (Multimeter) to see if your Alternator is charging take the whole car to Autozone or some place they will test you charging system for free.
Or, remove the Alternator from the Car and take that to be tested.

 

Note: If the Battery and Alternator really are fine then the last alternative is that you have a short circuit some where that is drawing current when you are not runing the Engine.