With a 8 month old battery one day my battery indicator light came on. Found a few dry cells and refilled with distilled water. Recharged battery and drove again but light still came on. Battery and alternator checked out OK at auto part store. Replaced voltage regulator as brushes were at minimum length. At this point everything worked fine for the next 3 months, even during a 800 mile round trip. Then battery problems started again. The battery was replaced and still runs down. The dash battery light isn't coming on this time. Had to be jumped and on the way home the lights were very dim, no blinkers or dash lights. Suddenly, at about 60 mph, the lights all came on. Any suggestions as to what is going on with my 1987 190d.

Yes but it sounds like a broad spectrum of possibilities. I might grab a used alternator from a wrecking yard if cheap. Without being certain that the alternator is flakey or not. There is not much sense doing anything but cleaning up connections etc. You could do that first I suppose and see if it catches the issue. If the voltage rises on the battery terninals above 13 volts with the engine running you know the alternator is charging.

First of all before anything though I would disconnect the glowplug circuit every time I was not using it. There is a chance it is staying on and draining more power than the alternator can keep up with. Or self energising randomly. This will provide symptoms like you have and is not an unknown occurance. Disconnection of the glow plug circuit when not in use or constant monitoring of the voltage present at the actual glow plugs is the only way practical to evalute this. The light in the dash is not good indicator of when the circuit may be on in a case like I mention. If it remained on with defective glow plug controllers life would have been much simpler for a lot of owners.

You could hunt for quite awhile for the issue without some form of baseline to start with. You want that glow circuit out of the loop. Originally your dry cells in the battery indicated an overcharging condition existed.Changing the regulator seems to have repaired that. Thats if the acid levels were normal when the battery was purchased. Alternators on cars are not desiged for heavy continious duty normally. Thats why a defective battery can take an alternator with it in failure.