I have owned my 1980 240 D for three months, and have worked steadily to get everything working properly. I've had several successes, but one problem persists: my battery occasionally loses power.
This has happened three times in the 2000 miles I've driven the car. Each time it's happened, it has been after the car was started several times successfully in a short period of time. In each case, the car showed no evidence of a problem before the ignition was turned off: the battery light didn't glow--except when the car was idling slowly because of the air conditioner (when the idle speed is even slightly increased, the light always goes out), and the blower fan, radio, headlamps etc. worked fine while the car was running.
However, a few minutes later, when I tried to start the car there was so little power left in the battery that the engine would not turn over. Even a jump from another car hardly started my car. Yet, when it started, I had plenty of power to run the air conditioner and radio with no problems.
Today, after AAA jump-started my car, I drove 100 miles home without incident. I would suspect the battery, but I don't understand why it only intermittently fails. In the two previous cases, after returning home and charging the battery, it ran successfully for 4 weeks the first time and 2 the second.
If it can hold a charge for weeks at a time, how can it be bad? On the other hand, if the alternator can supply enough power to run a air conditioner on high for an hour, why isn't it charging the battery?
Can anyone suggest a scenario that would explain my car's behavior? Better yet, could anyone suggest a diagnostic approach to finding the problem? Thanks in advance for your help.