Why is my battery dead?

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.

Ken
1980 240D

[akry]

When the car is dead, are all other electric stuffs(windows/locks/mirror)operate normally without any slow downs?? I would suspect something else. Have you check the ignition?? Or maybe your battery is not charging properly when the car is in motion?? Just a thought....

Andy Kuo

------------------

• 1992 Mercedes-Benz 400SE
  
• Pearl Grey/Black Leather
  

ICQ#26950002
Mercedes Owners ICQ ActiveList ID#61730549
Mercedes S-Class Page
http://akry.homestead.com

[Ted2222]

I once had a battery that would intermittently fail to start my car. Never knew from one start to another when it would fail. Sometimes it wouldn't start once or twice in a day and sometimes it would work for several weeks without fail. Finally replaced the battery and solved the problem.

Hope this helps you. Good luck.

------------------
Ted
1979 240D

Akri:
When the battery is dead, it's completely dead. It can hardly power the dome light or warning buzzer.

Ted:
Did you discover your battery was defective through testing or "experimentation"?

Thanks for your help.

Ken

[mreid]

I have had dozens of batteries act the way you describe yours. I would replace the battery before I did anything else unless there are other symptoms that make you think it might be something else. Starter maybe.

------------------
82 300SD 100K
91 Caprice SS
92 Jetta TD

I used to have an MGA that killed batteries. Lucas=Prince of Darkness, etc. After years of wondering I finally figured out that the back suspesion was so stiff it slammed the batteries hard enough for the lead panes to come loose from their internal morings and short out. If bounced wrong, the battery would be dead. A little wiggle and presto, it worked again.

The cure for my MGA was to buy an off road battery that was designed for heavy bouncing, but I'll bet just a new battery will fix your problem.

Agreed, new battery should do the trick. I had the same problem (as did the previous owner, judging from his records). Mysterious battery drainage in the middle of the night (those darn electron gremlins). What did I find out:

Old Exide battery = Baaaad!
New Interstate battery = Mmmmmm, goood!

Brian

I took my car to a local alternator repairman who specializes in rebuilding and servicing Bosch equipment. He says that my alternator shows an intermittent failure to produce sufficient voltage, and that it varies from a high of 12.9 v to a low of 11.8 v. A load test of the battery indicated that it is fully functional.

I am taking the alternator off tonight, and he says he can test and replace or rebuild any defective parts tomorrow.

Thanks to everyone who offered advice.

Ken

Ken,
While you're in there taking the alternator off, make sure that your battery cables are in good shape, and (very important) that the connections at both ends are clean and tight. Diesels are much harder for a starter to turn over than gas engines, and having all cables and connections in top condition prevents starting failures at inopportune times. My sister in law has a very ratty 1980 euro 240D with over 200,000 miles on it, but it starts even in Chicago weather because her tech is so vigilant about maintaining the glow plugs, starting system, etc. Good luck!