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lobu 05-22-2008 04:06 PM

Battery keeps dying W123 300D Turbo
Hello there!

Just changed the alternator. Cleaned the wire harness connectors. Check the fuse box for draw (no draw in any fuses). Had the battery taken to autozone (took battery out of car). They say holds charge.

Very little energy left in battery to start the car. Very slow cranking.
When car turns on for the first time after it was shut down. I don not rev the engine more than 900 rpm. I turn on the dome light. The light emits very dim. I then give the car some pedal action. It will rev to about 3k. Somewhere between 1k and 3k rpm the dome light will become very bright. At this point all electrical is active.

While the dome light is dim and I drive very slow and light city driving; The car will not have current flowing in the electrical system supplied from the alternator. Instead the current draw will be from the battery. I now this because, at first radio will work and lights will work. Then everything dies. But if I drive hard and accelerate with higher RPM's for like 10 seconds, the car is then fine and all electrical works.

Lately, even when I drive hard to activate the electrical system, later the battery doesn't charge, no matter how long I drive.

Dunno what could cause this. Any suggestions.

rrgrassi 05-22-2008 04:10 PM

Have you had the alternator tested? They have been known to be bad out of the box.

lobu 05-22-2008 04:19 PM

The car exhibited the same behaviour with the prior alternator. So I had changed the alternator in veign. The harness from alternator to junction box is simple. But is there a special charging circuit or relay or something?

winmutt 05-22-2008 04:22 PM


[edit] just saw second post. ground straps?

lobu 05-22-2008 04:27 PM

Cannon be belts, My car would be screeching and eventually my radiator bombarded with frags of rubber.

Ground Straps? Where are they located?

lobu 05-22-2008 04:27 PM

Cannot be belts, My car would be screeching and eventually my radiator bombarded with frags of rubber.

Ground Straps? Where are they located?

rrgrassi 05-22-2008 04:29 PM

Have you tried a new battery? I had AutoZoo test the battery and Alt in my car, and they said all was good. I was having the same issues you were. Needless to say a week after the test, I was replacing the alternator, but the batterywas not holding a charge. Replaced the battery and all is fine now, electrically anyway.

lobu 05-22-2008 04:47 PM


Did you have that problem with the system not being activated until a certain rpm? What would that be the cause? Low battery?

I mean the dome light goes from dim to bright, in a blink of an eye.

lutzTD 05-22-2008 05:14 PM

the alternator needs to have a good battery to work. the battery excites the field initially in the alternator, once that happens your lights should never dim while the car is on. sounds like you are draining the battery with the glow and start and after a while the battery recovers enough voltage to excite the feild on the alternator and viola, bright lights. check your battery voltage before during and after the glow/start cycle. it should hold 11-12 volts, if not you may have a bad cell

lobu 05-22-2008 05:18 PM

I got it. Sounds logical. Good thing the batt is under warranty. It's bocsh. I don't have an idea of who will warranty that battery. Any ideas?

rrgrassi 05-22-2008 05:59 PM

Take it back to where you bought it. If you are not the original purchaser, call a part house and ask them who carries Bosch branded batteries.

carnut 05-22-2008 06:13 PM

Another item that "excites" the alternator field winding is the red battery light in the dash cluster. The red light supplies just enough of an electrical load to enpower the regulator. So, is the red battery icon illuminated while waiting for the glow light to go out? We had this issue years ago on GMC vans with a volt gauge and no red "idiot" light. A bulletin was issued saying to wire in a lamp socket and bulb to load the alternator. If these GMC's were started on a cold morning to defrost windows and not revved up, after a week, the battery was virtually dead.

lobu 05-22-2008 11:28 PM

bulb is burnt out lol. I guess now a sense of urgency is upon me!

Diesel911 05-23-2008 12:35 AM


Originally Posted by rrgrassi (Post 1863037)
Have you had the alternator tested? They have been known to be bad out of the box.

This answer concerning a rebuilt Alternator nudged my brain a little.
The next time I get an rebuilt/new Alternator/Starter from the local parts place I am going to ask if they will test it before I leave the building!
Why should we bring these things home and put the labor in to install them and have to take a defective one back and start all over again. Next time I want it tested before I leave the store.

jt20 05-23-2008 03:29 AM

it might help to set up a voltmeter to your battery and watch the decay as you power up / turn on accessories etc... to help diagnose. Not that voltage is the answer, but it is a start. Make sure the regulator and capacitor on the back of the alt are in good shape.

ps I recently found a bolt.. yes an F*&%'n bolt in my alternator that was draining my battery over the course of a few days.

and.. if a battery is not properly charged, the charging system of the car will never bring it up to snuff.

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