Possible regulator/battery/alternator problem - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 12-01-2010, 12:09 AM Thread Starter
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Possible regulator/battery/alternator problem

I wonder if someone could help. I have posted previously about this problem but at that time I had assumed it was a mechanical problem (engine/transmission).

It's a 1971 280s auto, and the car died over a month ago. I had been driving it daily and noticed the oil pressure gauge dropping noticeably over a few minutes when idling but coming back up to full pressure when driving. Previously the oil pressure gauge had been pegged at 3 always, at idle and at speed.

A further minute or two later the car died. There was an audible thump which made me think the transmission had failed and maybe it has, but if this were the case it wouldn't possibly affect engine oil pressure, and would it have caused the engine to die?

I have not driven the car or touched it since except to start it not long after it was towed home. I haven't had time. I do think about it a lot however and after assuming this was a mechanical engine or transmission failure. Now I am wondering if it could be electrical.

I had the car towed home, and it started once more after leaving it alone for an hour or two, full oil pressure, but then 30 mins later wouldn't turn over, the starter just made a clicking sound.

Not long after there was no electrical power whatsoever.

This is what has been making me think it could have been caused by a rapid discharge of the battery, caused by or perhaps compounded by a regulator or alternator failure, or maybe some kind of short.

Here's my questions.

1. Is a relatively rapid discharge of the battery while the engine is running (assuming it is caused by no charging and some accessories on) enough to cut the ignition entirely once it reaches a certain low enough voltage?

2. Would the battery voltage dropping while the engine is running cause the oil pressure gauge needle to drop? If the voltage dropped quickly enough, would the needle indicate a visible drop of .5 or more on the gauge over a couple minutes.

3. A low enough battery voltage would cause the starter motor solenoid to click but not turn the motor, right?

I am going to charge the battery fully tonight and see if the engine starts and that the car still drives. This will tell me if it's a mechanical failure or not. I'll also check the voltage across the battery terminals to see if it is charging.

The only problem with this possible interpretation of the symptoms is that I never had a low voltage indication on the dash, the red light below the speedometer never lit until the engine died.


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post #2 of 3 (permalink) Old 12-02-2010, 12:12 AM
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I don't want to sound sarchastic, but have you looked in the engine bay? Could the thump have been a belt coming off?

Also, my 108's oil pressure drops at idle and pegs as I accelerate. It's normal.
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post #3 of 3 (permalink) Old 12-05-2010, 11:56 PM Thread Starter
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Thanks, the car is on the road again.

I was right and wrong. The original problem was indeed electrical not mechanical, upon swapping in a fully charged loan battery, it started and drove away, however the multimeter confirmed I had no charge coming from the alternator, so I swapped it out for a reconditioned unit on Friday.

I was wrong about the apparent drop in oil pressure having anything to do with this. You're right, the gauge was and is right, it's normal.

Now to tackle some other problems on the list.


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