This might be in the running for dumbest question of the month but . . .

Last night I'm a mile from my house after a halfway long trip and the car died. I knew my battery had been getting weak but I didn't think it was that bad. Just before it died, I'd stopped at a store and it struggled to start up, but did.

So today, I get a new battery (great deal at Costco - $74 after tax compared to $115 at O'Reilly's), it starts right up and I think to check the charger. The battery at rest reads 12.77. With the engine running, 12.54.

This can't be good. Must be a clear sign the alternator is dead, eh? Perhaps I taxed it overly by making it charge a weak battery for too long.

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1986 300SDL, 362K
1984 300D, 138K

if the alternator is functioning properly, the voltage between the terminals with the motor running should be between 13 and 14 volts.

replace your alternator. your battery may have been OK to begin with.

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Ben
1987 190d 2.5Turbo

I doubt it. It was 6 years old.

Regarding the alternator:

Oh hell yes. Put in a new (rebuilt) alternator and it's reading 13.86 at idle, 12.61 when off. It's been so long since I've checked charging voltage I'd sorta forgot for certain. What was I thinking.

Purists might scoff, but I went with an O'Reilly's rebuild. My OEM parts guy is in Berkeley, 40 miles away (my former home) and I wanted it running tomorrow morning (whoops, this morning). And I have to ask myself if I should go with top dollar parts on a rig with 357 K (!?). Was about $90, had an odd feature - they used insulated bushings on the mounts (not sure why), I had to attach a ground wire to the post they specified, made a ground wire out of a connector made for the purpose on one end and one I made from some scrap alum for the other, as I needed something big enough to attach to the 8 mm bolt I found to attach it to the block.

I didn't really need the extra complexity but oh well. I wasn't sure how to attach the 12 ga. stranded wire to the alum but it worked out. Used a smallish nut and bolt, the strands wrapped around and tightened with a lockwasher - the strands were sorta bulging out so I torched them and soldered it into a solid mass. Now I'm wondering if it's thick enough wire. I mean it's some serious current involved in the alternator, right?

This site talks about checking for voltage drop on charging issues, not that I have one yet but it would be nice to forestall one. They mention the ground strap sometimes being needed at the bottom of the page:

Voltage Drop Testing

I think I'm in good shape - I'm getting a voltage drop reading of .017 volts, engine running of course. The article says if you're below .1, it's good enough. .4 or .5 or above is when it will be a problem.

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1986 300SDL, 362K
1984 300D, 138K

cmac2012,

Hell yeah to you too

-Ben

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Ben
1987 190d 2.5Turbo

I wasn't trying to insult you. Just that I realized (after putting it in) how clearly the alternator was what was needed and felt a bit sheepish for having any uncertainty to begin with.

One more thing on the battery, I've read that putting a full on dead battery into a car, jumping it and depending on the alternator to charge it up is not a good way to go. Supposedly creates an unnatural surge of current as the battery is sucking so much in a short period of time. A trickle charger is a safer way to go, at least according to that source.

Not sure, but maybe my dying battery abused the alternator. At any rate, they were both old and no way do I want to be stuck on the highway far from tools and decently priced parts for lack of some $170 in parts.

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1986 300SDL, 362K
1984 300D, 138K