1994 944T 265K

Sitting at a light I noticed that the car was idling rough. The tach was not registering but would occasionally jump up to 3500 and hold there before dropping back to zero. The gas gauge had dropped to empty. Raising engine speed smoothed out the engine as well as tack and fuel gauge back to normal.

Drove the four miles home and as soon as I pulled up to the garage and the engine dropped to idle it stalled. Very dim dash lights and no response from the starter. The battery was dead.

I put the charger on and several hours later the battery was charged. When I start the car and measure across the battery terminals I get:

600rpm 12.4v
1500rpm 13.6v
2000rpm 13.9v
2500rpm 14v

Higher engine speeds do not increase the measured voltage.

I do not know what the measured voltage should be. What do these numbers at different engine speeds indicate?

The battery is 7 years old so it doesn't owe me anything. Dragging it into AutoZone for a test seems appropriate.

Looking for other input. Thanks

Edit: The AutoZone test indicated the battery was "Good". It had just been on a charger all afternoon.

Randy

The electron replenisher is the alternator. Check the belt and the wire that bypasses the rubber isolators to ground it.
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Art Benstein near Baltimore

Brickboard members are the most likeable, smartest, and best-looking people on the internet. [Citation needed]

Art,

I should have included the fact that the alternator was swapped out last week.

I found the car in the garage with a dead battery. I put a charger on the battery and checked it in an hour and found it still charging to the max (10+amps) and the alternator too hot to touch.

I was able to start the car but the alternator sounded like a bearing was bad. It had not sounded like that prior to finding the battery dead. I can't guess at why it had this new sound, but a little research suggested that a diode might have failed.

I had a 100amp Denso that I had from a pick and pull visit. The slip rings were in great shape and I installed new brushes. I had it checked at Auto Zone and it was found to be good. I installed that alternator last week.

After reading your post I double checked the belt tension as well as reseating the ground wire. Checked again with meter and the results were the same as in my original post.

Should the voltage readings be higher? When I turn on the headlights (I normally have them on all the time) the idle voltage reading drops to 12.

Randy

Yes, at idle the voltage should be above 13.2 or so. Definitely in the thirteens at least, even charging a drained battery in the heat of summer.

If not a loose belt or missing ground, the trouble is most likely in the alt. An exception could exist if there were some sort of crazy high current load of short-circuit magnitude dragging the voltage down. Of course this could be touching plates on a sulphated battery as well, but given the geniuses at Autozone can't identify it for you (even with the incentive to sell you a new alt), I'd plan to make some more swaps from the fleet at home to eliminate those two items.

In my opinion, reading the "research" diodes are blamed (right or wrong), for anything not solved by visual inspection.
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Art Benstein near Baltimore

Two atoms were walking down the street one day, when one of them exclaimed, "Oh no - I've lost an electron!" "Are you sure?" the other one asked. "Yes," replied the first one, "I'm positive."

After I put the battery back in I drove the car to AZ and had them test the charging system. Battery checked out good but the alternator showed a bad diode. OK, whatever, it was the same alternator that checked fine before I put it on the car last week.

I took a 1990 740 beater on trade last week and was pleased to see that it had a new rebuilt alternator on it. Not a Denso 100amp but it bolted right up.

The meter shows 13.8v across the battery terminals at idle so I think I am back on the road again.

Thanks for your responses.

Randy

So, you escaped twice with a free test and no significant sale at AZ. You'll have to lay low for a while or they're going to see you coming. :)

Glad to hear you're charging again!
--
Art Benstein near Baltimore

Q: Why are electricians always up to date?
A: Because they are "Current specialists".