Need confirmation on Electrical Diagnosis
 

Here are the facts:

1) The battery in my 85 300D has sounded weak when cranking since I pulled the car from winter storage in the garage after March. I charged it up though and the car starts right away every day. I figured driving would top off the charge if I had not charged it fully with the charger.

2) Yesterday was the first hot day when I had the A/C on and also had it out for an extensive drive into the city, about 12 miles on the highway. Not that far but more than my usual commute of 4 miles one way. I was going across the city at rush hour and was on a long downhill stretch. The traffic was inching along and I had a way to go downhill, so to conserve fuel, I turned the car off and coasted in neutral. When I got near the bottom of the hill and saw that I might make the next light, I tried to start it back up but the battery was near dead. Yikes!! Luckily there was a gas station on the right and I was able to drift right into a space. I called a friend, got a jump start, it started right up and I drove home without making any more stops. I did not use the A/C. When I got home I turned the car off and tried to start it again. No problem. It started up.

3) I tested the battery with a hydrometer to see if I had a bad cell. The readings were 1220, 1190, 1215, 1200, 1200, 1200. I tested the voltage after a short charge and it read about 12.70 V.

4) The car started up this morning, it started after work, and when I got home I did another reading. 12.57 V with the car off. I started the car up and did another reading with the engine revved and got 13.93 V

The reading of 13.93 V with the engine revved tells me that the alternator is OK, correct?

Did having the A/C on push the battery over the tipping point and drain it? Is it just not taking a good charge anymore and the extra load was too much for it? I cannot see a date on the battery but from what I can tell from the PO records, it was installed in July of 2001. Six years on a battery in my mind is a good long life.

Thanks for any input. I am planning on picking up a good Interstate battery tomorrow.

Richard

Very odd situation.

The AC being on should not drain the battery. I don't see a connection there to the no start.

Did you try to start the car AFTER you coasted into the gas station and BEFORE you jumped it? Any chance the car was not in neutral when you tried to start it during the no start? Perhaps there was some other random intermittent ignition issue.

Yes, 13.9v means the charging system is working....at least during the test. It still could be an intermittent problem.

Before buying a new battery I would load test the old battery. Autozone and many parts stores will load test them for free.

The A/C on, battery drain is puzzling me as well. I did try to start it after I drifted into the station and before I jumped it. The battery was still near dead. I was back in Park at that point in time. When I was coasting down the hill I was in Neutral when I tried to start it.

I would take it to Advance Auto to have it load tested but they do not sell Interstate batteries. I think the guy who sells them will load test it for me. I am going to try to get up there tomorrow.

Have you checked the voltage with the AC engaged?

Comes up at 11 yrs by my math. Truly time for a replacement.

advance or autozone, etc will test any battery; doesn't matter the brand

ok, everybody reading this should note something about the older diesel alternators...
they are very weak. if the battery is not in tip top shape, and fully charged when parked, the alt simply cannot handle accessories, and charging the battery back up.
also note, that idle conditions and driving conditions with demanding accessories are not the same. sure you read 13.98v at idle with nothing on, try it with the engine hot, the battery weak, the a/c and blower on high... alt fan spinning full speed...
I'd get a new battery, and check the regulator brush length.

For batteries
 

You can purchase a load tester from Harbor Freight. It wasn't very expensive at all. Volt Meters are great but when batteries are getting old or sick the only way to be sure in your drive way is to pop that load tester on it and hit the button for 10 seconds or so. My battery in my 85 300D lasted me like 5 years and then it would not take/keep a charge no matter what the charging system was. (Bad cell or two)

Just a thought.

Very good point. I need to do that.

First mistake, should not coast an auto tranny in neutral. 2nd mistake, turning the car off. You lose your servo braking in an emergency.

But seriously, I think the battery is OK. You may have a lose connections somewhere or problem in the NSS. Check the battery terminals and has the battery checked for free.

I had a load test done today. The little graphic on the printout shows my battery way way down on the scale and way below the range where it is recommended to replace the battery. In terms of hard numbers, my battery is rated at 850 CCA but is giving an output of 213 CCA.

I ordered a new one and he will have it in tomorrow.

SG is low throughout the range of cells yet showing a standing charge, battery chemistry is incorrect.

Good point. It maybe a slipping fan belt ~ been there.

All this over an 11 year old battery ???

"I cannot see a date on the battery but from what I can tell from the PO records, it was installed in July of 2001. "

Are you saying that the Specific Gravity readings look too high for the voltage that is being reported?