Battery/Charging Mystery
 

OK, I am at my wit's end, and I need some input.
Here is the scenario;
1981 300SD, after several days of normal driving with AC on, lights etc. the battery will suddenly be down to below 10v and the car will not start.
This has happened three times in the past month. After getting the car started with a boost battery, and driving home about 6 miles, the battery is charged enough to last a few days. This happened last Saturday night at Walmart, and my wife drove the car around today. When I got home from work, the battery was down below 10v, not enough to start the car.

The first time this happened, I had the battery checked out at Interstate Batteries (where I purchased the battery last August 04) and they determined that the remaining reserve capacity was a little low, and the total 750CCA may be marginal for the August Texas summers, so they replaced it with a new 850CCA battery.

I went ahead and removed the alternator to have it bench-tested. I had it bench load tested at three different shops, and all three showed that the alternator was putting out at least 14 volts, and 55 amps.

Now, a couple of weeks after I replaced the battery, the starter solenoid failed. I was able to start the car a few times by tapping the solenoid. As most may know, removing the starter on a 300SD is not one of the most pleasurable tasks, so instead of replacing the solenoid alone, I replaced the whole starter/solenoid assembly. I am now feeling that I have “beat the gremlins” and feel pretty god that my problems are behind me.

Tonight, I checked the voltage at 9.75v. I then disconnected the positive lead, and checked the current draw while the car was off. As soon as I connected the battery I could see the interior lights cycle, then the radio, antenna and then down to .02 current draw after a couple of seconds. This seems low, and in-line so no clues.
Tonight I am charging the battery on a 2amp charge, and I will check for the next few days.

Thoughts?

External Voltage Regulator
 

I would say it could be the externally mounted voltage regulator. Its on the back of the alternator. Two screws to take off and look to see if the bars are worn unevenly or worn down.

Should be less than $55 to replace. I use dental floss or fishing line to tie the bars down while installing.

Just a thought....

I have had this happen many a times, and its usually the culprate.

Ryan

I had a similar issue with my 300D a few months ago. After replacing the battery for the second time, I was poking around and noticed one of the electrical connectors going to the AC receiver was not insulated and was more or less suspended in mid-air by the wire. I checked the connector and found it was live. I taped the connector and my issue went away. I assume the connector was periodically making contact with a ground and draining the battery. I don't know if this is helpful, but I would take a good look around for any potential intermittent grounds. Good luck.

Up by the battery is a terminal block. Usually three terminals. Two from the alternator and one going to the starter solenoid. Remove the screws and clean the connections. I have seen oxidation there keep the alternator from charging the battery. Drives ya nuts till you find it.

you could have a drain with the car off....thats easy to verify..

or you could have a flahey regulator...I had one that gave those exact syptoms...90% of this time it was fine...out of the blue it would dicharge the battery overnight and sometimes it would overcharge with 17.9 VDC.

You can get regulators for as little as $29... and it takes 5 minutes to replace.

on my 82, this block is on the right frame rail, also holding a bracket for pipe for coolant return from heater system. Not only can this get corroded, but it can get oil, sand and water mixed in. Could be a partial short as it gets really nasty. It's easier to mark the wire locations, and completely remove the block to clean it, rather than do it in place. Coat the cleaned terminals with ox-gard or equivalent and make sure the cover closes or make a new one. I had this problem also, and cleaning the block really helped.

Thanks for all of the great info. On Sunday I did check and clean the connections at the terminal block. They weren’t corroded, but I clean to bright anyway. I am tending to think that this is the regulator. I will check today.

Thanks,

robert

Did you idle for an extended period before the battery was low? My alt doesn't put out enough at idle to keep up with the ac, radio, and aux cooling fan and it drains the battery in about 5 minutes.

You may want to look into the condition of your battery (and/or alternator), if it drains after 5 minutes of idling.

That was my point. :D