Electrical issues (not charging)
 

I have a 1976 300D that is not holding a charge on the battery. Things that keep this from being an easy (for me) fix: Battery is good and did well during load test; alternator is charging the battery; long drive with electrical going, battery dies next start; car started fine this morning (28 degrees F.) after sitting for two days; and finally, the problem started when I tried plugging in the block heater one morning and battery got drained (don't know if that was just a concidence or if that was the cause).

I have put the battery on a charger overnight and once longer, it takes a full charge, after driving around for a while, it was at 13.3 volts when I had it load tested. On a similar note, I am unable to locate the fuse box for the dang thing. I found four fuses on the firewall (three 16 Amp and one 8 amp), but think that there has to be more than that somewhere on the car. Any suggestions would be helpful. I am thinking that I should replace the wiring harness and the ground (ground is attached to a pretty rusted area of the body).

Regulators can do that....I went through that 2 years ago on my W116....turned out to be the regulator....

It was intermittantly draining the battery.....othertimes it worked perfectly.

W-123 Fusebox
 

Look on the driver's side firewall just inboard of the hood hinge , it's there , two big black knobs holding the rectangular cover on ~ it's tricky so PAY ATTENTION to how it goes and _don't_ force it ~ it'll crack or break if you do .

Get a longer battery ground cable of # 1 gauge (or thicker) and replace the crummy short one , connect it to the engine block proper then add a grounding strap from engineor trany to the body ~ watch your charging rate <magically> increase and cold starting improve too .

Mac,

Also check the alternator brushes. They could be worn to point where they are working intermittantly.

P E H

hmmm i wonder if this applies to me too...as i have had trouble the past couple of days with my electrical charging system...i am taking the car up to get a definitive answer from my mechanic...autozone/advance's tests were, to me, inconclusive.

Intermittant failures are just that...intermittant...

Hmm...3rd test, by my MB mechanic, showed that ALT was putting out good AMPs...no problems...power draw (when keys were in ign) was 7amps...this is about normal..im told..the ground wire was a little loosehe widend the connector and reinstall..

You have a 115 chassis so you don't have a big square fuse box like the 123 chassis, but I believe it is in the same location. I can't remember, because it has been years since I even sat in a 115. IIRC the fuses you found were for the power windows and the radio. I'm thinking intermittent regulator or a key off current drain like maybe the power antenna. Good luck.

flatline the battery a few times and you will kill it...a battery costs what? a regulator you can get for about $30...odds of it failing WHILE you are testing it is mighty slim.....