Recently I have been having alot of problems with my charging system of my 300sd. In the last 3 weeks, I have made many unsuccessful attempts to solve my charging problem. The problem seemed to start around the time I was having problems with my glow plugs. Anyway after I got the glow plugs replaced, I still had problems. I would charge the battery by driving the car on the highway for about an hour, but the next day would come back to a dead battery. I could not understand what was happening, so I bought a new Interstate battery. Problems ceased for about 5 days, and then they returned again. Had to have car jumped frequently!! So I concluded that the alternator was not functioning properly. I had the alternator rebuilt and had a heavy duty voltage regulator installed. When we tested the output of the alternator on a machine, it worked perfectly. But when it was installed back in the car, we only read about 12.4 volts on the battery, not good. So after checking the fuses and other possible loose connections, we concluded that the wire harness leading from the alternator to the distribution block had stopped functioning properly and it was decided I needed a new harness. This was on Friday. I was instructed to go to another shop where they should be able to make me a custom harness to supplement for factory one until I could obtain one. I called dealer Saturday and they told me that the harness which I needed was $260. I said that is ridiculous!! So after the shop told me they couldn't make a harness because they didn't have the special connector, I decided to make my own. I made my own harness from some alligator clips, 10 gauge wire and crimps. I then went back to the alternator specialists and had them test the output. Still only 12.4 volts at the battery!! I was very frustrated now and couldn't understand what the problem was. So we disconnected the negative battery terminal with the car running and tested the output. 14.4 volts at idle!! It was great but when we connected to the battery again, we still only got a reading of 12.4 volts. So we concluded the battery was bad, and that I needed to exchange it. Anyway, today I got my multimeter out and tested the output the same way we did yesterday. This time I tried both the factory harness and my custom one. Only 12.2 volts on the battery at idle and when I disconnected the ground cable, I was only getting 4. something volts to the cable. I don't understand what is going on?? I tested the cable exactly like we did yesterday and got a completely different reading. What in the world is going on?? Somebody please help me!!

Thanks in advance,

Knightridder521
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1985 300sd 315,000 giving all these problems now!!
1986 300sdl 285, 000