I was hoping somebody could shed some light on an electrical issue on my recently acquired '80 300SD. The battery doesn't seem to hold a charge unless it hits around 2500 RPMs. For example, if I drive it around the city constantly on a fully charged battery, it'll end up drained after a couple of days. The wipers run slow (slower with the lights on), the defroster fan doesn't blow as strongly as it should and each start gets tougher and tougher. Once the battery really loses power the battery light will start to try and come on and there's a buzzing/ticking type sound coming from the instrument cluster.

However if I drive it to work (about 18 miles mostly hwy driving), the car starts out acting like I previously described until I get to 60 or 65 (right around 2500 RPMs) then everything seems to work as they should. Also the glow plug light comes on for a second or two right before the wipers and fan kick on and behave properly. The car then starts up and runs fine after I get to and from work. If I don't end up driving on the hwy for an extended period of time, the battery will go dead.

The battery, belts, starter and voltage regulator hae all been replaced since I bought the car. The alternator was tested and is fine. I just replaced the voltage regulator last weekend, which I thought would solve the problem, but to no avail. Is there anything else I'm overlooking before I go hunting for bad grounds? Thanks for your help.

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1979 300SD
1975 240D

A large ground strap bolts onto the engine near the starter then runs to the frame of the car. Pull this clean it well with a wire brush on a drill so all connection points are bright and shiny then reconnect. I spent a ton of time on my 300SD with the same symptoms although mine only acted up as yours is about once a week and that finally did the trick. Enjoy your car it is a great ride
Dave S

Dave:

I'll give it a go tomorrow. I'll let you know as soon it's done. Thanks a ton.

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1979 300SD
1975 240D

Not to make an easy repair more complex (I like simple solutions).
But how often have you all replaced the brushes in the alternator ?
It is a 5 minute job and I think MB was genius in its method,
no rebuild needed, but works like new.

I have put two or three sets of brushes in my 300,000 mile 300D.

Don't see many posts about the alternator brushes.

I think we're talking about the same thing when I referred to the voltage regulator. I did replace it last weekend.

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1979 300SD
1975 240D

oops, I missed that point. Yer right.

Dave:

I did clean up the ground and it definitely put a stop to the light show in my dash. As for the charging, it looks like the belts just needed a readjustment. Everything was pretty rusty as the car had been sitting for 4 years, and the belts loosened up considerably since they were replaced. Thanks for your help.

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1979 300SD
1975 240D

You are around here in portland?

Your w116 doesn't happen to be red does it?

I am always on the lookout for other 617 powered vehicles...I go to portland state so I see a lot of them on the roads.