I just went through this same sort of problem with my 85 300D.
My temp gauge would work, then suddenly jump up, then peg up against melt down at the top of the gauge.
thought I had some sort of a ground problem between the gauges in the back, but too lazy to get into it............... until
I turned the key one morning and nothing, dead like some one stole the battery.
horn worked, head lights nice and right. turn key, no glow plug or anything else lighted up, but a weak battery light. remove key and Batt. light stayed on
.
Bottom line after a bunch of checking, hair pulling and finally posting a ? here. It was the ground strap between the engine and body of the car. It was tight, clean etc........the bolt that clamps the ground to the engine/transmission had enought paint it wasn`t getting a good ground.
Sanded off some paint for a good ground, and all is well.
The ground strap is on the drivers side under the car. 17mm bolt on engine/trans. and I believe a 15mm bolt on the body.
Your SD should be the same.
Charlie