Ground Points: Corrosion causing bad ground contact
Normal Fix: Use a small wire brush to clean the ground area as well as the round terminals
Here is a Pic of an S4 Ground Points and hopefully we can surface one for earlier models, which at that time I can add here.
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1981 Porsche 928 "Euro" Auto Gunsmoke Metallic Flat - Black Interior
1983 Porsche 928S "US" Auto Light Bronze (Copper) Metallic - Brown Interior **SOLD**