If you truly do have only 9 volts on the center post of the battery terminals you have a bad battery. I would connect a charger and see if the voltage goes right up to 12 volts. If it does, and then drops back down to 9 when you remove the charger the battery has big problems.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C