I had a similar problem when I pulled the instrument cluster on my 500SL. I touched the insulated wire on the right side (facing and probably the clock) to the frame. Right away I knew I'd messed up...but wasn't quite sure of what problem I'd caused myself.

Members of this forum told me I'd grounded out the cluster and suggested I run a new ground wire, which I did. I connected one end of the new ground to the post on the fuel gauge and the other to a good ground behind the cluster. Worked like a charm.

Hope this helps.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000