Most likely your OVP Relay. I had the same problem and I reflowed some of the solders on the board and to my suprise it fixed the problem. what model is your mb?

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• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*