This is either a faulty alternator or a failed diode in the instrument cluster. Try unplugging the alternator, if no change unplug the instrument cluster. If nothing works replace the OVP.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.