Have you measured the amperage the pumps are drawing???? If the pumps draw to much amperage the relay does not last for long. The main reason for frequent relay failure is clogged fuel filter.

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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.