Tightening the connection fixed it on the W126. I had put a set of dual electric fans on the radiator instead of the viscous clutch fan and just didn't tighten that nut sufficiently.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered