Well I did pull the OVP relay fuse on my HFM equipped E220 and guess what? No change to the engine, it started and ran fine, idle speed compensation worked in the normal way, only difference was the ABS light was on.

So I conclude that the HFM engine management computer is not fed from the OVP relay, presumably it has it's own protection from high system voltage.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)