On my 77 280E, I'm trying to find the source of power to the AC switch over valve (increases idle speed with ac on via vacuum to distributor retard). I believe power comes from one of the relays mounted on the left wheel well, but which one? There is no power at the valve pigtail.

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Not on your car but on my '76 W115 it was from one of two fuses on the center of the firewall in a stand alone fuse box/holder with a brown snap on plastic cover. See anything like that?

Good luck!!!

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No such thing on the 123 car, one fuse box on the firewall just behind the relays box and no lead to the change-over valve.

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