Did you check the cimate control servo? Im not sure if the 84's still did use that expensive unit. But if they do, it sounds like that could be the culprit. The unit is usually planted by the firewall on the pass. side. It has four water hoses going to, and a host of vacuum lines going to it on the top half. It is usually mounted near the aux. water pump.

good luck

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K