if the egr computer was totally fubar the tach would not be working, but it may be slightly fubar ? Seems like water gets in somewhere, maybe windshield gasket, gets on the cabling to the egr computer, runs down the cable and collects in the connector and the computer module itself. All the 85's I have come across in the salvage yard except for 1, have had liquid in the connector area or evidence of liquid once being there. check your computer connections.

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000