I'm not sure if the SD is different from the TD, but I suspect they are very similar.

I would not bypass the temp. sensor. I would first find out why the aux fan did not come on. A very likely reason is that the aux. fan is NG. Follow the wire from the aux. fan to a plug connector, disconnect it, and connect it directly to 12 volt supply (use wire leads directly to battery) to see if the fan runs....if the fan doesnt run, its NG. If the fan runs, then there is a problem with either the aux. fan relay, or the temp. switch at the rec/drier.

There are two sensors/switches located at the rec./drier...a pressure switch and a temperature switch. The pressure switch has two blade connectors directly on the body of the switch; the temp. sensor has two wire leads with connectors on the ends.

Good luck

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)