I went to a junkyard this weekend and picked up a sending unit from an SD for $2.15. It looked brand new because the latex was white instead of yellow (like the original in my car). I plugged the "new" one in and the tach is back in action. $2.15 is cheaper than a pack of smokes and a new sending unit.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold