There are two types of temp. switches. The old style (small threaded, no plastic cap) and the new (larger threaded plug, red plastic cap). If the R/D has ever been changed out (most likely at some time), it could be either one(depending on which type R/D). Go to FastLane and bring up a 1984 300SD (I know yours is a 300D)>climate control>A/C Temp. switch. There are pictures of the non-pressurized at the top and the PRESSURIZED new style at the bottom. Hope this helps.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)