I need to replace the a/c dryer pressure switch on my '84 300D. Do I have to discharge the system to do this? It still uses R-12. Thanks.

Yes, it has to be discharged. Take it to an a/c shop that can evacuate the R12 properly, then replace the switch. Pull vacuum and put that same R12 back in. (provided the R12 was not contaminated with something else by previous owner).

Cars this age generally have other leaks. Other replies here may suggest additional items to review/replace.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

On some of the 123 cars, one of the switches was a thermal switch, so it could be replaced without loss of refrigerant. Even though it was a thermal switch, it still in effect measured pressure as temp and pressure are directly related.

If the shaft of the sensor is very small, it is probably a thermal switch.

Good luck,

Larry,

On the 84 123, there are 2 switches at the RD. One is a pressure cutoff switch (subject to leakage, ask me how I know) and the other is a temp switch that closes and triggers the electric fan in front of the radiator.

The temp switch and the pressure switch could leak around the threads, the temp switch is solid, it can't leak "through" the switch.

A more likely leak is between the metal case of the pressure switch and its phenolic (looks like phenolic anyway) circular innards.

This was on both of my 84 123's (CA model). Federal should be the same, but I can't verify.

Eric

__________________
84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)