New Drier doesn't have high pressure valve

[my5thmopar]

I have all the parts to restore my A/C and recharg it with R12. The new receiver drier I bought doesn't have the high pressure relief valve. First is it necessary? Is there some part I have to add to the system? Here are photos of the old unit.

 

[slantsixdan]

Most filter-dryer units for our cars do not have a high pressure relief valve, they have a fusible metal plug made out of an alloy with a low melting point called Wood's Metal. Refrigerant pressure and temperature are proportionally related, so as pressure rises so does temperature; if the critical pressure/temperature is reached, the plug melts and the refrigerant vents. This seldom actually happens; it's a catastrophic failure event that you probably won't ever see if the system is properly assembled, evacuated, and charged.

I wonder if you're actually thinking of the LPCO, low pressure cutoff switch. This gets screw-threaded into the filter-dryer and cuts power to the compressor clutch if system pressure is below the setpoint to prevent the compressor failing from oil starvation. It is not supplied with new filter-dryer units; you transfer the switch from your old dryer or purchase a new switch.

 

[slantsixdan]

Transfer the switch (big round device with two terminals) from the old filter-dryer to the new one. Looks like your old one did have an overpressure blowoff valve rather than the fusible metal plug. If you see no such valve on the new one, it's got the plug, probably hard to see because painted over, but you can usually find it if you look closely for a small round irregularity in the surface of the paint, not too far from the threaded fittings and port for the LPCO switch.

 

[my5thmopar]

Thanks Dan! I'll check it tomorrow. I had the parts store reorder one before I knew this.