1973 Mark IV ac problem

[71817]

I have problem with my 1973 Mark IV ac.
The pressure switch is damaged. Could anyone tell from where I can buy a new one.
Please. look at attaches pictures

[Lee]

I don’t see that switch in your schematic, but I believe it serves to protect the compressor in the event Freon is low. If you know you have a full charge, you can jump those connections.

[TonyC]

Not an expert on '70s systems; but if my guess is right, the OP's pressure switch is called the "40° Ambient Cut-Off Switch" in the schematic. If the plug has two black wires with yellow stripes, then that confirms it. But Lee's description of its purpose sounds plausible.

Not sure where one can get a good replacement; but if the part stores do not have listings for it, E-Bay has been very good to me when I'd been in need of new parts. Keep in mind, however, that you may have to empty out the system to replace it, if it isn't already purged of freon.

---Tony

[Lee]

I saw the 40* switch, but I thought those were in order to disable the compressor in cold weather. I don't know why it would need to tap into pressure, but I've been surprised before.

[1Bad55Chevy]

I think it's just a terrible name for a low pressure switch. If it's 40* outside the vapor pressure of the refrigerant will be low and prevent the system from coming on.

The Mark iii doesn't have a pressure switch on the system so I am guessing this might be the first model for such a thing? My guess is they were being creative with the names of new sensors. IMO the name would imply it's a type of ambient air temp sensor which it definitely is not.

The real question would be how a r134a conversion would change the equation of that sensor...

[71817]

The shop manual tells about cut-off switch: The switch is preset to open the magnetic clutch field coil circuit when the system pressure drops below approximately 40 psi (276kPa)

[1Bad55Chevy]

Is that sensor on the high side? 40 PSI seems to be to low for it to be on the low side. I think 40psi would be normal operating low side pressure for a day where the weather would be mid 60s.

[1Bad55Chevy]

I zoomed in on your photo and noticed that sensor is not factory on that car since it's stamped "R-134a". I would assume the switch on there is some universal binary pressure switch like this.

https://www.summitracing.com/parts/fss-36646

You could do some research on the Four Seasons website and easily be able to match it.

Also noticed those are not standard 134A conversion fittings and looked to be part of the line. Does this car have an aftermarket AC system or custom lines?