1988 f250, 7.5L. Replacing the entire AC system(including low side pressure switch)converted system from r12 to r134a Finally got everything put together, pulled a vacuum on the system, no leaks (after letting it sit for 30 minutes it held at 30inHg) (am using machine at ford dealership) Charged it with 39oz of r134a (it held 52oz of r12) and started the vehicle and the compressor would come on and off very very rapidly. Like every couple of milliseconds, about as fast as a machine gun. I jumped the 2 wires going into the back of the low side ac switch (so I can keep the compressor running and see what my pressures were) the ac blows ice cold and my low side is at 30-35 psi, my high side is roughly 170-175psi. I left the wires backprobed into the switch for the time being and it will freeze you to death(I turn the ac off every so often to keep my evap from freezing) I’m thinking I have a bad ac switch (from orielys) when I bought the ac switch they offered an ac switch for r12 and r134a systems. The switches looked identical, the only difference I could think would be the passages in the switches were different( as r12 is thinner) so i bought the r134a switch since I retrofitted it. I’m thinking I either have just a bad switch, or maybe swapping it back with an r12 switch. Any input would help on why the ac comes on and off about as fast as a machine gun (when I’m not jumping the wires)

 

im not sure what CPS adapter would look like, the Ford Kit apparently has a switch with different threads, most likely to prevent confusion. The RCS, not sure what that is, probably for compressor longevitiy. The label is obviously not necessary for function. But according to this you need Air Deflectors, and a r134a condensor. You have those things?

These are the air deflectors

 

The on-off would be your condensor is still for R12 and it cant keep up with the new setup, or your switch is bad, Id get a ford switch personally!

 

At what temperature? I think the low pressure switch is turning off the compressor because you have not added enough refrigerant.


According to this chart you should need 46-47 ounces of R-134a.

 

Correct, not enough freon or refrigerant
Why would you add 39 ounces to a system designed for 52?
Add the amount on the underhood sticker
Then check the cycle times
The pressure will be higher with R134a with the required charge amount
Disregard that R12 to R134a chart

 

Was always told when going from r12 to r134 to go about 75 percent of the r12 capacity. The sticker under the hood for the ac capacity is missing so I had to do a lot of digging just to find out how much Freon this thing took. Thanks for the insight. Ill add some more Freon to it and see if it fixes it. Appreciate it.

 

you should not fill a system with the stated ounces of r12 , even with the mods to the hardware. When filling with r134a, that is.

THere is a ballpark, that chart intends to show it, but the math on the ounces when I checked would put him at 41. its all depending on what value used to convert. I used .8

 

Wait, you dont have an under hood sticker? Where did you find the one you have? You may have gotten a number from a crew cab.. or something... idk if that changed the ounces or not. For sure dont use a vans.

 

The under the hood sticker is gone. I got the specs from 4 seasons website
https://www.4s.com/media/5421/four-s...city-guide.pdf

 

Because you don’t put the same amount of R-134a in an R-12 system. What part of the chart should be disregarded and why?


Exactly

 

1989 F series manual ,52 oz was hte maximum charge of the r12

 

Should I add refrigerant first?

 

I beg to differ
You been doing this; long?
System capacity is system capacity regardless of refrigerant type
I got my Ford masters certificate and trophy in 1998, HVAC was a big part of it

 

Have you been pondering about an answer to Auroragirl’s post for the last 10 months and finally decided on one?

Who knows? You jumped in to revive a thread from Sept of last year with no information about what you are working on or doing.

 

Holy crap!! That was totally meant for another thread.