Hey everyone,

Hope all of you had a great Labor Day weekend. I spent my Monday enjoying myself working on the car until this. My car is a 1971 Challenger and I had A/C problems 2 years back. I had the system converted to R134 by Classic Air and they did a great job. I am no A/C expert, and unknowingly, when I charged my system the first time (I had access to a vacuum pump at the time and now I don't ) I overcharged the system, I put in as much R134 in as it called for R12. I forgot the ratio again, so I will have to look it up...again. Anyways, A/C was working great..then it stopped. Went to an A/C place, they evacuated the A/C system, recharged it and the A/C would just cycle on and off continuously. Well, after a long time I found an NOS Low Pressure cut off switch, thinking that may be the problem...and the thread diameter is to big!
The part I bought is part number 3441904, which is called out as being before November 1, 1970. After November 1, 1970, the part number is 3502016. Does anyone know if in fact the stud diameter is the change? I don't want to buy another switch for basically nothing again. Also, I can't be 100% sure my receiver/dryer is correct for 1971. While I am at it, I am also going to change the Pressure Regulator Valve (even thou I put a new one in when I installed the system in the first place) just in case I blew it up when I over charged it. I had to change my heater core this weekend, which got me to this point(good thing I pressure tested the core before I put it back in ) but the good news is, after 2 years, my A/C system still had full vacuum! When I disconnected the first line, it sounded like a vacuum cleaner for about 5 seconds . When I had it at the shop, and the A/C was just cycling, the shop evacuated the system and pulled vacuum and closed it up...and it held.
Again, any help on the difference between the low pressure cut off switches would be much appreciated!

Was the Dryer an NOS part or was it supplied by Classic Air? he deal is 70 style switch & dryer were whats known as "o" ring boss.. 71 is 1/8" pipe thread... The easy way would be to just let an A/C shop provide the switch, they will be able to see what you have & match it functionally.. Visually it may differ a little but if you've converted to R134 then chances are it already has different charge ports...

hey 1_Wild_RT, the dryer was the one that came with the car when I bought. It was off the car in the box with all the other A/C pieces. So I sent the dryer i had in to Classic Air to be reconditioned and that is the one I got back, but again, it was off of the car in the A/C parts box, so I can't be 100% sure it is the 'original' one. I am slowly pushing this car to originality as I come across parts when I need/find them, so I would like to replace the switch with an accurate one if it all possible. To your point, I think I am going to have to figure out the dryer as well.

You might want to talk to them & have them show you what they have.. The replacement switches are close.. Not perfect but generally very close..

Thanks! That is good to know, I will have to give them a call.

Thanks for your input 1_WILD_R/T, very much appreciatted