My AC seemed to not be freezing the cab as well as I thought it should. My buddy was replacing his AC Condensor on his Nissan and has the kit handy to fill it. So I bought a bottle and tried to see if I could add some. My compressor seemed to be short cycling, every few seconds the clutch would click on and off as the pressure went from 25 (clicked off)~45(clicked on) psi. The duration got much longer as I added a little, telling me I was definitely in need of a charge. My question is, when do I stop adding? The pressure didn't seem to go much higher as I added, though it did stay more constant around 35 psi or so. Is there a rate at which the compressor should cycle? The haynes manual said to feel two lines and see if they feel the same temp, but I was looking for a slightly better method if there is one.

Thanks,
Jim

 

Well, the outside temperature as well as engine speed and blower speed will affect cycling. What I like to to is, on an 80+ degree day, turn the blower on high, rev the engine to about 1400 rpm, and charge until there is no cycling and the low side pressure is about 35 psi. As the outside temperature and the blower speed decrease, they system will begin to cycle to prevent icing. If you overcharge the system, the low side pressure increases and you will loose cooling efficiency. This is because the pressure and temperature at around 35 psi are approximately the same. Because of this, I may tend to undercharge slightly; if so I just add a few more ounces. Also, at around 70 degrees outside and on med-lo blower speed, you can expect some cycling.

BTW, always remember to purge air from the line leading from the bottle of freon.

 

Ok, makes sense.

How do I purge the line from the Freon bottle? Just open the valve, then plug it onto the low side?

-Jim

 

Hook the valve to the can, pierce the can with the valve. Before hooking the line from the can to the low side, open the valve on the bottle slightly and let a small amount of freon escape. This get's the air out of the line and keeps it out of the A/C system, where it does not belong. Omitting this step is a common DIY oversight.