Ok. I have a F150 4X4 regular cab with a 5.0. I bought the truck with the A/C not working. I started trouble shooting with the guages and found the high and low pressure side both at 100 PSI with the truck running. Well with that bit of information I figured it was time for a new compressor. I took it down and had a buddy vac the system and bought a new compressor, new low pressure switch, new orifice tube, and a new accumulator. I tore the system down and flushed each hose and the condensor before reinstalling everything with new o rings.

I pulled a vacuum down to 28 for about 40 minutes and let it sit for 4 hours with everything ok I went to putting the oil in. I used the high and low side to suck the can in and began feeding in the 134A into the low side. Started the truck and after the first can the compressor kicked on and began short cycling. The next 2 cans went smooth letting the compressor suck them in slowly with no problem. The compressor however was still short cycling. I couldn't get a good pressure from the low side it would cycle between 20 and 45psi but the high side was steady at around 200 psi with the temp of 82 degrees. After putting in a couple of more ounces with no change I went for the garden hose and lightly misted the condenser with of course no change.

This is frustrating. It's my 4th conversion and I haven't ever had a problem like this. Could this new low pressure switch be defective?. This is day #3 and I, my friends, am stumped.

 

You may still be low on refrigerant. It should take ~80% of the R12 charge. The low side should be pulling down until it opens the LPCO switch. Disconnect the switch on the accumulator, and jumper around it. If the compressor stays running and you get cooling, you could try adjusting the switch using a small flat-blade screwdriver.
tom

 

Adjusting the LPCS I can get it to hold at 20 low and 150 on the high at about 74 degrees but that's it. The next adjustment up it starts short cycling again. I may try to replace the new LPCS.

 

If the lpcs is not in a constant state of cycle after you made the adjustment then it is doing OK why throw money at it? If you can only get to 74 it sounds like low charge, do you have a sight glass, do you see bubbles?

 

That's a negative on the sight glass and the 74 degrees was the temperature outside at 10:00PM.

I hooked the guages up and the low side cycled between 25 and 45 and the high side between 175 and 225. If I jump the switch the compressor will stay on and ramain in the 200 range with the low side going to almost nothing. Any ideas?

 

The LPCO is doing its job. The evap is supposed to stay above freezing, so the LPCO cuts off the compressor when there is not enough 134 coming through (evaporating and pulling heat) to keep the evaporator from freezing up. That means you do not have enough flow of refrigerant to keep up with the compressor. OR there is not enough load on the compressor & evaporator (and condenser) to keep the compressor fed a full flow of R134.
The LPCO will cycle the compressor, that is its job....
Maybe the ambient temp is not high enough to call for continuous compressor running. Either that, or you do not have enough refrigerant.
tom

 

How much 134 is the system supposed to get? I was told about 40 ounces. Is this correct?

 

b ray, there is supposed to be a manufacturers label under the hood somewhere that specifies the amount of refrigerant for the system. A number like 40oz would not apply for all systems, such as a van with a rear and / or roof evaporator, fan and ducts.
The amount you put in allows the system to work as hard as it can without over pressuring the compressor and as slow as it has to by cycling the compressor. If you put in too little, it will cycle, if you put in too much, the hi pressure reading will possibly get too high, AND you will lose cooling because there is not enough 'room' in the system for the gas to expand and extract heat.
tom

 

Point taken. I vacuumed the system yesterday to check for leaks and left it sitting overnight. The systems holding at 29 on the vacuum. THe r12 is supposed to be 3 lbs 4oz so that is 40 oz of r12 and at 80% that will be about 32 oz of 134A. Does this sound correct. I'll see what happens with the correct amount of freon in it will do.

 

Take it to about 70% of the R12 weight first, then add small amounts (1-2 oz) until you get a high side reading about 2.2X the ambient temp.

That will put you very close to the "ballpark".

When the charge is okay, the evaporator outlet will be about the same temp as the inlet.

Steve

 

I knew it was something simple. I put the extra pag oil in. I reflushed the system and it is now running at about 25 on the low side and about 225 on the high side in 94 degree heat. I want to thank all of you for your help. I need to find my vent temp guage and I'll tell you what it says (if you care).

 

You can store the remainder of any you have left in the freezer and it will not leak out so much.. vapor pressure at 0 deg is pretty low, so those molecules kinda bubble around and don't jump out of the can.
3 X 16 = 48 + 4 == 52. You should need approx 42oz .. or a little less. The 70% idea sound like a very good way to approach your optimum cooling.
tom

 

A can of freon (r12 , r134a or whatever) is not a "Pound", unless it has 16 ounces in it.
The cans sold today are 12 oz cans.

Did you change the orifice tube during your conversion?
Change the Accumulator?
Put in only the correct amount of Ester oil, too much oil is VERY BAD!

 

"A can of freon (r12 , r134a or whatever) is not a "Pound", unless it has 16 ounces in it. "
so what.

But, 3 lb 4 oz is 3lb 4 oz no matter what size can you are talking about.

That is what you call a conversion, from lbs/oz to oz so you can do the math and figure out how many OZ you should install.

Who is talking about cans?
tom

 

I didn't know I had to specify that I wasn't posting about your posts.

Many folks mistaken a can as a pound, as that is how its referred to when the bench racers are gabbing.

Perhaps some Ice Cold AC would calm you down.