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Countilaw 03-02-2007 03:41 PM

A/C cycles, 3lbs installed
Maybe some of A/C guys knows what is going on.

Problem: Compressor cycles

Vehicle: 93 Chevrolet Caprice 305 engine R-4 compressor. system holds 3.12 lbs of R-12.

What has been done:
(Converted system to R-134a)
New (not rebuilt) R-4 Compressor
New Drier
New Orifice
New Hoses
All new O rings
Reverse flushed Condenser and Evaporator

Install procedure:
Removed all old equipment and put a/c flush in Evaporator and Condenser. (let sit while I got everything else ready to install)
Pour shipping oil out of new compressor.
Poured 4 oz of Ester oil in new compressor and rotated compressor 10 revolutions and poured oil out.
Poured 4 oz more of Ester oil in new compressor and rotated.
Poured out all the oil and put in 7oz of new Ester oil and capped compressor.

Flushed out Evaporator and Condenser with dry air line at 60 psi. Then allowed remainder of flush to evaporate out.

Connected new hoses with new O rings lubricated with clean Ester Oil.
Installed New Compressor, connected high and low side hoses.
Rotated compressor by hand (socket and rachet) for 10 revolutions.
Connected guages and pulled 27 inches of vacuum for 45 minutes and then disconnected vacuum.
Let the system set over night (19 hours) on the guages.
Next morning, still holding 27 inches of vacuum.

Rotated compressor a few turns and began installing first can of R134a. Engine not running at this point. After about 1/2 can 6.5 oz of 134a I started engine and turned on a/c to Max with high fan speed.

Just as the first 13 oz of 134a started to empty, the compressor started cycling.

At the end of 26 oz of R-134a the compressor continued to cycle.
Low side pressure would increase to approx 40 then the compressor would come on. When the low side decreased to 25 the compressor would shut down. On the high pressure guage the reading would hover at 50 lbs.

At the end of each cycle, as the guage would drop to 25 the compressor would start knocking and then shut down. Once the pressure on the low side would climb back to 40, the compressor would come on and run quiet. But again the low side would drop to 25 and start knocking again.

Now I have 3 cans (39 oz) of R-134a in the system and it still does the same thing. High side reads 50 lbs, low side goes as high as 40 and as low as 25.

Any ideas?

grandpaws 57 03-02-2007 06:56 PM

Did you use GM [white]?Is it backward.Aside from tha,yur hi side is way low.Onr trick on changeover is to put a older ford cycle switch r-12 ,these are adjustable.Then you can adjst cycle time-cycle off say15 to 20 psi.One other thing my be junk you could not get out pluging orfice..Concerding your presure I would check orfice anyhow.

Countilaw 03-02-2007 10:33 PM

I used a new GM orifice tube and the short end goes in first, the long end is just inside the tubing. Just where you can grab it with needle nose pliers to remove it.

I made sure there was nothing in the system to plug it up by reverse flushing everything. (except the compressor).

I may have to pull it out and check it. :smash: There seems like something could be plugging the orifice because the low side pressure drops so much. But why is the high side pressure so low? I would think that if something was blocking anything in the system, the high side pressure would be off the gauge.

What is this Ford adjustable cycle switch?

grandpaws 57 03-02-2007 10:58 PM

If you lunched a former comp something you should know;you MUST REPLACE CON. ON134A SYSTEMS.They have a that holds junk,flushing won't get it.Been working ona/c all my working days.seen 134a con. cause comp to fail. As for presures keep low side above 15-20There is an adjust screw on many cycle switchs,look between spades.It don't matter what make of switch you have.two types r-12&134a won't mix, differen't threads.Some acul. have both.Whats air temp? that will change readings .So,pull orfice ,see if it dirty.Go from there.hope this helps

Countilaw 03-02-2007 11:13 PM

Didn't have a compressor crash, just wore out and started making noise. This is a former R-12 system I have just converted to a R-134A with this compressor.

Outside temp was 65 degrees. Temp at a/c vent was 35 degrees.

I guess I will just have to pull the orifice and take a look see.

Thanks .

grandpaws 57 03-03-2007 10:19 AM

blows cold
well at least its cold.Under lite heat load you can have a low high side .Mabey a diff. of 50lbs between low&high.Could you shocked comp with oil? Hope it helps

Countilaw 03-03-2007 10:37 AM

The compressor was new and I poured all the oil out of it. Was only about 2 oz in it. I poured in some new oil and hand rotated the compressor and poured the oil out. Did the same procedure a second time.

After I poured the oil out, in put 7 oz new Ester Oil in and capped compressor. After I got the compressor installed and all hoses attached, I again rotated compressor 10 revolutions and it felt fine.

I think your first suggestion may be true. There could be some trash partially blocking Orifice tube.

I am going to recover the 134a and check the Orifice.

I think the compressor is knocking when the low side pressure drops because of High head pressure. The 134a is stuck between the orifice tube and the compressor. My high side guage could be giving a bad pressure reading. I am going to check it with a different set of gauges.

Thanks for all the help. :welcome: If you think of anything else, let me know.

xntrik 03-03-2007 02:48 PM

Vintage Air recommends a binary pressure switch to save the system if it gets low, or has high pressures.

GMbowtye 03-03-2007 03:23 PM

you only use 80% of the r-12 capacity when you convert to 134a

xntrik 03-03-2007 04:08 PM


Originally Posted by Countilaw
Didn't have a compressor crash, just wore out and started making noise. This is a former R-12 system I have just converted to a R-134A with this compressor.

Outside temp was 65 degrees. Temp at a/c vent was 35 degrees.

I guess I will just have to pull the orifice and take a look see.

Thanks .

If your outlet temp is 35* then the compressor will automatically turn off before freezing your evaporator coil.

Your low high side pressure might be due to the low ambient. There is just no heat to remove from the cabin and the condensor is extremely efficient at 65* ambient.

You might not have any problem at all. Just low temps.

edited= 65 to 35 is only 30* delta drop. The system should be capable of 35* delta at 90* ambient, and 30* ambient at 100*. More if you are humming down the highway. Today your system is just not having to do anything.

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