Today I vac. the system down and added 2 cans of Duracool static. Then cranked the engine. The low gauge read 8# with the compressor running. When it clicks off the pressure goes up to 70#. Clutch pulls in again and the press. drops to 8#. I add another can of Duracool and get the exact same gauge readings. The dash air is about 60 deg. (90 deg outside) through this whole process.

You guys always talk about getting 20# on the gauge. Does that mean 20# all the time, compressor on and off? Should the pressure go up with the clutch disengaged?
Thanks,
Fred V

Fred
It should take approximately 3 cans.
Are you taking the low pressure side readings with the air conditioner on Max and the engine at 1500 rpm?

It is very strange that the pressure would be the same for can 2 and then can 3. Unless you are taking it at idle.

When the clutch disengages it is ok for the pressure to go up to 70. That is the equalized pressure between the low and high sides. However it usually doesn't go to that immediately.

If you really have only 8 psi with three cans then you probably have a bad compressor.

Emery Stora
77 Kingsley
Frederick, CO

Thanks for the advice.

First off the replacement compressor is off a '78 403 engine and I put it on my 455 engine. I found when I tried to put the belt back on that the pulley is larger on the 403 compressor and I had to get a longer belt. I've done my own AC work since the early 2000's so I'm not unfamiliar with it but still learning.

I did get a good solid vacuum on it and it held it for 20 minutes or so. I added 2 cans without running the engine as the can says to do. I then cranked it up and saw the 8 to 10# gauge reading at 1500 RPM. Adding the 3rd can made no difference. It is working and cooling enough for a July 4th short trip so I won't do anything more until after that. The compressor is cycling about every minute as it did before changing to the 403 unit.

Today I removed the blower to check for debris and found the internals to be clean. I'm getting about average air flow from the dash vents. I have, in the past removed the front of the AC box and cleaned it out. The evaporator at the time was not plugged up with debris. I have the heater cut off with a ball valve and the fresh air intake at the front of the box blocked off so all air is coming through the return air duct inside.

So, it is looking more like the exp. valve however with the old compressor I was getting the 20# gauge reading. That is why I'm asking these questions. The compressor was working fine on the coach it came out of.
Fred V

Fred, I'm wondering if the compressor may be cycling to soon because of an issue with the thermal cycling switch. It is easily bypassed temporarily to see if that may be contributing.

http://www.gmcmhphotos.com/photos/miscellaneous/p17173.html
http://www.gmcmhphotos.com/photos/miscellaneous/p17173.html

I haven't found the problem yet but today I jumpered out the temp. switch that cycles the compressor and it seemed to work fine. It put out 45 deg. air and the low pressure hose was not frosting up. Wed. I'll take it to a 4th gathering as a mobile hotel. The drive will be about an hour or so which will be enough to see what it does. If it fails then I'll work on it again when I get it back home. Expansion valve replacement will be the next thing. The system has a lot of green goo in it so I would not be surprised that the exp. valve was gummed up.
Thanks for all the advice,

fred v wrote on Mon, 30 June 2014 15:34

I haven't found the problem yet but today I jumpered out the temp. switch that cycles the compressor and it seemed to work fine. It put out 45 deg. air and the low pressure hose was not frosting up. Wed. I'll take it to a 4th gathering as a mobile hotel. The drive will be about an hour or so which will be enough to see what it does. If it fails then I'll work on it again when I get it back home. Expansion valve replacement will be the next thing. The system has a lot of green goo in it so I would not be surprised that the exp. valve was gummed up.
Thanks for all the advice,

If you have any kind of goo in there you are going to have to flush everything including the compressor and put in new oil too . The dryer will need to be changed also.

I haven't found the problem yet but today I jumpered out the temp. switch that cycles the compressor and it seemed to work fine. It put out 45 deg. air and the low pressure hose was not frosting up. Wed. I'll take it to a 4th gathering as a mobile hotel. The drive will be about an hour or so which will be enough to see what it does. If it fails then I'll work on it again when I get it back home. Expansion valve replacement will be the next thing. The system has a lot of green goo in it so I would not be surprised that the exp. valve was gummed up.
Thanks for all the advice,

Fred referenced a comment by Roy some time ago about mounting the bulb for the switch on the low pressure side somewhere instead of in the evaporator. Roy, would you please comment. Or anyone else who has tried this option. Thanks.

Rick