Thread: Any A/C experts out there?

Tweet

I am having trouble getting my aftermarket A/C system to take a full charge. It's a 134a system. The low side is at 15 pounds, should be between 30-40. The high side is 250-280, should be 190-200. It's a hybrid aftermarket system. When I did my motor conversion I had to get a new compressor, it's a Sanden rear exit with serpentine pulley. All hoses are new, the driers is new. The evaporator and inside blower and components are from Southern Air and were working fine before the conversion, the condenser is also from the old system. The system is free from leaks and holds 28 inches of vacuum and has been evacuated for over an hour, until closing the low side valve on the gauges causes no change in pitch from the vacuum pump. I am confident there is no air in the system. When charging, we placed a box fan in front of the grill to add air flow across the condenser. We did not add oil to the system per the instructions from Sanden. Some dye was added to help find leaks when it was first put together. The condenser and evaporator did sit open over the winter when I was working on the motor swap. It is blowing cold inside, 36-38 degrees out the vents. But I can tell it is still not right because it makes a definite growling sound and the belt jumps a lot when it compressor is on and periodically the compressor makes a real ratchet sound when run at about 2000 rpm for a period of time. My friend thinks it is the reed valves in the compressor slamming open and shut. I have already replaced the compressor once because of a leaking seal and the first one made all the same sounds. I'm pretty sure these symptoms are caused by the low pressure on the low side, but it won't take any more 134a.

Any ideas why? and suggestions of what to try?

thanks

Pat

Stu Cool,it's been almost 6 yr's.since I've done any AC work and even then I never did auto ac,but it sounds like you have either a bad metering device or a restriction somewhere.Denny is right,the comp.is working because of the pressures but a metering device that is closed or a restriction will cause low suction/high head. I have forgot about 90% of what I knew simply because I don't do the work any more but taking for granted that this compressor has a good suction valve then look for a plugged filter/dryer,etc., or closed metering device.I know so little about auto ac that I don't even know what type metering they use but your high side pressure isn't making it thru or another possibility would be an overcharge combined with too big of a condenser or way to much condensing airflow.Hope this helps.
And after reading your thread again,,,if you are using an aftermarket 134 system with part of an old R12 system then that may be a mismatch of system components there(such as coil size;cfm requirement;etc.,,)???But if not then look for a restriction.

Those pressures are ok, depending on what your ambient is and engine rpm. I don't know if you did it but a person should always spin the compressor over by hand MANY rev's once the system is assembled.

The noises that you describe can be due to unstable/loose mounting and/or loose drive belt. Check this first and then check your pressures again with engine at 2000-2500 rpm, high fan, max a/c, and the condensor fan running. Also, let us know the ambient air temp where the car is.

If its hot outside and he is getting those pressures at higher rpm, for 134A those are acceptable pressures for many oem systems.

Stu Cool,

I agree with DennyW. Your high side is too high. I would make sure the system is not overcharged. I am no expert but I am fortunate to be friends with one. I also own a telecommunications company and one of our clients in Dallas is the Sanden manufacturing plant located in Wylie. So I am lucky to be able to ask some of the engineers about the 508 unit in my sedan.

And like the issues you are describing, I too am having similar issues which occured just after I put my engine back together and had the system recharged.

When I called one of my buddies, he told me it sounded like overcharging was the culprit. He stated one of the most common mistakes with aftermarket systems is overcharging. He continued to say that overcharging is usually caused by filling the system by watching the gauges instead of simply filling by the correct measured amount of freon. For example, my Vintage Air system only takes 1.8 pounds of 134a refrigerant. He said any more than that will cause the low/high pressures to be high. He stated the low side should be around 6-12. This may seem extremely low, but he said these systems are very different than stock systems and run a much smaller low side pressure. And the high side should be no more than 2*ambient temp + 20%. So in texas with the current temp around 100, the high side should be no more than 220. Make sense?

When I had the system charged by the local shop (I know...I know what your thinking...but I was in a hurry to make a rod run and took the easy way out instead of doing it myself...another lesson learned!), I noticed a loud "screeching" sound when the engine was revved up. This turned out to be the high pressure switch cutting off the compressor and then allowing it to cycle back on as soon as the pressure lowered. This continued to happen until we drained the system of refrigerant.

It now works, but not as well as it did before when it was charged correctly. So this weekend I plan to vacuum the system down and start over. This time I will charge only the correct amount and I expect the problems to be solved.

Again, I am no expert, but I trust my friend and he knows his stuff! I will let you know how it turns out.

Denny's right on the money, I would suspect it's a stuck expansion valve, It does concern me that you said you did not add oil as most compressors are shipped with out oil in them. Your low side is positiveley too low, but the high side doesn't seem outa reason as with R134a with temps in the 90's I usually see about 40 on the low side and around 250 on the high side as an average at idle speed. rule of thumb on the oil is 2oz for evap. 2 oz for condensor,1 oz for hoses, and then the compressor will need whatever the compressor manufaturer calls for. Either way I would first put a new expansion valve on it, I say expansion valve instead of orfice tube because all of those units I've seen use an xvalve....

How can you determine his acceptable high side pressure without knowing his engine or compressor rpm, condensor airflow, and ambient temperature?

troy-cryer makes a lot of sense also,maybe try letting some gas out of it and see what happens,because if it's putting out 38 degrees at the vents there can't be a hole lot wrong as most oem stuff I work on will only put out about 45-55 degrees at the vents.....

Not a auto ac tech, but the metering device sounds like the problem. High head usually gives you high back pressure. A restriction will drop your low side pressure and the more refrigerant you add the lower it will go, hence slugging the compressor with liquid, not good. Is the suctions line sweating?
Is the metering device rated for 134A? 36 to 38 degrees to cold, kinda also sounds like improper size condensor. Wish I could help more, just thought I add my 2 cents. Stu Cool.

Wow, make a post and spend the day at a Blues Fest and come home to 11 replies. You guys are the best!

Here's a little more info. The systems is all rated for 134A, ambient temp was about 90, 36-38 degrees was with windows up on high, recirculating inside air. The compressor was evidently shipped oiled as it had a sticker on it that siad DO NOT ADD OIL.

I am thinking I do have a restriction or a bad/clogged expansion valve. I will call Southern Air on Monday and ask them about a replacement.

I will keep you posted about what I find.

Pat

Originally Posted by Stu Cool

Wow, make a post and spend the day at a Blues Fest and come home to 11 replies. You guys are the best!

Here's a little more info. The systems is all rated for 134A, ambient temp was about 90, 36-38 degrees was with windows up on high, recirculating inside air. The compressor was evidently shipped oiled as it had a sticker on it that siad DO NOT ADD OIL.

I am thinking I do have a restriction or a bad/clogged expansion valve. I will call Southern Air on Monday and ask them about a replacement.

I will keep you posted about what I find.

Pat

if your system is cooling down to 36 degrees there is no restrickens. on a hot day 250-300 lb. hi side is not to high, and if its cooling at 15 lbs. low side then thats great. most comp. make noise when they cycle, and other wise the belts can make dif. noises. as far as the ac goes if you drove it any dist. yesterday and it cooled and didnt ice up, i dont see a problem. if you dont have the proper mufflers in the hose you'll get all kinds of noise. sucking and pumping.

Stu,

Glad you had a good time at the run. Thats what this is all about!

As a follow up on this post, I wanted to stress again that my friend says forget conventional wisdom about stock A/C systems due to the fact these aftermarket systems work differently. This really hit home when he told me the low side should be between 6-12 psi. I immediately responded "that sounds too low to cool at the temp I want". He then reminded me to forget stock systems which have larger capacity systems.

Here is a link to the Trouble Shooting guide found on Vintage Air's website which describes the expected low and high pressures. I would suspect Southern Air and Vintage Air's systems to be similar in how they operate with respect to pressure, so it should work for you as well...

http://www.vintageair.com/DownloadsS...ng%20Guide.pdf

Again, I am no expert, but so far my research points to too much freon is not a good thing on these street rod systems.

OK Guys, based on earlier input I went looking for my expansion valve and for restrictions in the system, and I think I may have found a restriction. On the new hose that runs from the direr to the evaporator on the high side, the 90 degree metal fitting is a smaller diameter than the old hose fitting. The old fitting was 3/8" OD, the new fitting is 5/16" OD and then steps up to 3/8" at the connection point. Is this enough of a difference to cause a problem? The high pressuer side is pushing along through a #6 hose between the drier and evaporator and now has to go through a smaller pipe than before. I think the hose maker used what he had instead of what was actually needed and I did not even notice it when I went to install it because the fitting on the end was still the right size. Thanks again for all your help.

Denny those readings look like what I am looking for with 90 degree ambient, Troy that Vintage Air trouble guide is very helpful. From that I have a restriction, a bad expansion valve or am over charged. The growl and chatter could make me think we have air in the system, but I know it has been well evacuated.

Thanks again

Pat

Stu Cool,

HTML Code:

 http://www.csgnetwork.com/r134apresstempconv.html

Heres a site with a interactive chart that you can enter different psi's for
134a and get the proper evaporator coil temperature.

And there is a lot of other information there on all sorts of suff.

Pat, I don't think that is your restriction, the restriction I'm talking about is on the high side, (inlet side) of your exspansion valve, where there is a small screen in their that gets plug up with dirt and prevents you valve from metering properly. If you are able to Isolate or disconnect the compressor on both sides high and low from the metering device, you can take a tank of nitrogen and blow back threw the exspansion valve on the low side to the high and most of the time you can open up the restriction.
Note: at 15 psi looks like a evaporator coil of 14.94 degrees F. I hope this helps.

If you suspect a restriction in the expansion valve, check the temp of the line on either side of it. Normally the temp should be same on either side. If there is a restriction there, and you have the proper R134a level, you will see a temp difference.

Also, you never told us how much r134a you put in the system.