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SOLVED: Strange A/C Pressures - troubleshooting help needed

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Thanks in part to the @thehackmechanic's awesome book, I finally put together all the required items to install a used clardy A/C system in my '76. Everything with the exception of the evaporator is new, including a Sanden SD508 (thanks @02hobiedave!), new parallel condenser, receiver/drier, and a new expansion valve. I have some NOS R12 cans for the system. After ensuring it held vacuum overnight, I went to begin charging it. Unfortunately, a problem that I need some help troubleshooting.


  After taking in about half a can (~6oz), the pressures are not making sense. My low side is running around 80, and the high is around 100 (80deg day). I am not seeing the normal drop in low pressure once it starts taking in the refrigerant and it's clearly not taking any more. I can hear refrigerant moving through the expansion valve and see bubbles in my receiver sight glass. The only two items I can think of are either the compressor or the expansion valve. While both are new, it's certainly possible they are defective.


For the compressor, it engages normally, makes no weird noises, and I can turn it with a wrench easily. Everything I've read about SD508s is that they are CW rotation (which it is doing). Is it possible it's somehow reverse rotation?


For the expansion valve, is there any way to test it without removing it from the system? I still have the original one, but prophylactically replaced it. Maybe I shouldn't have! Any thoughts or advice would be greatly appreciated!


(And yes, if I have to open it back up, I'll take it to the local AC shop to evacuate it properly)

Edited by 02Coastie

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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That is very odd. 

I've installed at least six of these inexpensive Chinese-made Sanden 508 clones and have never had a problem, so given the choice of suspecting the compressor or the expansion valve, I'd suspect the expansion valve. I don't really know of a way to test it. I've never really trusted the new Uro Parts expansion valves, but I can't say that I had a bad one either. I tend to try to find NOS Egelhof valves or reuse old ones if they're clean.


As it says in the book, if you know that the compressor is engaging, when you start to charge, you should immediately see the low side spike upward as the refrigerant gets drawn in. You need to moderate this with the blue knob on the manifold gauge set. It'll quickly stabilize and you can run the knob wide open. It should settle down on the low side somewhere between 20 and 40 psi. 80 is way too high.


I do have a Clardy system in a box here at the house, but I've never had one in one of my cars. I worked on one in a friend's car and was very impressed by how cold it got on one can of R134a. They do have a nice advantage in having the expansion valve outside of the evap assembly.


That's a tough choice. I had a problem in my E9 that I convinced myself was a clogged expansion valve, pulled the whole damn thing apart, and the valve appeared to be fine, at least visually.



The new book The Best Of The Hack Mechanic available at https://www.amazon.com/gp/product/0998950742, inscribed copies of all books available at www.robsiegel.com

1972 tii (Louie), 1973 2002 (Hampton), 1975 ti tribute (Bertha), 1972 Bavaria, 1973 3.0CSi, 1979 Euro 635CSi, 1999 Z3, 1999 M Coupe, 2003 530i sport, 1974 Lotus Europa Twin Cam Special (I know, I know...)

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"My low side is running around 80, and the high is around 100 (80deg day)."


Not an AC system expert, but wouldn't the pressures being so close together mean either the expansion valve is sticking open, or the compressor is leaking and can't build pressure on the high pressure side?


Any idea which of our expansion valves some retrofit kits use?  Part number 901 573 915 00 fits 1977-1983 320i, and we sell about 200 of them annually with less than a 1% defect rate.

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Thanks to both of your for the thoughts. I was able to swap in the original expansion valve. While I have no way of testing it, it did allow me to blow a small amount of air through. After putting a small amount of r12 back isn’t he system, I got the same results. It seems like the compressor is moving *some* refrigerant, but not nearly enough to build proper pressures. I’ve reached out to the vendor for a warranty replacement. We’ll see how that process goes.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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2 hours ago, ray_ said:

Can you verify the a/c gauges are reading correctly?


I hadn't thought about this, and the short answer is no. All my other vehicles are R134A and I'm under the impression you don't want to mix gauges due to cross-contamination issues. Having said that, I did let the car run for a good 15-20min with the R12 can hooked up the first round. After that time, virtually no more refrigerant was taken in and the pressures remained the same. Additionally there was not even a little cooling at the evaporator and the condenser felt normal.


I can't think of any other scenario, other than a bad compressor that would lead to this condition. From what I can tell, something is preventing the suction portion of the compressor from functioning.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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Yeah, you're probably right, I think.


I'm not sure there's potential for cross contamination since you should be evacuating the manifold lines with your refrigerant. Maybe an issue with mixing various compressor oils but...




Stop reading this! Don't you have anything better to do?? :P
Two running things. Two broken things.


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  • 2 weeks later...


If you have bubbles you need more Freon. Sounds like everything is working you just don't have enough in the system. Unless your coils on the evaporator are frozen or frost on vents. then you might have expansion valve issue.  I'm not there but from what you have written down. Hope it helps


1972-2002 "polaris"

2000 M5 "Dieter"

2004 330i "ILKA"

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half a can 

I agree with Dave-

and, does everything have enough oil in it?


From 134 (all I've messed with, so far) if it's got less than 1/2 a

charge, pressures don't start to do what you'd expect.  Most

systems say to put in about half, THEN start to look at how pressures are doing.



forgot the oil, once.  That didn't last long.

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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On 5/28/2020 at 9:20 AM, 02hobiedave said:


If you have bubbles you need more Freon.

I would definitely agree with you. Unfortunately after leaving the can attached for nearly 30min, it only took about 1/3 a can. This repeated when I swapped out the expansion valves and tried again. I'm on deployment now, but when I get home I'll try a new compressor (I have to buy another one anyway for the CS). I'll definitely report back.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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  • 11 months later...

Just to close this thread: I am an Idiot.


Bottom line: The compressor is totally fine, I just used bad equipment to charge it.


In my attempt to keep my A/C charging rigs separate (one for R134a and the other for R12), I used a rig that I had bought second hand but otherwise never tested. It turns out the low side charge hose was missing the little metal plunger that acts on the schrader valve. Apparently the pressure differential was enough to introduce some refrigerant into the system, but obviously not enough to charge.


This morning I set about the task of replacing the "faulty" compressor with a new unit. As I was checking pressures to see if I needed to take it somewhere for evacuation, I saw zero gauge pressure even though I heard a little "psst" when I bumped the valves while attaching the lines. This lead me to inspect the lines and realize they were damaged.


I swapped them out with some spare good ones, put two cans in and Sabine is pumping out 38deg air. Couldn't be happier.


Fortunately, my E9 is in need of a compressor, so the extra one won't go to waste.


Lesson learned: Check your equipment.


Thanks to everyone for the help!

  • Like 2

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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  • 02Coastie changed the title to SOLVED: Strange A/C Pressures - troubleshooting help needed

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