Model: 2000 Spider
Engine Size: 2000cc
Refrigerant Type: R134a
Ambient Temp: 75F
Pressure Low: 70
Pressure High: Unknown
Country of Origin: United States
After about five months I finally got my custom A/C completed. This is on a 1980 Fiat Spider. The components are as follows. 15K BTU (supposably) evaporator unit, 12" X 20" condensor (largest that would fit this car), Sanden 505 compressor, R134a, 16"SPAL radiator fan.
First question. I pulled a vacuum on the system for about 30 minutes and emptied a 12 oz can of R134a into the system. Afterwards I get 70 PSI on the low side and a 10F drop in air temperature. It was around 75F and I aimed an infared heat monitor at the vents and it said 65F.
As of now I cannot check both the high and low sides simultaneously. Both gauges are connected to the manifold and the hoses are not long enough to reach both ports at the same time. The low side is at the evaporator and the high side is at the bottom of the condensor. Yeah not where you would expect them to be but I had to build this system around my custom built turbo system and had to install the ports where they would fit.
I read somewhere if the pressure is around 70 PSI on the low side this indicates a low charge? It's my understanding the more refrigerant in the system, the higher the pressure on the low side. So if the system was empty, the pressure would be zero. Supposably 45 PSI is considering "full" on a R134a system. Anything above 50 PSI means something is wrong.
Is there anyway to estimate how much R134a a custom system will hold? What formula do automakers use to estimate how much their cars hold? I would imagine the volume of the condensor+evaporator+hoses+dryer.
I don't mind to pay someone to charge my system if they can get the system to perform at it's full potential. Can these recovery/charging machines these A/C shops have add and remove refrigerant until they find the A/C's sweet spot? Does that make any sense?
Here's some photos of my system:
Buy some longer hoses. You have to be able to see the high side when trying to charge by pressure, it's a lot more informative than the low side. There aren't any systems so small that just one can is enough. TXV systems don't need to be charged quite so precisely as CCOT's because the receiver-drier acts as a buffer and can hold several ounces of surplus liquid refrigerant before it starts to back up into the condenser and cause problems (*). Also you should use a physical thermometer not infrared to measure vent temperature.
(*) In order for that to work, the R/D needs to be mounted in a vertical position and plumbed with proper flow direction.
Edited: Wed May 11, 2011 at 10:55 AM by mk378
I can't even imagine charging by pressures without monitoring religiously both the low and high sides. Another wild guess is how much oil to add to the system. 70 PSI is way too high at 75*F, would expect more like 25 PSI with R-134a.
Had a board member here about 7-8 years ago with the handle, DetroitAC. recall he posted a paper on how they determine the oil and charge of a newly designed system. But have to use the search engine to find that post. Recall reading it, part experience, part art, part science.
A slight glass is very useful to check for pure liquid at 85*F with the engine running in the 1,500-2,000 rpm range while monitoring pressures. Doors open, blower at max, AC compressor on. When compressor is off, both low and high side pressures are the same, static pressure. When the compressor just starts turning, the low side goes down a bit and the high side raises a bit more.
At 75*F static pressures would be about 79 psi, so if you are running at a very low compressor speed, may only be dropping down to 70 psi to explain the very low dip in vent temperatures. But also extremely doing this without monitoring the high side, could be well over 400 psi. You or anyone, just does not know.
When you order the replacement compressor, get the head with the service fittings on the back facing aft as well as the hose fittings you have now. That will give you a place to measure things.
12 ounces was a severe undercharge. With a TXV system you can charge by watching th pressures carefully. There will be a plateau or flat spot in the high side pressures as the receiver fills up. This is a range where the receiver can handle 2 to 3 ounces of liquid refrigerant in excess of it's minimum fill. Once you have enough refrigerant in the system to see some cooling and low side pressures closer to 40 psi with the cabin fan on low speed, you can start adding small quantities of refrigerant while watching the high side. Add an ounce. Wait a couple of minutes for the system to equalize. Check the high side, and compare it to the previous reading. Keep repeating this cycle as long as you see little or no change. A the receiver fills, you will see the high side jump up 5 to 10 psi from the previous reading instead of remaining nearly the same. This is slightly overcharged.
Once you have found the maximum charge this way (you are actually overcharged about 2 ounces) - recover the system & weigh the charge. Subtract two ounces and you have a target charge weight. Now go look at similar sized vehicles that mount the compressor you are using - probably an early Honda in your case. Compare the charge weight you have & the oil charge you started with. When you find a vehicle / charge weight that is close to yours, compare the oil quantity that you started with to the recommended system charge for the other vehicle. You may need to adjust your oil charge. If you need to add more than one ounce of oil to your system reduce the refrigerant charge an equal amount.
Hope this helps.
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.
We've updated our forums!
Click here to visit the new forum
Copyright © 2016 Arizona Mobile Air Inc.