Charging A 134a System

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SJTD
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Charging A 134a System

Post by SJTD » Wed Aug 19, 2020 8:26 pm

The newer cars I've had with R134a all are charged by weight per the factory manual. And they all lack sight glasses. I've read on forums that this is because a properly charged 134 system will always have bubbles flowing in the high side. Seems odd if they have a receiver/drier. That thing never fills with liquid???

I'm skeptical. My suspicious mind tells me it's because that's how they do it at the factory. Fast and simple, suck it down, fill it up, no running of the system required and no need for a sight glass. So no instructions on doing it other ways.

I've read that VW's with their variable displacement pumps and fancy "expansion" valve (I can't remember the correct term at the moment) can't be properly charged by pressure. No explanation why and again, no sight glass.

I've "topped off" my Dodge by pressures, contrary to the factory manual, and it seems to work fine.

Some day I'll get my Wag with a Vintage Air system running. Been a while since I looked at them but I recall the instructions say fill it with a certain weight. No mention of using the sight glass on the drier. I plan on keeping the factory in-line sight glass too just for the heck of it.

So, can anyone splain why one can't charge a 134 system using pressures and the sight glass?

How bout it Ed, you've installed a few systems. Wadayou do?
Sic friatur crustulum

'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.


sonoraed
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Re: Charging A 134a System

Post by sonoraed » Wed Aug 19, 2020 9:43 pm

When R12 was common (and cheap) and typical ac systems held 3 or 4 pounds, topping off a system with a sight glass was fast and easy and worked very well.
In 1993 along comes 134A the glide rate was close and temp/pressure relationship was close to R12, converted a ton of R12 vehicles to 134A,add 3/4 oz of pag or ester, fittings,10% less refrigerant (or so) and off they went.
My first experience with 134A concerning sight glass bubbles and capacity was while installing AC in a 68 Mustang Fastback,didn’t know how much 134A to put in it so I charged it till the bubbles went away, pressures were over the top and vent temps went up as I added more refrigerant , so recovered refrigerant and started charging system 2oz at a time while watching vent temps,ended up 28 oz was just about right, still had bubbles but ac functioned as it expected.
Concerning topping off a 134A system we used to do all the time when system capacity were bigger, 30/200 on a 80/90F day was the target, but these days with some cars holding less than a pound the exact amount is critical , 3-4 oz too much and pressures are too high,2-3 too little and oil doesn’t flow with refrigerant starving compressor(kiss $1000.00 good by)
Hope this answers your question


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rocklaurence
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Re: Charging A 134a System

Post by rocklaurence » Thu Aug 20, 2020 2:59 am

Ive rebuilt and charged two separate FSJs and used pressure to fill them with 134a.


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Re: Charging A 134a System

Post by sonoraed » Thu Aug 20, 2020 6:28 am

rocklaurence wrote:
Thu Aug 20, 2020 2:59 am
Ive rebuilt and charged two separate FSJs and used pressure to fill them with 134a.
Not saying it can't be done, older systems are much more forgiving to over/under charging, also the old compressors with oil sumps (A6,Sanden,Yorks) didn't need much oil circulating other than for reed valves.
Newer cars with small refrigerent capacities need to be exact, plus now were seeing more cars with 1234YF which is very expensive (10lbs= $650 my cost) I wholesale 30 lb cylinders of 134A for $95 to other shops.
If I was out in the field doing my own car with nothing but a set of gauges would not hesitate,as a side note one of the best mechanics I've seen was a guy down in Baja with nothing but simple hand tools and a DVOM diagnose a fuel injection problem that had my plagued brother in law for a long time, his fee $20.00, would love to give him a job!


Topic author
SJTD
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Location: Lompoc, Sunland or somewhere between

Re: Charging A 134a System

Post by SJTD » Thu Aug 20, 2020 7:23 am

What were the gauges saying on the Mustang?
Sic friatur crustulum

'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.


Xplitive
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Re: Charging A 134a System

Post by Xplitive » Thu Aug 20, 2020 9:43 pm

When I did this a year or two ago, recall reading something about the function of the expansion valve and the sight glass bubbleS or no bubbles not being relevant. Now my interest is peaked and have to see if the article can be found.
Erik
1989 GW, 31s on rancho front and general spring rear


sonoraed
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Re: Charging A 134a System

Post by sonoraed » Fri Aug 21, 2020 6:34 am

SJTD wrote:
Thu Aug 20, 2020 7:23 am
What were the gauges saying on the Mustang?
Was about 25 years ago, I think 50 over 350-400 lbs when over charged, anything over 400 lbs freaks me out, I had a discharge hose blow off the fitting and dent the hood of a new J10 (I overcharged) , installing AC for the dealer way back in the late 70's, one of those things you don't forget!

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