[Rob B]

I finally vacummned my AC system down in my 70 GTO. Everything in the sytem is brand new from a completed frame off resto from late last summer. I also have the POA replacement valve with the low pressure switch. After discovering my guages on my manifold set are screwed up, I just vacummned the system down for about 20 minutes then charged it.
Originally the system is suppose to have 4 lbs of R-12 in it. When using R-134, the amount use is suppose to be about 15-20% less.

4lbs of R-12 equals 64 ounces. Minus 20% makes it around 51 ounces.
I charged the system with 3 cans of 134, 36 ounces.

I was planning on one more can making it a total of 48 but when I checked inside the car, the air was cold and felt perfect.
Does this sound about right for the amount of 134 I've used or should I charge it with one more can? The pressure switch is suppose to comes on at 22 PSI and cycle off at 44 PSI +/-3 PSI from what the paperwork says. My manifold guages are not reading accurate so I couldn't get a correct low side pressure reading so not sure what the pressure actually is but do know the compressor never cycled so I must be under 44 PSI.
Any input on this would be appreciated.

[Rob B]

anyone?

[Ollie]

Why guess. I would take it to a local AC shop and top it off.

[Rob B]

Originally Posted by Ollie

Why guess. I would take it to a local AC shop and top it off.

I'm not really guessing. I just need to get (borrow) another set of guages to check the pressures.The guages I have now are not acccurate. What I was wanting to know is when people on this board have charged a new system with 134 instead of R12, how many ounces did they end up using when they charged to the right pressures. I have great cold air but only used 36 ounces of 134 where I thought I should have used more.

[SlowSpitzer]

Napa has AC machines on sale for just under $4,000.
Put a thermometer in vent and see what it is actually doing.

[Rob B]

Originally Posted by SlowSpitzer

Napa has AC machines on sale for just under $4,000.
Put a thermometer in vent and see what it is actually doing.

Why didn't you buy the one for me at the auction last month???? I know, you want to get me a brand new one for my birthday coming up!!!! Your the best Steve!!

I took it for a drive tonight in 92 degree weather and the better half was complaining it was to cold coming out of the vents in the car so i guess I aint going to worry that much about it!!!
36 ounces is just fine.

[SlowSpitzer]

I would agree. You were concerned about compressor not shutting off. When you are idling in garage with it this hot, I doubt it will shut off. I would just run with it.

[david400]

I used 4 cans in mine ,, so i think you would be ok if you added another can , how ever if its working fine you mite want to leave well enough alone .I heard 134r doesnt work well if you get to much in .

[BVR421]

all you need is a tire gauge to check low side pressure. 45-50

[77 TRASHCAN]

The correct amount that the A/C folks reccomend is to put 90% of what the R12 was charged with. Some say 85%, I've used 90%, worked excellent.....

[Squidward]

The most important thing is that you go less than factory to start with, and 80% was a good place to start. With the engine turning at about 1500 rpm and ac on max, your high side should be roughly 2.5 times the ambient temp (i.e. 80 degrees = 200#).

Another rough check on charge that doesn't apply to ALL systems is to see what your static pressure is when ac is NOT running. Your static pressure should be close to the value for ambient temp (80 degrees should be around 80-ish pounds).

Of course, you'll need working gauges to check this. I suspect you are still undercharged a little. Unless you are driving with the windows down in AZ, your clutch should cycle infrequently. Did you put your cans in a bucket of hot water while charging? That's the only way I've found to come close to emptying the cans after the first one that charges into a vacuum.

[Rob B]

Originally Posted by Squidward

The most important thing is that you go less than factory to start with, and 80% was a good place to start. With the engine turning at about 1500 rpm and ac on max, your high side should be roughly 2.5 times the ambient temp (i.e. 80 degrees = 200#).

Another rough check on charge that doesn't apply to ALL systems is to see what your static pressure is when ac is NOT running. Your static pressure should be close to the value for ambient temp (80 degrees should be around 80-ish pounds).

Of course, you'll need working gauges to check this. I suspect you are still undercharged a little. Unless you are driving with the windows down in AZ, your clutch should cycle infrequently. Did you put your cans in a bucket of hot water while charging? That's the only way I've found to come close to emptying the cans after the first one that charges into a vacuum.

I figured that I'm a little under charged right now. My high side guage isn't even working and the low side appears to be about 10-15psi off(guage needle won't rest on zero anymore). I suspect they got knocked off the hook they were hanging on in the garage. When I drove it, it seemed the compressor was cycling a little more than it should. Since I only have 36 ounces in, I'm going to charge it with one more can which brings it to a total of 48, well below the 68 ounces called for from the factory for R12. With a 20% conversion rate for 134, that would put me in the 54ounce range for a full charge.

[indymanjoe]

cant you "zero" your gauges?

[Rob B]

Originally Posted by indymanjoe

cant you "zero" your gauges?

These aren't the best set of guages in the world. I put another can in, smacked the guage a couple of times and seemed to be a little more accurate. Took the car for a good drive on the highway and worked flawlessly with minimal cycling of the compressor.