Jig A Low

Hey guys, now that I'm about done with the rebuild I figured I'd get the A/C charged up and running. but I seem to have an issue I need some input on.

here's the deal:

100% new system (comp, hoses, o-tube, evap, conden, drier, low pressure switch)

recharging with R-12, vacuumed down to 30 hg and let the vacuum pump run for about an hour, then let it sit for an hour with no leaks.

charged with the right amounts of oil in the compressor and drier.

charged with 48 oz. of r-12

ambient air temp 70*

suction side pressures 40 psi (compressor off)--down to 25 psi (compressor running)

high side pressure 250 psi compressor running.

air is cool but not cold as it should be

the condenser is sealed to the core support good, running a new clutch fan.

the high side pressures seem real high to me ??

the compressor cycles on and off (about 50/50 cycle), shouldn't it run all the time ??

REELAV8R

If you are running the stock clutch fan make sure the engine is up to full temp with hood down before you start.
This ensures that the fan is fully engaged and pulling air through the condenser. Also ensure that the RPMs are around 1000 or so for good air flow.
If you have insufficient air through the condenser what you will see is high on high side and low on low side because you don't have enough freon in yet due to your concern with the high side pressure.
If electric fans make sure they are running.
Once you get the system filled your compressor will continue to run.
Right now it sounds like the low side switch is kicking it off due to low pressure then the low side pressure rises while it's off and allow the switch to run the compressor again until pressure is too low again and repeated over and over.
Just filled my 77 yesterday. Initially my cooling fans were not running and saw exactly what you're looking at.
I also put a digital thermometer in the air duct to see how cold it's getting. I got 43* yesterday with 70* ambient temps.
Ensure that A/C selector is in max.

hardlanding

also put in a shut off valve to the heater core, i can tell you i get a 10 degree difference by adding that

Jig A Low

does anyone know what the charge amount for the system is (R12), I tried searching for it but all I can find is info about converting to 134a. I saw one post that said between 2.75 and 3.00 lbs = 48oz. but I'm not sure if thats even close.

Ralphbf

What REELAV8R said, except with electric fans they should run when ever the AC in on, or be powered through a high pressure switch, not a high pressure cut off switch.

REELAV8R

Doesn't really matter. Fill it until the pressures are correct.
When you top a system you don't determine how much is missing and add it for a certain total, same here.
The listed amount just gives you an idea about how much it should take.

Agreed, mine had become disconnected from the control relay, usually they are on whenever the A/C is on. They have to be or the condenser would overheat when not moving and the high pressure switch would shut the compressor off.

'75

R12 system is easy to tell when full, watch the sight glass, you should only see an occasional bubble going past when its been running and stabilized. Seems like you have an air flow problem across the condenser, you might try spraying it with a garden hose and see if your hi side pressure comes right down.

Jig A Low

thanks guys, I'll try the garden hose idea, unfortunately there is no sight glass on the '81 so it's a bit more difficult.

I hate to keep throwing freon in it, this stuff aint cheap and I would rather not be wastefull and overfill it. I'll put in what I have to but need to do it in a controlled fashion