I've read numerous procedures most are generally the same with minor additions. After evacuation, your under a vacuum and the low pressure switch is open. Some state to run the vehicle with the AC full blast and start adding the freon. Some on the other hand state to jumper the PS first than start adding the freon. What's the point in forcing the compressor to run without freon?

Is there any reason not to add enough freon in gas form till the PS is made before starting the vehicle and turning on the AC?

 

You dont indicate which year vehicle you have, earlier cars do not have a low pressure switch. I would think a system in a state of vacuum will take in enough gas to activiate the low pressure switch, if you have one, causing the compressor to engage. If not you can jump it.

 

I'm not a big fan of running a compressor totally void of freon but as long as it's evacuated and it has an oil charge, it probably won't harm anything either.

I normally will introduce the Freon and the compressor will start engaging (cycling) when the pressure gets to the point of enabling the pressure switch. On systems where the compressor runs continuously, I will pressurize the system with at least 1/2 of the recommended charge (by weight) before I allow the compressor to engage.

Either way, I prefer to charge my system by weight and pressure readings vs just going just by pressure. Your mileage may vary..

 

It's a '78 and does have a low pressure switch. My question is, what's the point of jumpering the switch or forcing the compressor to run? If it was running and the pressure dropped below 28psi, it would open up, why defeat that for charging?

 

i'm sure someone will tell me i'm wrong but in my experience forcing the compressor to run speeds up the whole process. the system takes the Freon faster. also in my opinion just the nature of a c-3 with the open hood covering a portion of the condenser I use a floor fan and get the whole process over with as soon as is practical. less chance for error

 

Have you charged yours after evacuating? It's under a vacuum, how much time are you saving instead of waiting to get over 28psi? Just seems crazy to defeat a safety switch.

I understand that using a fan helps speed up the process. How often are you doing this?

 

My $.02, after evacuating the system, turn everything off, with both your hoses attached to the car. Turn the bottle of freon upside down and open the high pressure port. Open the valve on the freon bottle and As the liquid freon is going in, you will see your gauges go up a bit. Not enough to charge the system, but enough to work the low pressure switch. Once the gauges have equalized, close the high pressure, turn the bottle rightside up, start the car, open the low pressure valve & complete the charge. If the freon bottle starts to freeze up during this final phase, drop the can in a bucket of water. That will keep it from freezing and speed up the last part of the charge.

 

the vacuum is gone almost instantly when you open the valve. if you don't like bypassing the safety switch then don't bypass it. i have the older style system with no low pressure switch.and the compressor runs the whole charging process without any problems. have charged many many older and modern cars over the last 30 years

 

Correct procedure. I will add liquid, through the high side only, as long as it will go in with the system off but not go over the required amount. With R-12 you will not be able to get too much in this way. There should be a weight of refrigerant for your system listed on the sticker on the compressor. The vacuum will allow more to be added. mike...

 

That's EXACTLY how my dad does it.
Using real HVAC gauges you pull the vac and wait to see if it holds.
He makes me wait an hour.
Then, Put the 30lb bottle on a scale and weigh it as it goes in (your bottle size may vary).
On a hot day you can get nearly a full charge on liquid (by weight) then finish the job by starting the car and cutting the A/C on to get the last bit drawn in as gas. Way faster.

 

Thanks guys but I have just one more question, I think.

Not sure if we're talking about R12 or 134A but either way on a system that's supposedly operating perfectly. Cold engine, not running, what do your read on the low and high side? Just curious but maybe nobody would make note of this?

 

No set pressure, it will vary with ambient temp.

Neal

 

I am an HVAC guy. some thoughts:

A compressor is a pump. A pump can move a liquid very efficiently (like a water pump on the engine). A pump can move a gas very efficiently (like an A/C compressor). However a pump will NOT last long if it tries to move a two-phase solution. Two-phase is a mixture of gas and liquid. A water pump that spins too fast can cavitate (shear water apart forming gas bubbles). These gas bubbles will tear up the impeller. In an A/C compressor if the compressor starts "slugging" (trying to move liquid), liquid refrigerant is not compressible. Something will give. The compressor will STOP if you are lucky (it fills with liquid and the belt and/or clutch begins to slip). If you are unlucky, the compressor will split apart.

Compressing a gas builds up a LOT of heat. If you have an air compressor or have been around an air compressor, have you noticed how hot the compressor gets when it cycles on? That is why air compressors have a duty cycle. It needs time to cool off. What about a refrigerant compressor? How does it cool? Not enough heat can be discharged by radiation or by convection to cool the compressor. A refrigerant compressor is cooled by the returning cool gas from the evaporator.

I have a two vettes, a 73 and a 2000. My brother has an original A/C optioned 1966 convertible. This is the advise I gave him when he went through the system on his car:

His car holds almost four (4) pounds of refrigerant (R-12 system). I suggested he should put at LEAST half of the system charge in BEFORE energizing the compressor. YOU DO NOT WANT TO RUN A COMPRESSOR DRY (i.e., with no refrigerant in the system). As suggested above put liquid in the high side of the system (turn the can upside down). Eventually the system will equalize and not accept any additional refrigerant. If it will not take it, put/pack some ice on the condenser. Refrigerant will condense in the condenser and lower the system pressure. Do this until half the system charge is in. Now WALK AWAY. remove the ice or let it melt. wait 24 hours. Some of the liquid WILL end up in the compressor. Over 24 hours any refrigerant in the compressor will likely migrate out into the system.

After waiting, crank the car, engage the compressor, and use the suction pressure to finish charging the system. you are done!

 

Hodge;

Why wait 24 hours to finish?

Neal

 

No reason really. I suppose you could take a heat gun and heat the compressor shell to boil out any refrigerant. Knowing how impatient my brother is I just wanted to make sure he allowed time for the refrigerant to migrate.

I am not an expert on automotive A/C but I do know HVAC (at least I think I know it). You would be amazed at the strange things you see happen with home air conditioning.

 

Before starting the compressor, you may put the liquid refrigerant through the high pressure side, the compressor discharge valves will not let the liquid back into the compressor. let it flow until it stops, then close the gauge valves, start the system and finish the charge. R12 put in the amount on the label on the evap housing, R134 ,put in 70-80% of the label.

 

that fan that plugs into the cigarette lighter is looking better all the time.