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Billy
Billy, Trainer/Master tech.
Category: Car
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Experience:  Trainer 8+ years, master tech. 10+ years, SAE member
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On an automotive AC system, should I pull a vacuum on the

Resolved Question:

On an automotive AC system, should I pull a vacuum on the system if it started out with 25 on the high and 30 on the low when I hooked up the gauges? I have been trying to add freon and did not pull vacuum, but it's been about 20 minutes, and the compressor won't stay running, and the high side is at about 50. I know without the compressor running, it won't give me accurate numbers. Should I have emptied what was left and pulled vacuum first?
Submitted: 3 years ago.
Category: Car
Expert:  Billy replied 3 years ago.

Billy :

Hello and thank you for using Just answer! I'm Billy and i will assist you with this. Feel free to ask if you need any further info.

Customer:

do you think I should have pulled a vacuum with the high pressure being only 25 at start?

Billy :

It is always advised to recover remaining freon, and then vacuum before putting new freon....

Customer:

could this be the reason that the high is not going up very well?

Billy :

And if you have the equipment....it is better to fill the correct amount (weight), as opposed to measuring the pressures.....so the pressure test would be to confirm.....

Billy :

Yes, it could be.....

Customer:

should i have also added an oil charge?

Billy :

not necessary....unless the system was leaking.......

Customer:

it must have been leaking since the pressures were so low, and it was blowing warm air

Billy :

was the compressor running?

Customer:

it started running after I started adding freon. Still though after 15 or 20 minutes, it wouldn't stay running

Billy :

was the compressor running?

Customer:

i assume it's because the pressures are too low

Billy :

ok......so it was a "good" leak, and adding oil in this case is advised.

Customer:

how many oz charge?

Billy :

yes of course.....if the leak is small, then the pressure would be high enough to run the compressor, as for the oil, you should,

Customer:

should I add a 2 oz charge or more?

Billy :

50 cc of oil would put you on the safe side....

Billy :

around 2 oz

Billy :

when the system is vacuumed, it would quickly take the needed amount of oil and freon.....

Customer:

Ok. thanks

Billy, Trainer/Master tech.
Category: Car
Satisfied Customers: 3279
Experience: Trainer 8+ years, master tech. 10+ years, SAE member
Billy and 20 other Car Specialists are ready to help you
Customer: replied 3 years ago.
I evacuated it, pulled vacuum, then charged it back up. The high pressure side still doesn't go above 50, and I stopped with the low between 40 and 45. I don't think it would be smart to go above that, do you? What do you think the issue is with the high pressure? The compressor does stay running now, but the air is only cool. not really cold.
Expert:  Billy replied 3 years ago.
What are the pressures with compressor on?
Customer: replied 3 years ago.
the pressures with the compressor on are 45 for low, and 50 for high. I double checked it with another set of gauges too
Expert:  Billy replied 3 years ago.
with compressor on, try adding freon, on the low side, and get back to me
Customer: replied 3 years ago.
that's where I was adding it. the van was taking it, but the high never went up. just the low. I stopped at about 5 lbs, which is high since it calls for 2.5 but at 2.5 it was not even blowing remotely cool
Expert:  Billy replied 3 years ago.
Can you try and replace the low pressure cycling switch....or at least hook the compressor directly, until you can finnish the service (adding freon), as the low on 45 is still acceptable, but the high being the same, means the compressor is not working, or else you have an open expansion valve......
Customer: replied 3 years ago.
the compressor is running constantly, but I already added nearly 5 lbs of freon. it only calls for 2.5, unless my scale is not functioning properly, but I think it is. Should I just keep adding until it's cold? It is taking it.
Expert:  Billy replied 3 years ago.
no.....if you have added more weight than you should, then stop.

Having the compressor on all the time, means, either the compressor is not compressing properly, or the EV (expansion valve ) is stuck open, like a normal pipe....so the circuit had same pressure on both sides, thus neither getting hot on the outside, nor getting cold on the inside..........
Expert:  Billy replied 3 years ago.
I would double check for any leaks.....as you might also have a big leak (most likely on the HP side, given the symptoms)........
Just thought of that, and would like you to know, so you check....

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