charging A/C

[Atl_Monte]

I recent swaped the motor in my 95 Trans Am and was trying to charge the A/C I started by putting a can of oil, but when I got to the freon it wouldn't take and the A/C compressor doesn't come on when I turn the A/C on some body told me that the low pressure switch is keeping the compressor from kicking on due to low pressure, and that I have to fround this switch during charging, but wanted to get a second opinion, if this is correct how do I ground the switch, if this is incorrect what is the correct method?

[fastTa]

Did you evacuate the system first? When you re-charge the refrigerant, you need to have the blower on high and the A/C on max. You are re-charging the low side, so as soon as the low side pressure sensor senses, I think it is 24 PSI, it will allow the A/C clutch to engage and the compressor will start to cycle.

Whoever told you to jump the low side pressure sensor in order to get more freon in the low side should consider taking up another career.

[Atl_Monte]

yea I think I did have the blower on high the A/C on max but I'll givei t another try, the fans come on but not the compressor...

[Jeff_in_Atl]

Jumping the low pressure switch is a common "trick" to charging an A/C system. I've done it many times.

You are supposed to vacuum the system out before charging it to remove the moisture out of the system. Moisture and Freon do not mix. So if your system stayed apart for more than a few days you should get it vacuumed down before charging it. I have gotten away without vacuuming it to replace a part where the system was only open for a few hours. But it didn't work as well as it should have. I ended up vacuuming it down later.

If you vacuum it down the system will usually suck all of the Freon in without even cranking the car. I will put the Freon in the high side in liquid form (can upside down) if the system is vacuumed down and the car is NOT running. The high side is liquid anyway. If you have to run the car to get the system to charge connect it to the low side and do not turn the can upside down. You do not want to put a liquid in the low side. The compressor can not compress a liquid and it will blow the seals.

Another trick to getting it to charge that I would try first before you try jumpering the low pressure switch is to get a large bowl of scalding hot water. Connect the Freon to the low pressure fitting. With the can facing up and the car running put the can in the hot water. It will force the Freon out of the can and usually get enough in the system to trip the low pressure switch.

[fastTa]

The thing is the low pressure sensor is there for a good reason and that is to protect the compressor. Nowadays most of the R-134A you buy also contains the oil charge. When you jump it it obviously does not matter how much oil or freon is in the system and the compressor will just run. That is not good at all for the delicate seals on an automotive compressor.

It is OK to jump the pigtail with a paperclip very briefly to simply verify that that the A/C clutch relay, the A/C clutch itself, and the compressor are functioning, but you should NEVER leave the low pressure circuit jumped and leave the compressor running. You can quickly ruin a compressor. One 14 oz can of R-134A is normally sufficient to get the 24 PSI in order to satisfy the low pressure sensor.

There are lots of 'common' tricks that mechanics use, but they are not always correct.

[Atl_Monte]

just gave it another try and for somereason the system just decided to take it this this...these are 12 oz cans, this is a 2 pound system, one can equals 1 pound correct?

[fastTa]

just gave it another try and for somereason the system just decided to take it this this...these are 12 oz cans, this is a 2 pound system, one can equals 1 pound correct?

16oz is one pound. So 3 12oz cans will give you 36oz in the system. That will be just fine.

[Atl_Monte]

16oz is one pound. So 3 12oz cans will give you 36oz in the system. That will be just fine.

duh, 16 oz in a pound, a pretty dumb question on my part, but thanks...

[OldSchoolFormula]

How do you jump the p[ressure switch being here are three wires going to it? Red, Black, and Gray? This is for a '94.

[Atl_Monte]

I don't know I did like the above suggestion and connected a can to the system the contents of the can wouldn't fully go in so I go a pot of boiling hot water and set the can in it while still connected, after that it was enough pressure in the system to trigger the pressure switch, thanks all

[OldSchoolFormula]

Well, the system has a charge in it, I just dont know if Im dealing with a bad clutch here or if its a bad pressure switch. Thats why Id like to jump it.