AC compressor short cycling...

[Twodose]

AC compressor kicks on and off every couple seconds, when compressor kicks on, pressure goes down to about 16 psi, when it kicks off, it goes to 70 psi, the pressure goes up and down form 16 to 70 as the compressor kicks on and off.

1999 GMC Sierra 1500 5.3 slt.

Edited June 26, 2022 by Twodose

[richard wysong]

low freon charge

[Twodose]

Yes, I unplugged the connector to the pressure switch and jumped it, it only had 6psi, I charged it to 30 psi, put the plug back on and it works fine, used ac, pro, it took a 20 oz and a 12 oz, the get it to 30 psi, is 30 psi enough? I thought the pressure switch was bad, but it must me OK.

The compressor is running for a long time now and shuts off when I either turn the ac off, or turn the fan off, so it seems it is running fine, old gmc’s never die, they just go faster…lol.

It’s funny because the temperature on the 1999 sierra is 48.4 @ 30psi, and the temperature on the 2011 sierra is 55.6 @ 36 psi.

I don’t know what the pressure should be, according to the can below 50 psi is good.

 

Edited June 27, 2022 by Twodose

[CamGTP]

The whole system probably only holds maybe 2lbs on R134a, so if you dumped in 32oz of freon that means you added alot of freon to a system with lots of air and moisture in it. I'd be leery of that causing a problem.

 

It's a 20+ year old truck and they are known to have A/C compressor problems and they are known to get a lot of crap stuck in the fins of the condenser infront of the radiator. I'd back flush that with water or a long air nozzle attachment made for that.

[Twodose]

What do you think I should have done, to do it correctly?

[richard wysong]

the system should have been recovered and vacuumed for at least 30 min to remove any moisture and air then recharged with the amount of 134 that the system requires. usually that's on a sticker on top of the accumulator. I also add florescent dye  to make it easier to find any leaks down the road. you are probably ok where the system wasn't empty so air and moisture didn't have an opportunity to enter the system, but you may have slightly overcharged the system. Cams advice was right on, clean the condenser as best you can, removing the upper radiator shroud should help and you may find leaves etc in between the rad and the condenser. the compressor problems are usually just leaks and clutch failure

[Twodose]

How can it be overcharged if the pressure is only 30 psi?

[richard wysong]

what is the freon charge on the sticker? most pickups are around 2lbs and that's what you added to the system that already had enough in it to cycle the compressor so if the capacity is 2lbs whatever was still in the system is how much its overcharged. pressures don't matter when doing a recharge as temp, compressor efficiancy, blower speed ,air flow thru condenser all affect pressures. that's why you go by freon weight

[Twodose]

When you pull a vacuum on the system, does it suck the oil out also?, I wouldn't think so but I don't know, and do you have to add any oil back to the system after running a vacuum pump?

Any oil need to be added after evacuation?

Looks like it needs 1.8 lb, truck only has a/c in front.

[richard wysong]

not just pulling vacuum but during recovery some oil is removed from the system and is seperated by the machine so it can be measured and replenished. so you may have an 8 oz overcharge which probably won't hurt it but it will cool better with the correct charge in it. the worse that could happen is either the relief valve will pop and you lose all the freon or the compressor may bind up and damage the clutch or belt but I wouldn't worry if it's working ok

[txab]

As noted above, during the vacuum process oil will not be lost. Moisture that is in the system will boil off and be removed. During recovery with a proper recovery system oil may be removed and trapped by the unit.

 

134a is pretty sensitive. A few ounces overcharge can be as bad several ounces undercharge. if you've ever used 134a with a sealant in it you are screwed long term

[Twodose]

So you never have to worry about adding oil to the system, for the life of the system unless you do a recovery?

 

Also 1.8 lbs would be 28.8 ozs of freon.

 

 

[txab]

If you replace parts of the system you may have to replace oil that is lost. Oil is trapped/stored in the various components. If you  had a serious leak at a component that actually let oil escape.

 

You really need to have a set of AC gauges to measure both high and low side. High side pressure reading is as important as the low. If your head pressure rises too high you may trip the high pressure cut out switch. If you open your system you need to apply a vacuum. Air/moisture in the system will not allow it to operate effectively. Both are non condensable in an AC system

[Twodose]

I have guages and a vac pump ordered.

 

 

 

[Twodose]

You stated that “If you ever used 134a with a sealant in it you are screwed long term”.  I don’t know why anyone would make a product that was detrimental to a system in any term short or long. I know the product I used stated it has sealants in it, but my understanding was that it was more of a conditioner for the seals, than a sealant that could damage the system. Why did you say that? Just curious.