Hey all,

I noticed lately that either my ac is not as cold as it used to be, or I just hadn't ever used it when it was really hot (I normally use windows when my wife isn't with). Its still cool, just not ice cold. I have also noticed in the past that at idle my ac compressor will lock and unlock regularly. I was reading about ac issues now to try and see whats up and discovered that the locking and unlocking probably means my system is low on pressure. Now I'm looking for advice on how to deal with this. Should I try a do-it-yourself can from autozone? Ive been told in the past they are dangerous and not the correct way to service the system, but I've been reading and a lot of guys say they use them with great success. Is it worth a try or not?

 

  Dripply is right.  The only time I've seen the low side pressure that high was 55psi @108° F.  Tests in a Jeep  have shown that @ 90° F a system can have a 75% charge in it with a low side of 20psi high side 185 and a vent temp of 48°F and with a 100% charge the low side is 23psi high side 187 and duct temp is 47.4°.  Now if this system was over charged by only 10% there would be a large increase in the high side pressure. 

  This is a Cycling Clutch Orifice Tube or CCOT system.  Since the orifice tube has a set metered opening the only way to control the pressure is with a pressure switch at the accumulator.  The pressure switch is set to close circuit (turn on the compressor) @ 45psi and open circuit ( turn of compressor) @ 25psi.  This will keep the refrigerant boiling in the evaporator and a duct temp of 35°-45°F.

    It is important to have as close to 100% charge as you can without a charging station.  The accumulator acts as a liquid/vapor separator and a lubricating/cooling point for the compressor. It is filled with 30-40% liquid refrigerant mixed with oil in the bottom and the rest is vapor.  In the accumulator along with a desiccant bag for moisture control there is the out going tube in the shape of a U.  The vapor is sucked in one end and the other end goes to the low side of the compressor.  At the bottom of that U is a small hole covered by a screen filter.  That hole acts as a venturi and oil and small droplets of refrigerant are drawn up.  The oil is needed to lubricate, and the refrigerant is used to cool the compressor.  

  If the system is under charged the refrigerant will be a low pressure gas by the time it exits the evaporator and will not carry any oil to the accumulator. The oil will collect at the bottom of the evaporator.  There will also be no liquid refrigerant to cool and no oil to lubricate the compressor.

  If the system is overcharged there is a chance that the accumulator will be come filled with liquid refrigerant. This liquid can make its way to the compressor and we all know you can't compress a liquid there for compressor damage.  

 

 

Edited August 10, 20168 yr by IBMobile

Thanks for the in depth explanation @IBMobile! So any idea what would cause my vent air to still be so warm? (Sorry if you answered that above, I didn't see it)

@IBMobile post is correct. 

 

Now this why to some degree you have to look at both measurements weight of freon vs pressures on the gauges. For example the Subaru I repaired yesterday. It had a bad expansion valve. Regardless of the failure I had to completely purge the system to do the repair then recharge. Now on the radiator support it states the amount of freon in kilograms of 0.6 to 0.7 kg. Each 12oz can is 340g. So that means 2 cans should be on the mark. Now in the case of this Subaru it does not even come close to normal table pressures. At full charge it was 28-32 PSI low side and only 120-150 PSI high side. Outside temperature at the time was about 75*F. Vent temperature hovered at about 44*F on high fan. So the table is a good reference but in the same token if you not aware of capacity you can potentially overcharge the system attempting to reach a pressure that is unreachable because of design.

 

 

You can see the expansion valve needle stuck open...

Edited August 10, 20168 yr by Mopar1973Man

Hmmm so I guess this is where I run into trouble since I don't know how much is actually in my system. I know I added approximately 32 ozs to get to the pressure I'm at, but I don't know how much was already in when I started. All I know is that it only had around 10psi at that point. All that said, is there anything simple that would likely cause my discharge temp to still be high? 

This is where experience helps a bunch in adding on top of existing fill. You base off of pressures more so but also have ot be aware of outside temps at the time. Like This morning I've got to run and do another A/C job this morning. It will be cold when I do it. Current temperature here right now is 46*F. So now the owner admits they added to the system but not work quite right. Small sedan car. So max rule for me is 1.5 can to 2 for a full load. Now considering pressure and temperatures just gotta work with it. Like my previous example it can't always base on pressures. 

Edited August 10, 20168 yr by Mopar1973Man

You've over charged the system.  Total charge is 32 oz. You had some in it to start with then added 32oz more. That could be the reason for that high side pressure reading being elevated.   

8 hours ago, IBMobile said:

Now if this system was over charged by only 10% there would be a large increase in the high side pressure. 

10% is only 3.2oz

Edited August 10, 20168 yr by IBMobile

Ok that makes sense. I guess more what I'm asking is, is there anything other than level of charge that would cause the discharge air to be warmer than it should? Or is it all in the amount of freon, pressures, ect?

2 minutes ago, IBMobile said:

You've over charged the system.  Total charge is 32 oz. You had some in it to start with then added 32oz more. That could be the reason for that high side pressure reading being elevated.   

 

Really? It seemed like it needed that much to keep the compressor from cycling though....what is the best way to let a little out and at what time? Engine running or no? Compressor engaged or not? High side or low side?

Edited August 10, 20168 yr by leathermaneod

Basically the higher the pressures go the warmer the discharging temperature. 

 

Example: Normal house refrigerator would have a low side of about 2-5 PSI. Our A/C typically is about 35-40 PSI max typically. Again look at the lo side temperature ring and you'll see lower the pressure the colder it gets.

 

Again there is variance in pressure because of outside temperatures and vent temperatures in the cab. So initial startup of the system with high cab temperature might cause a spike in the hi side pressures as well driving up the lo side some. As the cab cools then the pressures will fall. There is fine line of balance between undercharge with compressor cycling and overcharge with the low side flooding out. It's also possible to see the compressor spit. Typically this occurs on a hot day and initial start up the lo side might have liquid or oil and compressor will near hydro-lock and spit out the pop off vent in the rear of the compressor. Sign you gone way too far.

 

14 minutes ago, leathermaneod said:

I guess more what I'm asking is, is there anything other than level of charge that would cause the discharge air to be warmer than it should? Or is it all in the amount of freon, pressures, ect?

 

The blend door not closing all the way.  The screen on the orifice tube dirty causing a restriction.  Flow of air over condenser blocked ( not only the fins on the condenser need to be clean but also the intercooler and radiator)

A build up of foreign matter on the evaporator will reduce the temps. No GOOD way to check that though unless you pull the HVAC box.That would be last on my list of things to check and if only a few degrees off would not get checked unless something failed inside of it like mine did.

Well thanks once again for all the help guys. Looks like maybe the only thing left for me try without a crazy amount of work is to let some pressure out of the system incase I did overcharge it. Any specific way to do that? 

  Best done with the manifold gauges.  Start vehicle, take pressure and duct temp readings.  Open low side port very slowly and only slightly to discharge refrigerant.   After short time (1-2 minutes) close port and check readings.  Repeat until there is a drop in high side pressure of about 10-15 psi. This number is contingent upon how much the system was over filled. The greater the over fill, the higher the pressure, the more of a pressure drop you need.  If temp goes down the system was over filled.  If not you have other problems.

  The easiest thing to check is the blend door not seated with heat coming from the heater core.  

Look herehttp://www.heatertreater.net/?gclid=CJbTxIOSt84CFZNZhgod4DQC2Q There is some good info on the blend door. If yours is still oe they have a part that will help fix it if it is broken. Mine was and I used this to fix it while I had the HVAC box out.

Thanks again guys! Looks like I'll have to see if autozone rents the manifold guages. Not at the in-laws anymore lol Is the blend door located behind the glove box? I mean how exactly would I check it?

Not behind the glove box. It is on one side of the tranny hump. Cant remeber which at the moment. it is  PITA to get to, but if that is your problem it is worth the trouble. You need one of those right angle screw driver thingys to get the screws out.

Can you watch it open and close to see if it's working properly?

I dont think so. If the little connector is broken or cracked chances are the door is not indexed properly anymore. The heater treater deal comes with what you need including instructions on how to index it. It has been so long since I did mine I dont remember the process. It is probably spelled out pretty good on their website.

Well so far from what I'm finding, it looks like messing with the blend door will be way harder than trying to get the system pressures corrected and see if maybe it is overcharged. Since my high side is already a little too high, I think I'll do that first and see if it helps. My dash is sooooo brittle already I really hate to mess with it at all. I cant afford to be replacing a dash and heater core and evaporator right now lol. 

Nvm all that, I see what your talking about now. You dont have to take anything apart except the motor off. Thats looking a little more do able now. 

Alright so now I'm really getting annoyed with this. We let pressure out and let pressure out and it kept staying locked at 2000 rpm. Finally got down around 20 psi low side and the comp kicked off. I figured no big deal just put a shot back in. Well we ran to the store and got a can. Get back and the darn think won't even stay locked at 1500 rpm. The new 12 oz can ran out without even getting it back up to 25 psi. Now it's sitting at about 24 psi at idle and 150 or less on the high side. air gets to about 42° which I'd be happy with, but rev it up at all and it kicks off and on. It gets so cold at idle that the line iced up around the low port. What do I do??? Put more in again??? Looking back now I'm wondering if the switch was stuck from being over pressurized and that was it wasn't kicking the comp off before. Maybe I really do need to just add more. Just don't get why I went through a whole can withought getting very far. Also what kind of freon do you guys use? So far I used AC prob because I heard good things about it, but I'm up to $100 in freon now and still not done. Starting to think I would have been better off just getting someone to do this....

Edited August 11, 20168 yr by leathermaneod

Test the system with A/C control set on A/C dash vents and not on A/C MAX dash vents and fan speed #3.  You will see your low and high side pressures come up.  

Example:  Air temp 78°F,  with fan speed set at #3, A/C control set at MAX dash vents(recirculate air), engine at idle, the low pressure side would go down to 19 psi and the compressor turn off, the pressure climb to 38 psi and the compressor turned on. The high side never went above 165 psi. Vent temp was 38°F. 

         With fan speed at #3 and A/C control set at A/C dash vents (out side air), the low side pressure stayed at 29 psi and the high side pressure stayed at 190 psi with no cycling. Vent temp was 42°F.