1999 Tahoe a/c not getting cold enough after 85 degrees

[MO351]

My a/c works fine below 85 degrees outside temp, but above that the a/c doesn't get cold enough to keep up and cool down the interior. 1999 Tahoe 5.7L 4x4 with rear a/c (no rear heat). Brand new clutch cycle switch. While adding some freon from A/C Pro http://www.oreillyauto.com/site/c/d...=Search_N1783_1353721_4066&pt=N1783&ppt=C0050 I noticed the low side pressure wasn't rising. So I replaced the clutch cycle switch which doubled the pressure and now it shows the pressure is correct (system is fully charged) via the pressure gauge that comes with the a/c pro can. I've checked the pressure with the outside temp ranging from 72 to 90 degrees. I just can't figure out why the air temp out of the vents won't get cold. It cools down, enough to cool down the interior when the outside temp is below 85, but above that it doesn't cool down well at all. I don't have a thermometer to be able to measure the air temp from the vents. It sure doesn't get as cold as our 2003 Ford Windstar which puts out freezing cold air.

The low side line gets cold and gets condensation on it. The high side line gets quite hot. Dye in the system, no leaks (except a possible, occasional, slow leak at the low side schrader valve). One thing I have noticed is the clutch doesn't cycle. It stays on unless I turn off the a/c. I put the starter relay in place of the a/c relay and the clutch continues to not cycle.

Is the gauge that came with the a/c pro freon reading low and I just need to add a bit more freon? Thoughts.

 

[SunlitComet]

Usually if it is just running all the time when you turn it on yourself and it wont cycle off it is because the refrigerant charge is low. I say this because as you describe the temperature of your lines it sounds like the orifice tube is working as it should and at this time does not appear to be clogged.

 

[83 Bimmer]

First things first, you need to make sure the proper amount of refrigerant is in the system. It needs to be evacuated, vacuumed, and recharged, then check the pressures and see where they are at.

Sent from my HTCONE using Tapatalk 4

 

[SunlitComet]

You should only need to discharge and evacuate if moisture entry is suspected. For example if it has been untouched since the factory build-up and is just a little low from say a leaking core valve, use the special tool to just replace the valve core and not lose the charge from have to discharge into a recovery tank. Won't have to worry about the oil amount in the system really either. Top it off as needed and call it done. But do not use a refrigerant with leak sealer in it. It can clog up the orifice eventually.

 

[83 Bimmer]

I disagree. Unless you want to just start guessing and changing stuff that you think might be bad, you need to have the proper tools to diagnose the a/c system. You need to have a gauge connected to the high side and the low side, and those gauges aren't going to do you any good if you don't know how much refrigerant is actually in the system. If you have too much or too little the readings are going to be way off. The problem with those "diy" kits is you have no idea you can very easily overcharge it and an overcharged a/c system is a very dangerous thing.

Randomly adding some refrigerant to the system all willy nilly is like seeing your check engine light on and just replacing an egr valve because you heard that is a common problem.

Get some manifold gauges or find someone who has some

 

[MO351]

Finally got a hold of my neighbor who does industrial HVAC. Turns out it was WAY over filled. Pressure was too high on the low side causing the system to blow hot instead of cold. Pressure was around 55 psi. He said it needed to be around 28-38 psi. Evacuated the excess freon and got it to around 31 psi. 90 degrees outside with the front vents blowing around 45.5-46 degrees and the rear vents blowing 50-50.5 degrees. Sooooo much nicer than the 90 degrees it was blowing before evacuation. I also found this website http://www.ackits.com/aacf/ptchart.cfm

DO NOT go by those freakin' gauges on the do it yourself cans. For example, the a/c pro one has a red V with an arrow that you put on the outside temperature and if the pressure gauge reads within the V then it's at the correct pressure. WRONG!!! The gauge was reading 50-55psi and telling me it was correct. Go by the PRESSURE (PSI) on the gauge. Know what psi you need, which is around 28-38 psi, and use the gauge that way.

If the clutch runs and runs without cycling it's an indication that the pressure is too high which also creates heat, rather than cold, because of the excess pressure in the system. The lower the pressure, the colder the a/c gets. If the pressure gets too low then the cycle pressure switch turns the clutch off to keep the a/c from freezing up. If the pressure is too low, the clutch won't turn on.

 

[83 Bimmer]

Great, glad you got everything figured out, I hate not having ac!

 

[SunlitComet]

I disagree. Use of the proper hvac gauge set can tell you a lot about your system. If you know how to use and read them properly. Including whether or not your refrigerant is high, or low, or there is a plug somewhere(handwork involved there too), or your compressor is broken among other things. There are refrigerant charts you can use as well as there is a pressure/temperature chart just for these vehicles that can help determine if your charge is correct or not. So when he or someone does get a gauge set they will not have to get access to a recovery system as well as a vacuum pump and refrigerant and a scale as well as possible have to to discharge it twice and loose the cost of the first refrigerant, unless you suggest just throwing money at it, just to find out the likely cause with say a 80% success rate using just a gauge set. Having a multimeter would take care of 15% of that and a thermometer would take that success rate to 100%.

---------- Post added at 08:14 PM ---------- Previous post was at 08:12 PM ----------

Great, glad you got everything figured out, I hate not having ac!

Yes and he did it without discharging, evacuating and recharging the entire system according to his statement. AMAZING!

---------- Post added at 08:21 PM ---------- Previous post was at 08:14 PM ----------

BTW using that particular chart does not take into account the systems that have a fixed orifice device and thermal expansion valve for two different evaporators in the same a/c system.

 

[83 Bimmer]