01 Tahoe - Hot AC Mystery

2001 Tahoe LT with 155k miles, runs great.

AC has started to blow warm/hot in the last month (first time in 21 years that it hasn't been ice cold).

Pressures are good:
-Max AC - Low 30psi / High 155psi
-AC Off - Low 70psi / High 75psi (75F Outside temp)

-Compressor turns with no unusual noise or vibration (and stops when AC turned off)
-High side line is warm, low side line is ice cold (develops frost after running in the garage for a while) split happens before/after the orifice tube location.
-Blower fan blows great
-Replaced cabin air filters
-Blend door actuator functions correctly
-Dashboard AC controls all function (light up) as expected when pressed (no flashing)
-Front and rear AC both blowing warm
**Interesting observation: No condensation dripping under front or rear AC units - No dampness or water inside vehicle anywhere.
-I cleaned the condenser (which dropped the temp on the high pressure line from hot to just warm)
-Engine cooling fan runs normally
-Pulled and checked all AC related fuses (all good) and swapped the relay with the horn (both good)

I've checked everything I can think of. My suspicion is that the evaporator cores are not getting cold, but I can't figure out why/how because the low pressure line on the engine side of the firewall is ice cold.

What am I missing? Any other ideas?

Your problem is staring you right in your eye and system told on itself with the frosted low side line. Your evaporator is freezing up because of low charge so no evaporation is taking place. Any A/C or Refrigeration system MUST have the evaporator temp above 32 deg or the system will not work because the humidity will freeze on the evaporator almost instantly. You are running 5-15psi low on the low side. You are just about at the low pressure cutoff switch pressure now. You are low on charge. The first place I would check is the valve cores at the service ports. Give them a snug. They like to leak slowly over time especially when they have refrigerant gauges attached to the ports and the gauge set hoses are moved around and those gauge adapters will back out the valve cores ever so slightly and its usually the low side. ALWAYS give valve cores just a slight snug after having gauge set hooked up.

rockola1971 said:

Your problem is staring you right in your eye and system told on itself with the frosted low side line. Your evaporator is freezing up because of low charge so no evaporation is taking place. Any A/C or Refrigeration system MUST have the evaporator temp above 32 deg or the system will not work because the humidity will freeze on the evaporator almost instantly. You are running 5-15psi low on the low side. You are just about at the low pressure cutoff switch pressure now. You are low on charge. The first place I would check is the valve cores at the service ports. Give them a snug. They like to leak slowly over time especially when they have refrigerant gauges attached to the ports and the gauge set hoses are moved around and those gauge adapters will back out the valve cores ever so slightly and its usually the low side. ALWAYS give valve cores just a slight snug after having gauge set hooked up.

Click to expand...

Thanks for helping me see what should've been obvious (this isn't my day job). I went to charge it and found the high pressure valve core (ball valve) is leaking slowly. Is there a way to pull and replace that valve core without discharging the entire system (like there is with the low pressure valve?

Not sure that the high pressure valve was the original culprit, but after installing/removing the gauges from it a few times, it's surely leaking now. Any other tricks to just reseal the ball without replacing that I'm missing?

Thanks for the help!

Pull core and all is lost, period. Sometimes can tighten and all is good. If only leak, air cannot get in when pressure leak is pushing out. Tighten and add refrig. Or, pull a vacuum and let sit for minimum 30 minutes and charge and be cold again.

Scough19 said:

Thanks for helping me see what should've been obvious (this isn't my day job). I went to charge it and found the high pressure valve core (ball valve) is leaking slowly. Is there a way to pull and replace that valve core without discharging the entire system (like there is with the low pressure valve?

Not sure that the high pressure valve was the original culprit, but after installing/removing the gauges from it a few times, it's surely leaking now. Any other tricks to just reseal the ball without replacing that I'm missing?

Thanks for the help!

Click to expand...

Im not aware of any tools for automotive that allow valve core to be pulled with system charged. (There are for industrial and home a/c units though) Just snug the cores up with a valve core tool. Any auto parts stores around you should have one. They arent expensive. You really need to get a gauge set on there and pospt up pressures found. Overcharging is just as bad as undercharged, only different symptoms.

Blows hot
Blend doors function correctly

One of these is wrong lol

Regarding the low charge and leak: you won't be able to replace that valve without an evac and recharge. Which... Should be done at this point anyway to get the proper charge in the system, being that there's an unknown amount in there. The pressures are a guide, but on the automotive side of things - one too many variables for that to be your bible. Condenser flow, debris in the expansion valve screen, compressor age/wear, engine RPM. If you have a clutch fan, add that one to the mix as well.

Oh, and replace the caps while you're at it. Those cores aren't as gas-tight as you would hope. Fresh caps with a good rubber seal in there will add another barrier of protection.

Update:
Ended up finding a leak on the accumulator at the low pressure switch connection. Replaced the accumulator, the low pressure switch, and the high pressure valve. Pulled the vacuum and recharged today. Front AC is blowing cold! But the rear is blowing warm. Suspecting either the rear blend door actuator or the rear expansion valve. From what I can tell the electronic overhead controls are all working, the system responds to any changes.

Next step: pulling the panels around the rear AC to see if the actuator is working and if the expansion valve is working. Any easier diagnosis steps or something else worth troubleshooting?

Thanks for the help! Definitely focused my search.

Scough19 said:

Update:
Ended up finding a leak on the accumulator at the low pressure switch connection. Replaced the accumulator, the low pressure switch, and the high pressure valve. Pulled the vacuum and recharged today. Front AC is blowing cold! But the rear is blowing warm. Suspecting either the rear blend door actuator or the rear expansion valve. From what I can tell the electronic overhead controls are all working, the system responds to any changes.

Next step: pulling the panels around the rear AC to see if the actuator is working and if the expansion valve is working. Any easier diagnosis steps or something else worth troubleshooting?

Thanks for the help! Definitely focused my search.

Click to expand...

You can just pull the panel, remove the blend door actuator back there, and move the door by hand to see if that’s your problem. It’s pretty easy unlike the front actuators under the dash.

Yeah the rear blend doors are just like.... Watch them move
Both actuators are the same, so if one doesn't work, pull it off, see if the flap moves freely by hand, then you can just swap them over. If the issue follows the actuator, then you've got your fix!

GM Specs
Rear a/c has a txv instead of an orifice tube that is on the front.