My 99 Tahoe's AC is cold up front but hot in the rear

[hekg]

Hey all, I'm trying to figure out if I can fix my rear AC because it's extremely hot down here in South Florida and my two year old boy gets hit in the face with hot air when I attempt to cool him down in the back however my front AC vents blow nice and cold for me.

Where should I start looking first? I just picked up a set of AC gauges and hoses as well as some cans of R134a refrigerant at Harbor Freight. This is my first time doing thing so I don't want to just start adding freon to the system.

 

[L31MaxExpress]

Hey all, I'm trying to figure out if I can fix my rear AC because it's extremely hot down here in South Florida and my two year old boy gets hit in the face with hot air when I attempt to cool him down in the back however my front AC vents blow nice and cold for me.

Where should I start looking first? I just picked up a set of AC gauges and hoses as well as some cans of R134a refrigerant at Harbor Freight. This is my first time doing thing so I don't want to just start adding freon to the system.

That one is easy. The expansion valve in the rear is either plugged up with crap or otherwise stuck closed or potentially even the lines under the truck are damaged or have become obstructed internally. Either way its a restriction in the rear refrigerant loop.

On a system that has an orifice tube up front and an expansion valve in the rear, if the refrigerant level is low, the expansion valve will open fully, hog all the refrigerant flow and the rear will be ice cold while the front is warm. Tahoes do not have rear heat, so there are no blend doors to act up.

 

[L31MaxExpress]

Random thought too. Has the rear ever blown cold? Reason I ask is some hacks have developed a block off plug set to seal off the refrigerant loop in the rear should it develop a leak. This makes the ac system a front only system. Living in Texas it is a hack way to do it to me.

 

[hekg]

You were right. I replaced the expansion valve in the back and the AC blows cold again. Here's the kicker, the front is blowing hot now!
I also replaced the orifice tube because the front was not cooling that great anymore but now it's not cooling at all in the front but cold in the back.

I only replaced those two things though. Should I replace the accumulator as well? I just don't get why it blew somewhat cold in the front before I put the new orifice tube in and now it does't.

 

[HotrodZ06]

How much refrigerant did you recharge with it may be a bit low like L31MaxExpress said "On a system that has an orifice tube up front and an expansion valve in the rear, if the refrigerant level is low, the expansion valve will open fully, hog all the refrigerant flow and the rear will be ice cold while the front is warm." Just a shot not sure what else it could be but I know my wife's Escalade took a good bit more than my trucks do.

 

[hekg]

I added three 12oz cans and stopped there because the low pressure gauge was at 75psi and the high was at 225-230psi. I didn't want to overfill or cause damage.

I've read on a few posts that these trucks with rear air require 64 oz of refrigerant and that's what I was planning to put in until I saw the low pressure going that high since every video that I've watched always says to keep the low pressure under 50psi.

 

[HotrodZ06]

That does seem really high but also depends upon ambient temperature as well, higher temps will result in higher pressures. According to the service manual four door utility with C69 (rear A/C) take 4# or 64 oz.

 

[hekg]

I'm in South Florida and today it felt like it was around 100 and extremely humid. Maybe I should go ahead and keep adding refrigerant as long as the high side doesn't start climbing too high say to the 400 psi range.
Yes this is my first time messing with AC so hopefully it won't end up costing me more than it would've just to take a it to a shop.

 

[HotWheelsBurban]

Automotive air conditioners will only cool to 30° below ambient temperature, and the average car AC could cool a 3 bedroom house. Don't know for sure that these things still are completely true, but these are two things that that they taught us in the AC Delco training courses back in the 70s and 80s. This was before R134a, still R12 back then, and the newer refrigerants are supposed to be more effective and efficient.
A lot of shops back then would have a big box fan, like you'd use to cool the shop, in front of the car, blowing on it. Dad did this on several of our family's cars when recharging in the summer, and he felt that it helped.

 

[L31MaxExpress]

I added three 12oz cans and stopped there because the low pressure gauge was at 75psi and the high was at 225-230psi. I didn't want to overfill or cause damage.

I've read on a few posts that these trucks with rear air require 64 oz of refrigerant and that's what I was planning to put in until I saw the low pressure going that high since every video that I've watched always says to keep the low pressure under 50psi.

You are still way low on refrigerant. Not uncommon to see more than 350 psi on some systems when it is 100F outside. Takes a very strong fan setup and parallel flow condenser to get the pressures down. Keep a water hose handy while charging, if the pressure creeps up, mist the condenser. Once the system is charged the cool refrigerant returning to the compressor will help drop the pressures. 64oz is what you need. Typically lose 1-2 oz per can.
Take a short flat blade screwdriver and wedge it between the idle stop screw and the throttle lever and get the engine up to about 1,500 rpm while charging.