Suburban Rear AC not circulating refrigerant

[gmbellew]

on my 1990 suburban, i have been fighting AC issues since my sanden gave up the ghost.

All lines flushed and blown out (including rear lines and rear evaporator core, blown from where the expansion valve connects), evaporator core flushed and blown, new condenser, new orifice tube, new accumulator, new rear expansion valve.

the front components of the system seem to be working ok. it has a full charge of R134, compressor cycling well, front AC is cold, but the refrigerant doesn't seem to be wanting to circulate through the rear lines.

Best i can tell they are just teed off and the only thing that would prevent refrigeration from cycling through the rear is a blockage somewhere. i had good flush and flow going through all the components in the rear, so i don't know how there could still be a blockage.

it is possible something froze up in the back during charging. so i am letting the system come to equilibrium over night and will see how it works in the morning.

my only thought is somehow the new rear expansion valve isn't opening like it should. is there anyway to diagnose that? anybody had issues like this before?

 

[fast 99]

If you've checked and blown out all the lines and evaporator kind of only leaves the orifice tube.

Could be froze up. If so should work for a while after sitting or show frost somewhere after running for a period of time.

 

[gmbellew]

the rear high side tees off before the orifice tube, goes through an expansion valve and evaporator core, and then back to the front and tees in after the accumulator. the rear expansion valve is new....

the orifice tube cools the front before the front evaporator core and it is cooling great.

the rear high and low lines are the same temp above the rear tire. the low line is warm just off the tee into the front after the accumulator. the tee off the high side under the hood is hot. not sure why the fluid isn't circulating through the rear unless the replacement expansion valve isn't working.

anyway to try and diagnose the rear expansion valve sensing bulb to try and get it to open? seems like maybe it is stuck closed or not opening like it should. what does it sense to get it to open? how can fluid ever circulate if it starts closed?

 

[R Carnella]

Expansion valve could definitely be faulty, is it possible there's to much oil in the system?, just a thought.

 

[gmbellew]

Expansion valve could definitely be faulty, is it possible there's to much oil in the system?, just a thought.

I think the oil is good. compressor came with a full charge.

I just tried blowing a hair dryer on the expansion valve sensing bulb where it is clamped to the low side line in the rear to warm it up to see if valve would open. it didn't make a difference and the rear high and low sides are the same temp.

front is still operating normally.

nothing is frozen up in the rear. the rear lines are both ambient air temp after you get out of the engine bay.

I guess the new rear expansion valve could be bad. i hate to discharge the system again.....

anybody ever run into this on these rear AC systems?

 

[gmbellew]

been trying to figure out the operation of these rear expansion valves. the sensing bulb goes on the low side after the rear evaporator core. should the valve be open at rest (ambient air temp) and then start to close when the sensing bulb temperature gets cold to reduce the flow of refrigerant? is that correct?

 

[consti2tion]

A hair dryer may not be able to create enough heat, do you have a heat gun? It should read the pressure from the thermal bulb/probe if you can pull it out of the evaporator and test it that way.

I have the same issues on mine, well I did before I tore it apart to LS swap it. I will have to resolve this issue when I go to connect the AC system, which will be coming up soon.

 

[gmbellew]

I've ordered a couple different rear expansion valves. I plan to compare to the original and see if they look right and are working. the valve should be at least partially open (maybe all open) at room temperature. it seems it should close when it gets cold. then I will again make sure the lines to the back are free flowing (the high side tee seems to be the only other place for a blockage to the rear besides an inop expansion valve) and give it another go

 

[consti2tion]

Could you put the probe into a bowl of ice water to test it if that is how it functions?

 

[gmbellew]

Could you put the probe into a bowl of ice water to test it if that is how it functions?

It is a weird system. there is a return spring that tries to close the valve. there is the liquid in the sensing bulb and capillary tube that expands when warm to push against the spring and open the valve. the valve should allow air to pass when temps are warm when the sensing liquid is expanded. as the sensing liquid cools to near freezing from the low side, the liquid should contract and the spring should push the valve closed to restrict flow and prevent freezinf. the spring must be high pressure to keep high side pressures from overwhelming it and flowing through. my new valves should be here soon and I plan to play with then before I crack the system open again to change the valve.

 

[consti2tion]

Such a complicated system for such a simple function. Please report back and let us know what you figure out. Thank you!

 

[Hunter79764]

No direct experience on the rear evap, but I'm familiar with the layout etc. Any chance the cap tube is kinked anywhere? that could cause it to keep the valve in the closed position. If you gave it a good flush etc, I wouldn't expect trash to have plugged it but it could happen. Blockage is the only thing that makes sense, unless you had a ton of water or something that freezes up as soon as anything starts flowing, blocking the orifice. Either way, if you have a new expansion valve, I'd swap it out and make sure you install a new drier while the system is open. Even if it isn't water in the system, it's always best to install a new drier every time it's open.

 

[R Carnella]

How have you made out ? Just wondering.

 

[gmbellew]

How have you made out ? Just wondering.

I have some new valves that I tested to be sure they work. haven't had time to dive into the system again yet. I'll be sure to update.

 

[gmbellew]

took a half day off work and dove in. the replacement expansion valve wouldn't pass air. as soon as I took it out I tried to blow through it and it was completely stuck shut. the one I just installed today could pass air easily at the same ambient air. I also checked the high side t line and it was clear. replaced the orifice tube just for fun. got in and out quick and bagged the lines, so I didn't replace the new accumulator installed before. should all be ready for a vac and charge now. I don't have the rear blower and cover installed bc I am waiting for the superglue to cure for 24hr on a plastic piece that holds the motor mount screw that broke. I'll update if the new valve in the back doesn't solve the problem.