diagnosing intermittent front AC
 

1997 Suburban 6.5td (is Jim still policing the use of TD?) with front and rear AC. Front AC quits cooling after a couple of hours, rear AC stays cool. Front AC cools again after an hour with the engine off. Front AC has an orifice tube, rear AC has an expansion valve. No coolant flow through front heater core, front AC vent temps on the best day are such that I don't believe there's ice on the evaporator, no evaporator case drain problems, no airflow or distribution problems

I studied the relevant shop manual which shows the refrigerant circuit for front AC only (trucks) and words to describe how the Suburban with rear AC is different. Neither diagram nor words describe why, if I can trust my eyes, the rear evaporator discharge flows directly into the compressor while the front evaporator discharge goes through an accumulator. Rear AC uses an expansion valve while front AC uses an orifice tube. Does this have something to do with it?

I have to remember the next time it stops cooling to check if the front evaporator discharge line is cold. I've seen it sweaty but never iced - how can it ice so close to the turbo? I suppose I can check the front evaporator line downstream of the orifice tube but it's tucked under the turbo and I don't have an infrared thermometer.

Possible scenarios:

1) Debris accumulates in the orifice tube after several hours of AC use. The debris slowly settles back in the condenser when the compressor isn't engaged.

2) The compressor low side port, rear AC evaporator discharge or accumulator is at a higher pressure than the front evaporator discharge confounding flow through the front evaporator.

Are these scenarios plausible? What other possibilities are there?

Sixto
MB-less

"At first glance, a (CCOT) cycling clutch orifice tube system will have an accumulator and a cycling switch attached. This accumulator is located on the low side of the system, after the orifice tube. This is different than a (TXV) thermostatic expansion valve system that has a receiver drier located on the high side of the system, between the condenser and the expansion valve. Both of these devices are dryers containing a desiccant to clean and absorb moisture from the refrigerant."

Cycling Clutch Orifice Tube vs Thermostatic Expansion Valve

I never worked on a system like yours.

I use a surface temperature tester and pressure gauges to watch subcool and superheat in the system for troubleshooting. You need to know where your refrigerant is liquid and gas to understand what is going on.

Cleaning evaporators and condensers is routine maintenance.

Do you have good airflow from the front vents when the front air quits cooling?

Gotcha. That quote was probably in the manual I read but my eyes were glazed over :)

A surface temp reader should help. Gauges too but the valves are at the common points, not in the front or rear circuits so I can't compare circuits.

Keeping things clean is obvious and this rig is fairly clean but how can the effects of a dirty evaporator be intermittent? How can a dirty condenser affect front AC but not rear AC?

Airflow doesn't change from when front AC cools to when it doesn't cool. Switching to recirc doesn't help.

To be clear, the refrigerant tube should be cold downstream of the orifice tube, right?

Referring to this picture -

http://images.gtcarlot.com/customgal...e/49569676.jpg

The front AC accumulator is on the firewall behind the turbo. A refrigerant line goes from the accumulator, around the mastaba, through an inline canister to the compressor. What is the canister?

This picture shows the line from the rear evaporator taking the scenic route around the front of the engine attaching to the compressor at the same port, the bigger line without a canister across the front of the engine -

http://www.2040-cars.com/_content/ca...537065/011.jpg

Sixto
MB-less

I have wondered about that little canister too. I think it is to give a chance for any liquid to flash off before it enters the compressor, just guessing. My Fords have them.

It is an interesting problem, kinda like siamese twins sharing crucial parts.

You can't measure the pressure at all points but you can measure temps and compare those.

Theoretically the refrigerant temperature drops after the orifice but if it is heated from proximity to the turbo you may not be able to measure it, try it.

I have a 98 Suburban with the 350. The canister on the AC lines are mufflers - that's according to my neighbor who is the lead tech at the GM dealer.

The front evap discharges into the accumulator because of the orifice tube. I think this site does a good job explaining the difference between the receiver/drier and the accumulator.
https://macsworldwide.wordpress.com/2010/07/13/three-important-functions-of-the-receiverdrier-in-your-cars-ac-system/

It's hard to make a guess about what the issue(s) could be. I would probably start with putting on a set of gauges before the problem started to establish a base-line, drive it around until the front AC quits, then put the gauges back on.

Are you 100% sure there is no issue with the blend door?

As sure as there's no change in air volume through the dash vents when cooling goes away, and the front heater core is disconnected from the cooling system.

Sixto
MB-less