Suburban AC system rebuild from hell

[countryroads]

I have a complete AC rebuild in a 96 6.5L diesel suburban. Mechanic is stumped (He is reputable), and I'm $1000 down the hole so far with the wife still without the rig that has air. Not good for the marriage either =) of course, it was my idea to buy the beast so we could tow.

History
2 years ago - on the day we were to take a camping trip with the kids, the compressor grenaded in the driveway w/ a 33' trailer attached to it, grenaded bad enough we had to shut off engine as clutch was seized. Wife said Rear AC stopped working earlier that day. Broke and had to replace compressor without hooking up any hoses to the new one and taped up the old high/low side lines.

1 year ago - friend of my parents wanted some work - semi reputable mechanic. New compressor. Flushed out lines. Unsure whether orifice tube was replaced. Dryer replaced. AC worked for about 3 weeks. Compressor started leaking out of back side where pressure switch is located. Mechanic probably did not flush lines enough. Front and Rear AC worked for a short time.

4 weeks ago. After the burb blew the PMD, replaced Oil pressure switch and filter housing- had sat for nearly a year while I was broke, I got up the courage to work on it again. Had a temporary intermittent temp spike. Had a bad thermostat and truck runs great now without any temporary overheating; flushed entire coolant system twice including taking off overflow tank and washing it for an hour. Took to reputable local mechanic who does not like dual AC systems, but who I trust not to screw me. Front AC worked again w/ new compressor (4 seasons reman), replaced dryer. Rear AC still did not work. Intermittent front blower fan. Replaced it without problems. Pulled orifice tube (black one), had some small metal shavings in it. Rear expansion valve was completely plugged w/ metal crap.

After 3-4 weekends of working on the beast;
- Replaced dryer again, flushed out front/rear lines w/ flush kit, took it all apart, bought new TXV from dealership for rear air (cut small hole in rear airbox), replaced entire condenser, put in a new orifice tube from Napa for suburbans w/ dual air - PN # 407308 - white/beige color), made sure the new condenser did not have an orifice tube installed. No leaks in system and held vacuum fine. Put a filter on the suction side of the system where it connects to compressor. Rear TXV valve also has factory filter. Only thing not replaced is the evaporator and that looks like suicide to me. Evaporator was flushed out from top to bottom and blown out with air with no blockages.

Mechanic says gauges look great, around 56 oz in system without any pressure spikes. Rear ac blows 55 degrees. Front AC is not working. line from y-pipe to front evaporator is cold. Top line from evaporator to acculmulator(dryer) is not cold. Mechanic is stumped. We've done this twice now and the bills keep adding up.

My concerns: It seems like the compressor is not strong enough to run both systems; or the brand new TXV is stuck open not allowing enough pressure to circulate through the front evaporator and into the dryer. Rear TXV uses factory clip to attach to tube in back. Stumped, running out of patience and money and feels like a dead end.

Orifice tube is also a bit of a concern; napa might have the wrong part # ? Orifice tube I pulled out originally was black, replacement was white.

Also - Concerned problem could now be blend doors, but top of evaporator is not cold.

Any help you could provide would be appreciated.

Thanks

Brock

 

[THEFERMANATOR]

Add more charge until the line coming from the accumulator to the compressor gets cold. The dual system holds roughly 4 pounds, so the 56 ounces isn't quite enough. Also these systems are VERY picky about the oil and how much is in the A/C system. Many aftermarket compressors now come with lightweight PAG 46 weight whereas GM speced the system for thick PAG 150. When you go to PAG 46 you have to up the charge to keep the compressor running. And a full system charge is 8 onces of oil, NO MORE. I JUST went through this yesterday with my BURB as it had a leak at the high side test port. Replaced the test port and put back in roughly 3 pounds and the rear expanision valve was making teh pressures go crazy and warm air. Brought it up to about 4 pounds of charge and the pressures leveled out and it went cold. Try topping it up to 4 pounds(64 onces) first.

 

[dangerousdave]

OOOh, this is some good info. What a story, Brock; my condolences.
Subscribed for future reference.
Thanks for sharing.

I think if I had any metal shavings in the system, I'd have to replace the whole shebang; lines, evaporators, dryer, and condenser. Flushing is good for old oil in the lines but metal shavings are Almost impossible to "wash" completely out. I hope I never have to do it. ($$$)

When the dual AC in the Burb works right, it is a true pleasure.

 

[countryroads]

Add more charge until the line coming from the accumulator to the compressor gets cold. The dual system holds roughly 4 pounds, so the 56 ounces isn't quite enough. Also these systems are VERY picky about the oil and how much is in the A/C system. Many aftermarket compressors now come with lightweight PAG 46 weight whereas GM speced the system for thick PAG 150. When you go to PAG 46 you have to up the charge to keep the compressor running. And a full system charge is 8 onces of oil, NO MORE. I JUST went through this yesterday with my BURB as it had a leak at the high side test port. Replaced the test port and put back in roughly 3 pounds and the rear expanision valve was making teh pressures go crazy and warm air. Brought it up to about 4 pounds of charge and the pressures leveled out and it went cold. Try topping it up to 4 pounds(64 onces) first.

I'll have to try that. What should the pressure be for the system at idle (700 rpm) and at 2000 RPM? It sounds like I should really make sure the rear expansion valve is getting enough contact w/ the factory clip. Posting a youtube video here. http://youtu.be/_vWwm3o7SR0

Once i've verified contact w/ the expansion valve is good enough, i'll take back to mechanic and put some more freon into it. Any other suggestions ?

 

[Cumminsfever]

If I recall correctly, the suburbans with rear air have a special compressor. It has smaller pulley, and bigger internal displacement. Make sure you have correct compressor.

Does the front blow heat? Is something (dirt? blend door?) preventing it from pulling air across it?

 

[THEFERMANATOR]

If I recall correctly, the suburbans with rear air have a special compressor. It has smaller pulley, and bigger internal displacement. Make sure you have correct compressor.

Does the front blow heat? Is something (dirt? blend door?) preventing it from pulling air across it?

SAME compressor for dual or single air, a 15 CFM fixed displacement unit is used on 96+ trucks. The evap doesn't normally get metal in it as the orifice tube and screen catches it all before it gets to the rear. The pressures will vary alot depending on temp and how much airflow you have across the condenser. Make sure your getting good airflow across the condenser and that your fan and fan clutch are in top shape. Teh DURAMAX fan blade helps out alot with ac at an idle as the stock fan doesn't move much air. I know mine right now at 90 degrees outside was running about 225-240 high side, and about 35-40 low side pressure with the fan on medium for the front and low for the rear. Both on high ran it up to about 240-250 on the high and 45-55 for the low side. If teh charge is even remotely low the rear expansion valve will make the pressures spike and dance like a failed compressor. And make sure you have the right expansion valve instaled in the rear and that the capilary tube is SOLIDLY atatched to the outlet line(it is a good idea to wrap it with insulating tape so it gets a good reading on the temp). if the capilary isn't reading right, it will flood your compressor out and blow it in out in less than a minute. Also make sure you are using a GOOD HIGH quality compressor, the cheap ebay ones will never work right from my experience. I have a SANDEN on mine, but have used the global GPC brand with good results also for alot less money.

 

[THEFERMANATOR]

One other thing I thought of is, are you sure you have the correct condenser installed in it? You said you changed it, and there are 2 different condensers that will fit it. One for a single air unit, and one for a dual air unit. The dual air condenser is a good bit larger than the single air unit.

 

[countryroads]

One other thing I thought of is, are you sure you have the correct condenser installed in it? You said you changed it, and there are 2 different condensers that will fit it. One for a single air unit, and one for a dual air unit. The dual air condenser is a good bit larger than the single air unit.

That's a very good question on the condenser. The condenser I replaced was the same width and fit the same dimensions; the factory harrison condenser seemed a bit thicker. I asked for the condenser from Napa for a suburban and had them look it up w/ rear air. I would assume they did it right. Price was about $150. One benefit to the new condenser is the increased airflow to the radiator. I've noticed easily a 5-10 degree improvement in temp on the gauge when driving in traffic.

BTW, I hooked up the gauges tonight. At idle, pressures are 30 low and 150-175 high, outside temp 90 degrees. I had my wife hit the gas to 1500 rpms and the high side started bucking like you said it would from the rear expansion valve on the high side. I could hear the valve make noise in the back like it opened and could hear something from the compressor that sounded like a kick. I made absolutely sure the expansion valve has good contact w/ the capillary tube, and taped up the rear ducts w/ duct tape. It sounds like it simply needs more freon at this point, which would be excellent.

 

[THEFERMANATOR]

Sounds like you have the wrong condenser unless you got the new style condenser(the origanal was a flat tube whereas some new ones are multi pass round tubes and more efficient with a thinner core). Many places sell you the same condenser for a single or dual unit, but they are different and the smaller condenser will not give great ac output at low speeds. From your pressure readings it sounds like your pretty low still on charge. It takes about 4 pounds or about 5 1/2 cans to fill one up with dual air.

 

[countryroads]

Sounds like you have the wrong condenser unless you got the new style condenser(the origanal was a flat tube whereas some new ones are multi pass round tubes and more efficient with a thinner core). Many places sell you the same condenser for a single or dual unit, but they are different and the smaller condenser will not give great ac output at low speeds. From your pressure readings it sounds like your pretty low still on charge. It takes about 4 pounds or about 5 1/2 cans to fill one up with dual air.

You may be right on the condenser. It does have round tubes on the end, and it's made differently than the original one. Napa said this was the right part. It fit into the same spot perfectly; but the original Harrison unit was thicker. The part from Napa was a OSC 4544; which shows up on eBay for a tahoe. Getting concerned Napa screwed me. Going to take the receipt down tomorrow and re-check to make sure I have the right part. Thanks for all the help today.

 

[THEFERMANATOR]

Sounds like you have the new style condenser. I just checked and mine uses the 4544C, but they list that one for a 94-95. 96-99 lists a 4721. They show to be the same core size and fittings, so I don't know what the difference is if any. Looks like they superceded the 2 different old style units to the new style and have just one now. I know I had to add an electric fan to mine to get it to blow cold and keep the high side pressure below 325-350, but part of that was how I had to relocate mine for the swap.

 

[WarWagon]

The old condenser is 1.5" thick. The new style is 1.25" or so. Required new mounts on my 1995.

IMO have the wrong orface tube from NAPA and are low on charge. It should be the same color as this indicates orface size. Mine in the 1995 is black and white.

How old is the fan clutch? If it is over 5 years old you are not doing the AC system any favors as the fan looses 200 RPM per year from clutch age/wear.

In the how to section there is a TSB for AC controls. However your evap line temp is the major symptom here.

 

[countryroads]

Wow, those are some serious high side pressures. No wonder my mechanic undercharged it. I guess the normal truck would take 2 pounds and do 30/175. I had an extra 10 ounce can of freon from an old leaky honda civic. I started at 30L/175H and ended up at 30-35L/225H after I hooked up the gauges tonight at idle with it about 85 degrees out. The mechanic must have only put in a couple of pounds - 30 ounces or so. The front AC started to blow cold, but not as cold as the back. I'd say the front is at 65-70 degrees and the back is around 50-55 degrees which is awesome. What is the maximum pressure I want to charge the system to? It sounds like I should really be around 40L and 250H at idle for it to get cold enough up front. The evaporator line up front was getting cold to the touch, but not as cold as the line where the orifice tube is located right before it enters the evaporator on the front. I've now put back all of the rear interior together, and am waiting to finish the front AC with a bump in the charge. Getting excited that this AC nightmare might finally be over. Crossing my fingers. The kids will love it.

 

[THEFERMANATOR]

Sounds like you still need more charge. If the cabin inside is hot, and the fans are turned up. Seeing 50-65 on the low side isn't uncommon with 250-to almost 300 on the high side. I cannot stress enough how important a GOOD fan and fan clutch is for the dual air to work properly at idle. It doesn't take much of an airflow drop through the condenser to see 350-375 on the high side, and at 400 the popoff valve opens and blows off. With a GOOD fan and fan clutch, at 90 outside, cool inside the BURB with the fans on medium, I would expect to see 35-50 on the low side(the expansion valve will alter the low side ALOT) and about 250-275 on the high side.

 

[THEFERMANATOR]

A good simple method to verify the charge in most any A/C system that uses an orifice tube and accumulator is to check your line temps. Feel the high side line going into the condenser and it should be hot, roughly 180 degrees or so. The line coming out of it should be about 120 or so. If you don't have a hot line going into the condenser, then chances are it's still low on charge. If your getting a temp drop across the condenser, then next move on to checking the temp of the low side lines. You want to fell the line coming out of the accumulator and follow it over to the compressor. When you first turn the AC on this line should get cool, and once teh cab is cooled off that line should be fairly cold and have moisture on it. If it is still warm then your charge is low, if it is iced over then you are overcharged. Don't worry about the line coming from the rear evap when checing this way. The idea is to get enough refrigerant stored in the accumulator so as to have an excess in it but not enough so that cold liquid refrigerant can make it to the compressor. If the line coming from the accumulator is cold, but the system still cycles alot, then chances are it is low on oil. And if the pressures are really high and the line isn't cooling, then you probably have to much oil in it. Checking teh line temps is a good way to verify charghe and if you have the proper oil amount in it.

 

[bobbiemartin]

The front AC started to blow cold, but not as cold as the back. I'd say the front is at 65-70 degrees and the back is around 50-55 degrees which is awesome.

One thing to check is the recirculation door. The control unit and/or the motor can fail and the door will always be on fresh air even though the light shows recirc. You have to crawl down and look at the door while the engine is running to check it. If it is hot out and the door is in fresh air mode, the front air will not get very cold.

 

[WarWagon]

The GM orface tube system is charged on weight. Any attempts to use 'pressures' are only a wild guess. You get 1/2 pound leeway as this is the accumulator capacity. Most systems are on the overcharged side for a leakage reserve. So it isn't an even 1/4 lb on either side of a perfect charge! Too much charge and the system will not cool well and risk liquid getting to the compressor. A low charge starts to cool slightly better, however, the oil gets stuck in the evaporator and the compressor wears/locks up. The evaporators need to be flooded with liquid to cool every part of it, avoiding hotspots, but more importantly bring the oil out of the evaporator.

This said you need to charge the system by weight. Guessing by using maybe half full cans is going to get you poor performance by missing the narrow charge window the systems operate with. So have a shop recover the system then charge it to the proper weight. If you are doing this yourself recall most 12 ounce cans are not full 16 oz pounds. 5.3 12 oz cans means you need a good scale to do it properly for the last can. 64 oz = 4 lbs. Watch cans that have 1 oz super cool oil and 11 oz of R134a.

An electric fan that comes on with the compressor clutch on it's own relay really helps system performance. Relocate the trans cooler behind the condenser for room. Removing the heat (oil coolers) from in front of the condenser also helps these under sized AC systems.

 

[countryroads]

Thought I would let you guys know the AC is successfully working now. We had to add another 12 ounces. Low side 43 high side 240 at 90 degrees outside. Thanks to all that helped with this fiasco. Would probably not have been successful without the input on here. Vent temps front and rear 50-55 degrees.