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Old 05-25-2015, 07:50 PM   #1 (permalink)
cliffyp
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Strange A/C behavior

The rear AC in my 95 suburban is very consistent. 55 degrees at idle, almost 50 while driving.
The front will fluctuate from lower 50s all the way up to 70. I can't find any pattern to it. Sometimes it feels great, then it feels like the AC was turned off. Even when the front goes to 70, the rear is still blowing cold. All measured with an infrared thermometer.

I'm no AC expert so help would be appreciated. There is only one compressor, correct? So the compressor is fine since the rear always blows cold. Could it be the blend door is moving sometimes?

I did take it to an AC shop. The pressures where fine and the front was blowing cold while they had it, so they just said it works.

Any thoughts?


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Old 05-26-2015, 02:31 AM   #2 (permalink)
sixto
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x2. Same thing with our '97. Can an orifice tube work intermittently?

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Old 05-26-2015, 03:03 AM   #3 (permalink)
bshull
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What was the outside temp? The test of an AC system is temperature differential, outside temp vs output temp. Is it putting out the same temp front and rear?

I know on my burb it takes 10 miles or so with front and rear AC to get it cooled down and then it will freeze you out at the 20 mile mark.

Yes, one compressor but the front has an orifice tube and the rear has an expansion valve. I have had the orifice tube clog up while the rear works great. Then the front works great while the rear doesn't.

On a 95 burb the orifice tube is found on the passenger side of the engine bay just after the "Y" split located behind the condenser. One side of the "Y" works the front and the other flows to the back where the expansion valve is located.

To answer your question, "Can an orifice tube work intermittently?" Not really. You could have debris in the system that catches in the orifice tube. I had this on my burb, took forever to flush it all out.

I recommend you check into your cooling stack and make sure the condenser is super clean and the space between condenser and radiator. This can also help with coolant temps. It is also a good idea to look at the evaporator coils as well.

If the coils check out do you have an issue with your HVAC and door control. Usually it just fails.

Just a guess, you have debris in the system and it clogged up the orifice tube. Be very careful, I had this with mine when a compressor died. I could get the front working and the back suffered or I could get the back working and the front sucked as debris moved throughout the system. I finally had enough and spent hours flushing the entire system. Crappy work and it sucks. After all this you must vac the system down and remove all water. Fill with proper amount of PAG oil then add freon.

Be careful of the freon you add to the system as most on shelf stuff has an ounce of oil in it. Ask the parts guys for freon without oil, usually it is not on the shelf as it is just a bit cheaper. This, if not planned for will put too much oil in your system and cooling will suffer.

Start with the cheap, check the coils and blend doors.

My 2 cents,

Brian
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Last edited by bshull; 05-26-2015 at 03:06 AM.
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Old 05-26-2015, 12:10 PM   #4 (permalink)
Glagulator
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x2 bshull
The diesels do not have a heater coolant shutoff valve, although the gas shutoff valve only slows it down.
I put a brass shutoff ball valve on the heater hose to keep the heat out of the cab.
Every degree counts here.
Also make sure your recirc door is closing and there is no debris for it to hang up on.
Are you getting good air flow out the dash vents? Does it slow down when the temp rises?
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Last edited by Glagulator; 05-26-2015 at 12:13 PM.
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Old 05-26-2015, 03:34 PM   #5 (permalink)
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Quote:
Originally Posted by Glagulator View Post
Also make sure your recirc door is closing and there is no debris for it to hang up on.
Are you getting good air flow out the dash vents? Does it slow down when the temp rises?
Would look there first
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Old 05-26-2015, 05:11 PM   #6 (permalink)
nsdtk
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My 95 truck will freeze the ac grid in the cab and will loose cooling. Its a odd thing to get hot while the compressor is on turn it off and get froze out of the cab.
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Old 05-26-2015, 06:22 PM   #7 (permalink)
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Yup... Orifice tube debris from a failing R4 compressor... Reclaim the refrigerant, pull the tube out and inspect. If it's got black or gray gunk on it, plan on replacing the compressor and the condenser. If it's clean, pull the condenser and look for leaves and gunk in it.
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Old 05-27-2015, 11:34 PM   #8 (permalink)
cliffyp
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Quote:
Originally Posted by bshull View Post
What was the outside temp?
mid 90's

Quote:
Originally Posted by bshull View Post
Is it putting out the same temp front and rear?
Quote:
Originally Posted by cliffyp View Post
Even when the front goes to 70, the rear is still blowing cold. All measured with an infrared thermometer.
Quote:
Originally Posted by bshull View Post
I recommend you check into your cooling stack and make sure the condenser is super clean and the space between condenser and radiator. This can also help with coolant temps.
I'll check it out, but since the rear AC always blows cold, I wouldn't think this would be the problem.

Quote:
Originally Posted by Glagulator View Post
I put a brass shutoff ball valve on the heater hose to keep the heat out of the cab.
Every degree counts here.
Also make sure your recirc door is closing and there is no debris for it to hang up on.
Are you getting good air flow out the dash vents? Does it slow down when the temp rises?
I like the brass shutoff idea as an extra. I'll check the recirc door, but I would assume that it fully closes since the front does cool down into the lower 50's. Perhaps the recirc door is opening, or heat blend door is opening, and then later closing. Air flow is always strong. When the temp rises, you don't notice anything change other than warmer air. Feels exactly as if the compressor stopped, except it hasn't and the rear stays cold.

I guess I'll take it back to an AC shop and explain better what it does, mention the orifice tube debris and see where it leads. Thanks guys
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Old 05-27-2015, 11:55 PM   #9 (permalink)
Glagulator
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With credit to previous posters:
The rear air is working good so there is nothing wrong with the freon cycle. If there were trash on the OT then it would also plug up the expansion valve on the rear air, it has an even finer screen than the OT.
I have never seen an intermittently blocked OT but anything is possible. Having owned more burbs than anyone should be allowed, I can sympathize with you.
With it running and a/c on max/recirc, pull the blower fuse and see if the accum/dryer freezes up. If it does, then the problem is a door, solenoid, wiring, or the control head.
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96 SUBURBAN K2500 wife's truck 210k+ mi. orig. motor & trans, 3.73...
95 C2500 Suburban 6.5td HO w/p, timing set, HB & pulley, all rubber hoses, modded engine & main harness, converted to OBD2, PMD to bumper...
95 C1500 Suburban 6.5td rebuild tranny, convert to DB2, replacing as many parts as I can.
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Last edited by Glagulator; 05-27-2015 at 11:58 PM.
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Old 05-28-2015, 01:00 AM   #10 (permalink)
JMJNet
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Quote:
Originally Posted by nsdtk View Post
My 95 truck will freeze the ac grid in the cab and will loose cooling. Its a odd thing to get hot while the compressor is on turn it off and get froze out of the cab.
Usually, the low pressure switch on the dryer is out.
This switch is commonly failed due to location.
Fortunately, changing it is just a matter of screwing in and out without leaking any refrigerant.
Plus the switch is cheap.

Try this first and see.

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Last edited by JMJNet; 05-28-2015 at 01:00 AM.
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