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  1. #1
    Join Date
    Jan 2002
    Location
    Fort Worth\Dallas, Texas
    Posts
    1,703
    I got a 98 suburban with rear air. The a/c works good as long as you are going down the road, but if you come to a stop or you are driving in town it just blows hot air. The suction pressure when Idling is around 70 psi. I thought maybe the valves were bad, but that does not make since because it works fine on the highway.

    Any thoughts?

  2. #2
    Join Date
    Apr 2003
    Location
    Smokies
    Posts
    970

    CHECK

    your electric fan.

  3. #3
    Join Date
    Aug 2003
    Posts
    38
    Check for dirty condenser, that would be my bet.
    might be slightly overcharged or orfice tube may need replaced.
    But Im going to bet by it workin alright going down the road its a condenser problem.
    Also check clutch fan and shroud . You should be able to feel plenty of air going thru the condenser.
    Check inlet and outlet temps thru condenser, you should feel a
    considerable difference by hand.

  4. #4
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584
    Overcharged {70 psi} at idle .Or check the high pressure switch.
    Normal operation idle R12- 28-32psi
    R134a-38-40psi

    Something is in the system -non condensables.Check drier/metering tube.
    I would recover refrigerant, vacuum and recharge.
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  5. #5
    Join Date
    Jan 2002
    Location
    Fort Worth\Dallas, Texas
    Posts
    1,703
    Ok, I just slapped the gauges on again. Here is a little more info. The condenser is clean that was the first thing I tried.
    OAT - 77 deg. With the car idling and the fan on full speed in the front and low speed in the rear the suction pressure was 60 psi, 130 psi discharge, suction line temp 74.8 discharge air 74.8.

    With the engine reved to 2000 rpm suction pressure fell to 35 psi, discharge went to 175 psi, 63 suction line temp and falling, 62 deg discharge air and falling.

    I bought this car 2 years ago and the a/c worked fine the first year. It started doing this last summer. The system has not been opened since I have had it, and it has had no freon added.

    What should the normal discharge pressure be at idle. It seems low at 130 psi. I wonder if something is wrong with the fan clutch?

    Thanks in advance for any info!

  6. #6
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,619
    Thinks it needs gas ..........suction temp is high!

    I would evacuate and scale in a charge and see what happens. This suction can be high since i thinks you got 2 units on that big rig

  7. #7
    Join Date
    Jan 2004
    Location
    Burleson, Texas
    Posts
    1,741
    Fan clutch is probably the culprit..

  8. #8
    Join Date
    Aug 2003
    Posts
    38
    Do both units quit at the same time?
    If it was a fan or condenser problem your high side pressure would be 300 psi in a very short time.
    with only 175 at 2000 rpm you can rule that out.
    Check to see if you can feel which evaporator is messing up.
    You might be able to feel a difference on suction line where they tee together.
    I believe the front has a orifice tube and rear has a txv.
    The orifice tube should be right at the connection of the condenser, feel it to see if there is a temp drop.
    Then recover vac and recharge.
    If it still does it I am suspecting a weak compressor or bad end on orifice tube.
    I would put a new tube in it while the 134a was out of it.
    That might be a cheap 5.00 fix if your lucky.
    As for the high pressure due to two units I dont believe that would cause it. I work on semi with front air and bunk air all the time with same set up as yours and depending on the outside temp they will run 25 to 40 on the low side.
    Let us know what you find out.

  9. #9
    Join Date
    Jan 2002
    Location
    Fort Worth\Dallas, Texas
    Posts
    1,703
    Originally posted by gsjohnson
    Do both units quit at the same time?
    If it was a fan or condenser problem your high side pressure would be 300 psi in a very short time.
    with only 175 at 2000 rpm you can rule that out.
    Check to see if you can feel which evaporator is messing up.
    You might be able to feel a difference on suction line where they tee together.
    I believe the front has a orifice tube and rear has a txv.
    The orifice tube should be right at the connection of the condenser, feel it to see if there is a temp drop.
    Then recover vac and recharge.
    If it still does it I am suspecting a weak compressor or bad end on orifice tube.
    I would put a new tube in it while the 134a was out of it.
    That might be a cheap 5.00 fix if your lucky.
    As for the high pressure due to two units I dont believe that would cause it. I work on semi with front air and bunk air all the time with same set up as yours and depending on the outside temp they will run 25 to 40 on the low side.
    Let us know what you find out.

    Just to educate me, How would a bad orfice tube cause high suction? I could see it causing high head but I would think it would starve the evap.

    As far as, " do both units quit at the same time?", Neither
    one of them cool very well if the unit is idling even if it is when you just start it up. As far as a week compressor, I thought about this but it sure doesn't have any problem pumping at 2000rpm with a 35psi suction and a 175 psi head.

    Im stumped!

  10. #10
    Join Date
    Aug 2003
    Posts
    38
    If you have blowed the end out of your orifice tube it will let more refrigerant thru and give a higher suction and lower high side and everything quits cooling due to abnormal pressures.
    Same goes for your TXV on the rear unit, if it is stuck open too much will do the same thing.
    Most of the time if there is a failure in front or rear evap(txv stuck closed or blocked orifice tube) you can feel it at the tee in the suction lines, one warm the other cool.
    Have someone rev it up while you trace the lines to see if you can feel the difference.
    If one is considerly warmer than the other that will tell you witch way to go.
    I hoping that your end of your orifice tube is cracked just enough to let too much thru at idle, and then when you rev it up the compressor has enough suction to overcome it and go ahead and work.
    You might check the superheat off the rear unit, it should be around 10 degrees with a TXV ,that might rule that end out.
    Best to recover, vac, and recharge to make sure you do not have any non condesables in there and then go from there.

    I would not rule out a weak compressor yet, thats what it is starting to sound like to me, but we need to look at it from all angles first.

    Check it out and see what you find and let me know. Im pretty curious now to know what it is. I will give you all the help I can from this side of the computer!!!

  11. #11
    Join Date
    Jan 2002
    Location
    Fort Worth\Dallas, Texas
    Posts
    1,703
    Thanks for all the input gsjohnson. I will try to check some of the things you mentioned. If the rear has a txv where is it located. The liguid line turns up on the passenger side in the back and it looks like it goes straight into the sealed coil housing. There is a fitting underneath where the lines turn up to the evap on the suction and liquid line, but i am assuming this is so you can yank the evap out.

    Agian, thanks for the help.

  12. #12
    Join Date
    Aug 2003
    Posts
    38
    What sucks is you can not just yank it out.
    If I remember right you have to take about half the interior out just to get to it and it sucks.
    I did one last summer that the compressor actualy exploded inside.
    I dont remember if it was a tahoe or suburban.
    We replaced everything except the evaporators.
    Cool like a brand new one when we were done(cause it was).
    The fittings you see under the back are short jumpers(on the liquid line, I dont remember about the suction).They are aprx. 6 inches long, the other end is on the txv on the evaporator.
    Pray that that end is fine because it is a big job to get to it.
    Good luck!!!

  13. #13
    Join Date
    Oct 2001
    Location
    Waterford Michigan
    Posts
    2,668
    Discharge pressure is low for a 77 day with 50% humidity it should be around 200-225 psi at 1000rpm. You must realize the capacity increases with engine speed.

    [Edited by johnl45 on 05-06-2004 at 07:14 AM]

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