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'99 Suburban Front & Rear Temp Difference

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  • '99 Suburban Front & Rear Temp Difference

    I've noticed my '99 suburban's front AC is notable warmer than the rear. I can tell quite a difference simply holding my hand in front of the two units. Front use to be colder than the rear. Front is an orifice tube. Rear is an expansion valve. I did the last charge and it was done by weight. I can check the pressures and blown air temperatures easy enough but I'm not sure what that info tells me.

    1) If the charge is low, is it common for the two systems to blow significantly different temps? Meaning, is this ever indicative of a charge problem?
    2) I'm suspecting a partial clog somewhere. The evaporator and orifice tube being the likely place to look since the rear system seems unaffected. Orifice tube would be the easiest to replace. What are the symptoms of a bad orifice tube? I'm assuming they can go bad.

    Thanks for any suggestions on where to begin.

  • #2
    Actually if you are in a heat extreme alone this could be normal! Rear uses an X valve and front is CCOT this set up + others like it are capable of the rear taking more of the "bounty" of already condensed refrigerant than the front. It would also do that if low.

    You need at now ~20 years old if every last thing is up to max ability. Check list would include strong fan clutch they do diminish some % per year if and should be OE thermostatic type has exposed temp spring in the front of them.

    Condenser clean and clean in between it and radiator. Also this layout temp should pick it's spot once warmed up and about never change where dash needle sits if not and extreme conditions OK like 110F AND a load on the vehicle - hills or just working hard radiator can put heat against the breeze "radiant warming" I'll call it.

    If not extreme and all else checks out even the dials for rear controls overridden by the front I'm pretty sure and want to see if it's still full or close shut rear off totally and take pressures then see if they become credible. That's never exact just shows within range to work.

    If you wish to peruse it more and really know if low you'll need what's in it now recovered by equipment that can tell you exactly what it took out should be same or super close if you really knew how much was installed by weight and trust that.

    These can be off up or down maybe 5 ounces and behave they hold so much in total like I think 65 ounces total - check that spec is correct.

    If you find it was a bit low only and works fine with known charge in the same conditions is time to find where it went. Most common was the compressor Delphi nick named a "Belly leak" would or should show oily or dye now. Your call if you want to and really trust yourself and observations of EVERYTHING boost it back up but know to stop too much is never good or better.

    I'll quit writing a whole novel but quite familiar with this exact set up and vehicle.

    Just FYI and a suggestion. Get real known accurate thermos in the center vent front one in rear if you wish too cheap enough and read them a lot vs known outside temp coming in. When any go low and keep working at some point oil doesn't move for compressor and will be total nightmare toss a ton of stuff being dual systems decision time for you for the expense of keeping it dual or will reduce costs if you just plug that off - disabled and just use the front should have been available anyway. It's the same or close to the BTUs of cooling just splitting it up as dual. Yes if passengers are using the rear enough and it matters do it up or maybe at this age can find a strong 12V fan for a power port clipped to say one of the overhead grab handles and front vents pointing as far UP as they go.

    1 more: You can defeat excessive heat it's in if still just mist the condenser with water, raise idle then if all snaps to perfect decide from there . Sure it will work better doing that anyway how much better a clue.

    Lied - Another "More:" Use a super HD fan clutch but consider how annoying it will be feel the power loss and noise of it just me find that obnoxious but it does help A/C if mine would switch it back when not so hot out - really! Takes a 1" wrench that's made for it to air hammer it off. OMG near sure it comes off counter clockwise double check that with a new one feeling the internal threads to be certain.

    That's enough for now just say if it's super hot when you notice this again,

    MetroWest, Boston


    • #3
      It was actually 84 degrees at night cruising on a expressway. Optimal conditions. System was not stressed. Normally, the AC runs you out of the truck under these conditions. I have to roll the fans down to low and start moving the thermostat upward. Now, the truck never gets cold.

      Regarding your post. There is a big difference in how the truck cools using just the front system. In fact, the front has always blown warmer with the rear system off. I'll check receipts but I believe the fan clutch is HD. Shouldn't matter on an expressway at 70mph. The charge was printed on the accumulator, which was replaced when I moved to a Sanden compressor.

      I'll check what you suggest and see what I find.


      • #4
        You are right that those are optimal conditions for performance. Right again charge would or was printed on accumulator if original doubt the replacement if it was for a compressor change I'm not sure same accumulator will fit another so wouldn't state charge again. OE should be 64 oz. 134a, 11 oz. of PAG 150 oil with replacement of parts oil just might be the issue or original compressor did throw debris.

        It changed apparently so I'll just list out areas that are the common snags.

        Over oiled? Would reduce ability of condenser should be tossed with a compressor failure if it did and O tube checked for debris. Condenser can't effectively be flushed out should be new if/when any debris found if not totally flushed out front and rear any remaining debris will just compromise new one quickly again then the rear wins as said the TXV will steal the condensed refrigerant first starving the front.

        Accumulator fitting are probably impossible to remove without doing new hoses to front and rear the aluminum likely to ruin threads of connections if they behaved at all and didn't twist up lines.

        Rear issues: You can't get at the X valve nor evap there without cutting a hole just right in the case! Procedure may be in archives at the old site loaded with info on this system. With that now you lose me as I don't know in person anyone who fixed the rear rather deleted it back to front only set up.

        Other things reported not seen by me personally and wont this vehicle's set up ended this model year, 1999 just BTW.

        If battery was disconnected ever was reported blend door could stick shut would (my guess) then not work at all complaint would be extreme.

        By chance own this front only in a '97 HD P/U is mostly the same low mile use is still OE totally and works as intended part luck plus being in New England I disable A/C for off season it absolutely does leak down some and most do strongly believe shaft seals don't hold in sub zero temps systems will only click on thru off season also when defog or defrost is used if only confused by engine heat parked for a few minutes can fool it there's adequate static pressure to allow it to work!

        Just FYI thru at least model years '99 2/3rds of all reasons for A/C complaints are from leaks and low charge if not noticed and shut down/disabled is almost assuring compressors will fail due to lack of oil that doesn't move if not condensing the charge by designs are just too late to self shut down systems to save it common to assorted makes and models of vehicles!

        Also note this is a serious rust belt. Vehicles used all year round on heavily rock salted roads will have fatal rust issues showing that it will soon out cost any practical reason to keep the vehicle in use at all A/C would not be fixed for a final year of use over an A/C only problem unless by chance a remarkably simple and inexpensive fix.

        Good luck and consider if checking more while working some at all getting accurate temp devices both meat thermos types for vents and infrared touchless ones can check temps of lines and metal items accurately know the temp then you know what pressure is where at that spot I find totally useful info for diagnosis of a problem or just where it's failing,
        MetroWest, Boston