740 A/C system: low fan speed colder than high fan speed

Discussion in 'Volvo 740' started by Jamie, Aug 9, 2006.

  1. Jamie

    Jamie Guest

    I just converted my 1987 740 A/C system to R134. As of yet I have no
    evidence of leaks or of any system failure. As it stands, here are the
    quirks I am trying to understand.

    1) This system is definitely relative to the outside ambient
    temperature. The system cools approximately 35-45 degrees F less than
    the outside temperature. When the temp gauge in my A/C vent reads
    110-115F when I get into the car, it goes to 60-70F after a few

    However, if the fan speed is on number 1 (low), the temp gauge reads
    about 48 degrees F.
    On number 2 it goes to about 58F
    On number 3 it goes to about 65F
    On number 4 it goes to about 72F

    Of course the actual number depends on the outside air it's cooling,
    but the air is getting less cool as the fan speed increases.

    To test this I also did the same thing in my 2002 Nissan Xterra. It did
    not blow much colder, but it was a constant 52 degrees on all speeds.

    Is there a specific part that would be symptomatic of not being able to
    cool at all fan speeds? I am guessing the evaporator??

    Jamie, Aug 9, 2006
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  2. Jamie

    Jamie Guest

    The only other thing I can think of is that I partially removed the
    blower motor, then reinstalled it. I did not reseal it.

    It is a bit noisy, but still has some life. I don't know if it could be
    sucking warm air in through where the old seal was.
    Jamie, Aug 9, 2006
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  3. Jamie

    Allen Guest

    The faster the airflow over the cooling coils allows for less of a
    temperature drop (less time against the coils to cool). Thus at the lower
    speed you will have a lower temperature than at a higher speed. It sounds
    like the system is operating normally.

    Allen, Aug 9, 2006
  4. Converted R12 systems never work as good as they used to , as the condensor,
    evap, OT, and compressor are all sized for R12. You get about 80-85% of the
    performance when using R134a, which you seem to be getting.

    Getting the best possible airflow through a clean and un-obstructed
    condensor is the key to getting max cooling from it.

    R134a charge is between 70 and 85% of R12, the exact amount for best cooling
    is only determinable with H and L pressure gauages and acouple of

    Tim \(remove obvious\), Aug 10, 2006
  5. Jamie

    Jamie Guest

    OK, I made some progress on this:

    1 - A few days ago I pulled the A/C controls away from the dash because
    the slider knob was disconnected from the lever and the vents were open
    in the HOT position. When I slid the control lever over by hand (under
    the controls), I started getting cool air for the first time.

    Since then it's been doing as I described. So, I pull the controls
    completely off today thinking the slider knob was blocking the slider
    arm from going all the way into COLD mode. I disassembled the controls
    and re-attached the slider knob onto the control arm. I reassembled
    everything and at about 95F in the shade at idle I was getting about
    62F on fan speed 2 and 72 on fan speed 3 and 4. My aux fan is
    disconnected so I put a blower fan on the front and it made no

    2. Here's what made a difference. at about 90F a short while later I
    went for a drive down a shady road. The fan blew a constant 52-54F on
    all speeds. It was even blowing cold on high. Outside temperature and
    sunshine do make a difference. As does cooler wind across the coils.

    So, I plan to tint the windows and buy a thermoswitch for the aux fan.
    The wires were all corroded so I repaired and reconnected everything. I
    just need a place to plug it in - either the hose or the radiator tank.
    I have no idea which is better. The hose has no thermoswitch and the
    tank has a hole with a plug in it, but I don't know what part I need.

    Jamie, Aug 10, 2006
  6. Jamie

    Jamie Guest

    At first it must have been 100F around 7:00pm, because I just checked
    Weather.com and at right now at 8:04pm it says it has cooled down to

    Thank goodness for low humidity.

    Jamie, Aug 10, 2006
  7. I agree with all that, except the guages. The pros told me it is impossible
    to determine the charge of a 134a system by pressure, but I had to try it
    myself. They were right! My only attempt to charge a 134a system by guage
    was a miserable failure, although I was practiced at charging R-12 systems
    by ear (compressor cycling), by thermometer, by sight glass or by guage.

    Definitely start with the condensor, Jamie. Your symptoms point to either
    insufficient charge or the condensor is not working well. Okay... guages
    would tell you the difference in that respect, but listening to the
    compressor clutch will do that too. If the compressor is cycling quickly the
    charge really is too low. If it is staying on the condensor is choked. Wash
    it out with water spray and check that it is getting lots of air blown on
    it. If for any reason you don't have the aux fan running when the A/C is on,
    focus on that.

    Michael Pardee, Aug 11, 2006
  8. Jamie

    Jamie Guest

    Thanks Mike,
    It's been a hard read. I've been test driving late evenings, but the
    temp's been 97-100F. On a straight highway in the shade I'm blowing a
    steady 52-54F.

    My aux fan isn't working. I traced all the wiring, but no power to the
    fan. As for the compressor, I haven't watched it long, but it isn't
    cycling on and off like it's low. It seems like it is staying on -- but
    I haven't timed it. How long should the cycles be for a compressor in a
    normal system?

    Condensor -- I removed it, poured A/C flush in both tubes and tried to
    work it in. Then I took an air hose and blasted it out and the air
    flowed well. I just don't think I really got much flush in there. I
    used a funnel, but it's not like it free flowed. The air flowed very
    well. though.

    Other than some bent fins on the compressor from the aux fan breaking
    off and spinning on it, I really have no evidence of damage.

    So, the question is -- if it's 100F outside and I'm driving down the
    road, is 52F considered normal? That's about a 50 degree cooling.

    Jamie, Aug 11, 2006
  9. Jamie

    Jamie Guest

    Regarding the above, I did not test for power in the lines. I need to
    do that. I think maybe the fan fried, if it broke loose and jammed
    against the condensor. Just a guess -- it turns easily though.

    I may rewire a new fan. I could take the wire from the ignition, run it
    to a relay, then run out to a new fan. My question would be should I
    power it off the battery or by a fused line from the compressor power?
    Jamie, Aug 11, 2006
  10. That sounds pretty good to me for a converted system. You've pretty much
    narrowed it down: the condensor isn't able to throw off the heat collected
    from the evaporator. That is consistent with the compressor staying on the
    whole time, which often also occurs with a healthy system. The A/C has to
    have the interior pretty well cooled down or the outside temperature has to
    be fairly cool before the compressor will cycle.

    Supposedly, the big limitation in 134a conversions is in the condensor. The
    more air you can get flowing past that thing, especially when the vehicle
    isn't moving, the better you can expect the A/C to work.

    Michael Pardee, Aug 11, 2006
  11. Jamie

    Jamie Guest

    Agreed. I never took the time to learn how a car A/C works until
    recently. Once you learn what happens at each stage, it is very
    interesting and somewhat simple. I just wish it were a bit less


    Jamie, Aug 11, 2006
  12. Jamie

    Tim.. Guest

    I disagree- charging via the gauges in conjunction with thermometer is
    second nature after abit of practice,
    although its far easier and quicker to at least have a rough idea of the
    weight of gas you have put in.
    Yes, the condensor fan must be working, you cant expect it to work without
    adequate condensor cooling...

    Tim.., Aug 11, 2006
  13. Jamie

    James Sweet Guest

    Sounds pretty reasonable to me, particularly if the aux fan is not
    running and you have some bent fins.
    James Sweet, Aug 13, 2006
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