Engine Size: 350 5.7
Refrigerant Type: 134a
Ambient Temp: 90
Pressure Low: 47
Pressure High: 275
Country of Origin: United States
I have a 1988 chevy suburban with dual air. This past month ive been experimenting with the expansion valve and different size oriface tubes. Before on 90 degree 50%
humidity day vent temps would be 55-60. Now I'm getting about 45-48 vent temps low side 45-47 high side 260-275. At 1500 rpm the low side 35 high side 300. To achive
this I adjusted the rear expansion valve by turning an allen screw on the inside of the valve in all the way and backing out two turns. And ended up using a brown (.047 Dia)
oriface tube for the front. On the road I'm getting as low as 35-37 at the vent and that's on high blower! I reset the low pressure cutout at 20lb. So it won't freeze up. My
only concern is that the high side pressure has gone up. Is This going to be a problem in the future? What is the max high side pressure? I originally posted "won't cool at 85
ambient" I just want to conferm my results. Just to make sure I haven't missed
It would appear your "tuning" has increased the heat removal from the air passing over the evaporator.
Reducing the orifice sizes alone will increase the high side pressure, and the more heat that is now being removed from the cabin must also be released by the condenser.
I realize that, but will running with the pressures i have hurt the compressor? Especially if it gets hotter say 95-100plus degrees outside? I just want to know the high side
limit on a dual air system.
350 psi and above is going to shorten the life of a compressor.
yes cw=more spring pressure ccw=less spring pressure. Overall My goal was to increase high side pressure slightly in order to lower low side pressure. Say for example on 80-
85 degree day high side would be 175-200 low would be 50. So by increasing spring pressure I increased high side (2.8)-(3.0) ambient and it lowered the low side. The
original valve took 10 turns cw to bottom out, the new one I replaced took 5 turns cw to bottom out, so at first I went in cw until it bottomed out and backed out ccw 4-1/4
turns. Ended up going cw until it bottomed and backed out ccw 2 turns. 350 and up, ok I think I'm going to be allright, I'll just have to gauge it when and if we get 95-100plus
weather. It cooled off here in Memphis, usually this is the hottest time of year w/ 95-100 degree days with 50-75% humidity. I'll see how it goes and thank you all for your
input, I really do appreciate it!
FWIW, I just redid the A/C in my Geo Metro.
Replaced bad o-rings, the expansion valve with a new one, flushed, refilled oil with POE, new drier, new/bigger condenser fan, refill with R134A.
Results were lackluster. Low side pressure was ~50, high side ~230. Was getting ~ 20 degree drop out of vents. This was on a 95+ degree day in the driveway.
TXV was originally set for 3-3/4 turns cw to stop. I readjusted it to 2-1/2 turns and now my low side pressure is ~ 30psi, high side is ~220, now have 30 degree drop at vent.
My condenser and evap are tube/fin construction (low efficiency)
I had thought that with the higher vapor pressure of R134A that the spring pressure on the TXV would need to be lowered, but in my case (and yours) going up helped out by lowering the low side pressures.
I still need to figure out what my proper charge is. I've got aftermarket, dealer installed A/C and there's no sticker or information on it anywhere.
Edited: Fri September 04, 2009 at 1:18 AM by Probedude
I'd say with those numbers you would be seeing 32-35 at the vent. What are the inlet and outlet temps on the evap if they both feel cold then I would suspect a blend door
problem. Because even when you had 50lb on low side when 95+ you should have had at least 50-55 at the vent. But if the evap inlet feels cold and the outlet feels warmer
then I'd say a little low on refridgerant. If that's the case I'd try adding a little slowly until the outlet is as cold as the inlet. As far as proper charge I would ask the dealer who
installed the ac.
Edited: Fri September 04, 2009 at 6:17 AM by squeege
what! that doesn't make sense. Are you sure that your not backwards. If the pipe between expansion valve and the evaporator, is cold, then that is the inlet. If that pipe is
warm and the other is cold then I don't know what to tell ya.
Well, which one is it? I'm saying that if the pipe between the txv valve(the one you adjusted)and the evap is cold and the other line from evaporator to compressor is warmer,
then you are a little low on refridgerant. Because on 95+ day you should have 40-45lb on low side 209- 237.5 high side. I would go by the numbers on your gauges. Put in
a little at a time, if it starts to cool better at the vent, keep going until it no longer cools any better at the vent. That's what i did. On my conversion I put an aftermarket
parallel flow condenser, the sticker said w/rear air it should take 4.0 lbs of r-12. I really don't know how much 134-a I have in it because I didn't have an accurate scale to
weigh it in with. I used 30lb cylinder to fill with. I just went by the numbers on the gauges. I'm guessing it is around 3.2- 3.8lbs. All i had to weigh with was a bathroom
scale, not very accurate. As long as you don't exceed 325-350 on high side i wouldn't worry about it.
I Found this site. I just googled subcooling for automotive ac and found doc blocks. It is a real simple way to find out if you are under or overcharging the system by
Edited: Fri September 11, 2009 at 11:21 PM by squeege
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