Low pressure switch - cutoff pressure for R134a

[82DMC12]

After much reading, I think I may have a bad/clogged orifice tube but I will have to evacuate and put the correct amount of freon in before diagnosing.

Actually, as cheap as they are, and it's suspect, would it not be a bad idea to just replace it while my system is empty?

You're right, they are only a couple bucks. Has it ever been replaced, do you think? There is some risk of damage to the evaporator pipe which is resides in if it is hard to remove. If the pipe is straight and it's been out before, you should be able to get it out with needle nose pliers however there is a special tool you can buy that might make it easier and safer. Installing the tube can be difficult as well, you might have to make a tool out of copper pipe to push it far enough into the evap pipe. I posted a picture of the tool I made elsewhere, I can post it if you need to see it.

I'd say if you still have trouble after recharging, yeah, evac again and replace the orifice tube and the accumulator.

[David T]

You're right, they are only a couple bucks. Has it ever been replaced, do you think? There is some risk of damage to the evaporator pipe which is resides in if it is hard to remove. If the pipe is straight and it's been out before, you should be able to get it out with needle nose pliers however there is a special tool you can buy that might make it easier and safer. Installing the tube can be difficult as well, you might have to make a tool out of copper pipe to push it far enough into the evap pipe. I posted a picture of the tool I made elsewhere, I can post it if you need to see it.

I'd say if you still have trouble after recharging, yeah, evac again and replace the orifice tube and the accumulator.

Check if your high side pressure is too high and your low side pressure is too low. That would indicate a plugged (or partially plugged) orifice tube. Rare but possible. Look inside the hoses and pipes. If they are nice and clean and shiny it is doubtful the tube is plugged up. High risk of damaging that pipe. It is soft aluminum and you puncture it before you even know it. Too high a risk to "just replace it". Once you damage that pipe you have to disassemble the whole dash to R & R that evap coil. Big job! Better to not do it proactively. The orifice tube was put in and then that pipe was bent. Just trying to straighten it out can damage it and you can't remove the orifice tube unless you first get that pipe straight. "Good Practice" is to replace the dryer/accumulator during service and the cores of the service ports. I also use brass caps instead of the plastic dust covers for the service ports. If you do convert to -134 you are supposed to have a label indicating that and the amount. You should also install adapters for -134 over the service ports. Also a good idea to record the temps and pressures for future reference.

[Michael]

If I remember correctly, both high and low sides were reading lower than I wanted to see for the ambient temp. I then added 1/2 lb and saw pressures up but no discernable cooling improvement.

I guess I will just pull it down and put the correct amount and go through the tech manual diagnostic flow chart.

[Michael]

Pulling down now. Should I go with 2.0 lbs? The manual states 2.2 of R 12 so minus 10% would be 2 even.

[82DMC12]

I can't remember exactly how much I put into my car but I was just talking to another owner who did a r134a recharge and he used about 1.75 lb, going off of some kind of a generic R12 to 134a conversion chart. Also assuming 2.2 lb R12 is the correct charge.

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[Michael]

The manual does say 2.2. I may split the difference and go with 1.9.

Also, I think my mixing flap wasn't fully closing. I got under there while it was drawing down and I think I was able to get a little more throw out of the arm after loosening the rod keep.

[82DMC12]

Let me also point out that the little cans of refrigerant you get at the parts store are usually only 12 oz cans, not a 16 oz can. Just so you are sure you have enough on hand and get an accurate calculation.

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[Michael]

I'm using a recovery machine. Still in the middle of a leak test before I charge.

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[Michael]

Ok...after adding 1.9 lbs of 134a my pressures are:

Engaged at idle ambient at 90 degrees . No extra air over condenser

L 55
H 200

I had to adjust the low pressure switch because I couldn't get it to draw down to 22. Clutch disengaged...same ambient conditions. I did drive it home before these readings and I could hear the compressor cycling so I assume the lack of airflow over the condenser made the difference.

L peaks at 65
H peaks at 200

Obviously the low side is too high. I'm really leaning towards the expansion tube.

[82DMC12]

Take a look at this chart, it shows what the normal operating pressures should be at a certain ambient temp.

If you are in the sun with doors open, AC on, and the ambient temp is 90F, you should see AT IDLE low pressure about 45-55 PSI and high pressure at 250 - 270 PSI. If you open the throttle a bit like to 1800 or 2000 RPM , you should see the high side go up a bit while the low side continuously drops, and the compressor should stop at 22 PSI (as you have the low pressure switch set). Then you see low pressure rise, high pressure fall, and when the low pressure hits 40's again the compressor re-engages.

What exactly do you see when you do this?

Stick a thermometer in the dash vent and see what the vent temps are. You should also feel the low side metal fitting on the compressor be refrigerator-cold and sweating while the high side fitting is too damn hot to touch more than a moment.

The accumulator should also be cold and damp but not frosty.

You should also have condensation dripping out of the hose behind the accumulator after a few minutes of running the AC.

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