AC pressure readings (1 Viewer)

[amaurer]

I finally got around to checking my AC with a manifold gauge - the AC isn't working presently.

Low side pulled a vacuum... the gauge actually moved backwards. There are no markings in that area.

High side read quite low, like 75psi or so.

With the system off the low side pressure was in the "Good" region on my gauge. Fluid level in the sight glass looks perfect.

Sooo..... what?

 

[gotmud]

You're right, high side is low, as is the low side. Did you have to jumper the pressure switch to get the clutch to engage? If so, it sounds as though you are low on freon.

When properly working, your low side should be around 30-35. Typically the high side should be about 2.2-2.5x the ambient temp. This is assuming you are getting plenty of airflow across the condenser and have the air set to recirc.

 

[cruiserfj45]

Sounds like you have very little refrigerant. What do you mean the fluid level in the sight glass looks perfect? If you are referring to the sight class on the receiver dryer and it is clear then it is probably because you have no refrigerant instead of liquid.

 

[amaurer]

I had another thread about the refrigerant level; the sight glass is full and bubble free, but definitely not empty. It bubbles a bit briefly when turning the system off, as the FSM says it should. So that, I think, is fine.

 

[vipergrhd]

Expansion valve

I had another thread about the refrigerant level; the sight glass is full and bubble free, but definitely not empty. It bubbles a bit briefly when turning the system off, as the FSM says it should. So that, I think, is fine.

I think your problem is a stuck expansion valve reason why your low side reading is going to vacuum.your high side reading should be 100-120 degress plus ambient temp.even on a full system there is no flow in your system reason your high side will not build up also consider that the discharge side of the compressor could be shot.

never use the sight glass as an indicator of system charge,reason why newer cars no longer use them.the reason why it's bubbling when you shut off is that the pressure is trying equalize.

 

[amaurer]

Anything I can do to unstick it?

Tap on it? Spray it with some LN2? Touch it with a match? Etc...

 

[Wile E Coyote]

Anything I can do to unstick it?

Tap on it? Spray it with some LN2? Touch it with a match? Etc...

If it's indeed the expansion valve, in my little experience I've needed to open the system and just replaced the darn thing. Stupid cheap part not worth chancing on just cleaning. You can rig a shadetree recovery bottle for the freon which you can then recharge from after repairs.

 

[amaurer]

You can rig a shadetree recovery bottle for the freon which you can then recharge from after repairs.

Definitely more info on this please? I do have a vacuum pump.

 

[LAMBCRUSHER]

yes please, more on this. Good tech

 

[Wile E Coyote]

Definitely more info on this please? I do have a vacuum pump.

Spendy R12 makes the project worthwhile, if it's been converted to R134, I wouldn't bother. You use your manifold hooked to a cleaned container with proper fittings brazed on then purged with the vacuum pump (think small fire extinguisher or propane tank which are both DOT approved vessels for such pressures) placed into a bath of dry ice and acetone. The temp drop (needs to be -21deg F or colder) will cause enough pressure difference to allow freon to flow into your recovery bottle. Place the whole bottle and bath onto a scale which is then zeroed before recovery to measure how much freon is actually recovered and then returned. There's no other more accurate way to measure the charge.
To recharge, reverse the pressure difference using a hot water bath for the bottle.
Definitely a PIA job unless you are a truly stubborn DIY kinda person.
If you have reason to believe the system is overly contaminated, then that's another reason to not bother with recharging 'uncleaned' freon.

 

[Coolstream]

Another option in a similar vein: stop by you favorite mechanic and see if you can get them to give you an empty R134a 30lb cylinder. If you are recovering R12, get an adapter fitting to go from the 1/2" Acme fitting on the jug to 1/4" male flare (Santech # MT0432) this is so your R12 gauges will connect. If you're using R134a, skip that step. Pull vacuum on the empty container. If you can fit the jug into a freezer, great. If not, follow Wile's instructions about the ice bath. Now when connected to the vehicle, the refrigerant will flow into the cylinder. Don't forget to open/close the valve on the cylinder as appropriate.

 

[Wile E Coyote]

Poke around over here for more learnin' : Automotive Air Conditioning Information Server: Home

 

[amaurer]

Ooooh, awesome, this will be fun. My system is R-12.

I can get liquid nitrogen from work, that should get the vessel plenty cold.

 

[amaurer]

Also - what about the oil when doing this DIY method? Do I need to be sure to invert the tank when warming it or anything so that the oil flows back in?

 

[Coolstream]

It will only pull out a minimal amount of oil. You might put an ounce back in the system when you button it up.

 

[amaurer]

And should I expect to lose a lot of gas doing this? Some surely will not leave the tank I use... so if I'm using like a 2 gallon tank is that a lot to lose?

 

[Wile E Coyote]

And should I expect to lose a lot of gas doing this? Some surely will not leave the tank I use... so if I'm using like a 2 gallon tank is that a lot to lose?

I'd only expect you do lose about 2 second burst of freon while purging air from the manifold's hoses when making/breaking connections before charging or recovering.

 

[amaurer]

Maybe I don't understand, then... won't the tank be full of low pressure gaseous R-12 at the end of the fun? Its not going to return to a vacuum on its own...?

 

[cruiserfj45]

I built my recovery tank with a colman propane cyl. I just used a ice bath, no dry ice and it pulled the system down fine. Either run the truck with the hood down to heat up all the lines under the hood (make sure all gauge lines are clear of moving and hot parts) or go over everything with a blow dryer. This will help boil all the liquid and then it can transfer to the tank. I have done this several times and am able to recover at least 90% (best guess).

 

[amaurer]

I built my recovery tank with a colman propane cyl.

What size?