A/C not blowing cold, but pressures are fine

[Ron]

Ron, thanks for posting that chart. I remember seeing that somewhere but couldn't find it when needed.



Also, it answers this question. Both Dave's and my pressures are in range at the slightly different ambient temperatures. Although would high pressure range at 85 ambient be 167.5 - 195 rather than 167.5 - 185?

@90F: (170 + 200) / 2 = 185

Target = 176.25

[DMC-81]

@90F: (170 + 200) / 2 = 185

Target = 176.25

Cool. I see. My thought was you would take the range of values between 80 and 90 ambient to derive 85:

Low range: (145+170) / 2 = 157.5
High range: (190+200) / 2 = 195

@85 157.5 - 195

We still get 176.25 target though: (157.5 + 195) /2

I guess 195 looks right eyeballing down the increasing pressures, but maybe it's not a linear relationship?

[Ron]

Before you go messing with the pressure switch you should make sure you have the correct amount of refrigerant in the system. You can't do that by just monitoring pressures, not accurately anyway.

I can agree, when it comes to an old/questionable system, that checking temps (in/out of condenser, evaporator,...) should be next. But if it is fresh (ie everything clean inside AND out, air flow tight, etc.), it should be spot on if it matches across the board (including duct temp), using R-12.

Rare to have to touch the switch unless, as Ron mentioned, you change the type of refrigerant.

Or you don't want to play with hot water.

...or want to by-pass it to help eliminate a restricted orifice tube...

[Ron]

Cool. I see. My thought was you would take the range of values between 80 and 90 ambient to derive 85:

Low range: (145+170) / 2 = 157.5
High range: (190+200) / 2 = 195

@85 157.5 - 195

We still get 176.25 target though: (157.5 + 195) /2

I guess 195 looks right eyeballing down the increasing pressures, but maybe it's not a linear relationship?

Hehe..that was my quick guess too.

RE Linear: Check this chart out from the Resources Section:
http://dmctalk.org/showthread.php?12...ll=1#post13935

R12 @ 70F is ~70psi; less than 70F, pressure value is less than ambient temp value; above 70F, pressure value is above the ambient value.
The further from 70F, the greater the difference.

You might also find it interesting/helpful that:
134a @ 70F is also ~70psi.... And the further from 70F, an even greater difference....

Looking at better gauges, you will see these values are shown, and aligned.
Great rule of thumb tho, imho.

[kings1527]

Ok. That all makes sense. I found you could actually adjust the cutoff pressures on the switch from this video -- https://youtu.be/lt1oarL0soE. Should I be adjusting it down to 20? It'll be easy enough to put some more R12 in as i have the big tanks. Which is a better next move?

Why would you want to adjust it to 20 when it is at 30, exactly where the manual says it should be???

{Pass the popcorn, Rob. }

You'd adjust it down to 20 depending on which manual you're reading. The factory manual gives 25-32 on the low side for most ambient temps, but we all know how beautifully written the factory manual is. Go ahead and rebuild the transmission strictly by the manual and let me know how it goes. On the other hand, looking at Rob Grady's technical manual, the low side shows 20-28 for most ambient temps.

Take it all the way down to 20 if you want. You're not going to hurt anything. It'll definitely be colder. But is the LP switch adjustment likely the root cause of your problem here? Probably not.

[David T]

Running with the low side pressure too low can result in the evap coil freezing over. The cycling that is supposed to occur helps to prevent that but under the right conditions you can freeze the evap coil. Probably why the heater coil is open except in MAX also. If you notice the vents are not moving enough air but it is very cold you probably froze the coil. Shut the compressor off by going to VENT for a couple of minutes to melt the ice. If the evap coil freezes your discharge pressures will skyrocket, and you will slug liquid into the compressor. You will pop the high pressure switch and maybe the relief valve. With R-12 you should not go below 30 psi for any extended period and you should be cycling quickly. This is why a system low on refrigerant seems to work better just before it gets too low to run because the low pressure safety stops it.

[kings1527]

Running with the low side pressure too low can result in the evap coil freezing over. The cycling that is supposed to occur helps to prevent that but under the right conditions you can freeze the evap coil. Probably why the heater coil is open except in MAX also. If you notice the vents are not moving enough air but it is very cold you probably froze the coil. Shut the compressor off by going to VENT for a couple of minutes to melt the ice. If the evap coil freezes your discharge pressures will skyrocket, and you will slug liquid into the compressor. You will pop the high pressure switch and maybe the relief valve. With R-12 you should not go below 30 psi for any extended period and you should be cycling quickly. This is why a system low on refrigerant seems to work better just before it gets too low to run because the low pressure safety stops it.

You're not going to freeze the evaporator running it less than 30, or even right at 20. Trust me on that. And that being said, I wouldn't go less than 20.

[Project Vixen]

So I got some time tonight and added some more R12.

Temperature was about 75 degrees outside. Compressor started to cycle at about 5 seconds on, 10 seconds off pretty steadily. Pressures were 25L/170H. Not sure why the cutoff went from 30 down to 25 without adjusting the switch.

It's definitely running cooler, but not yet super cold. My obviously broken vent thermometer said the air was 75 and didn't change even when I turned on the heat, but it definitely felt cool. Probably good enough for now, but I'll let y'all know what happens after driving for a bit.

At some point I'll pull the orifice tube and put in a new one, but I should really install the new steering bushing first. Many thanks to all for the help!