A/C not blowing cold, but pressures are fine

[Project Vixen]

Hi All,

Ran into a problem with The Vixen a few days ago... the A/C is actually blowing warmer than just the vent.

Checked out the pressures, and it all seems fine. Running R12, and have about 30 on the low side, 185 on the high @ about 85 degrees outside. The compressor kicks in and cycles fine. It allworked fine last year

I did replace the hot water / heater valve last year, but I think even if that was stuck open it wouldn't make a huge difference (IIRC, that valve only applied when A/C was on Max)

Am I forgetting about a blend door or something similar that may be stuck?

Been a while since I have dug in that far in the dash so my memory is foggy.

Thanks for any help y'all can provide!

-Dave

[Bitsyncmaster]

Normal pressures and normal cycle but no cooling....Don't know how that can happen. Does the low side pipe/hose feel cold when running the AC?

All I can guess is to evacuate and recharge.

[DMC5180]

Dave,

Have you verified the Hot / Cold control is moving properly Left/Right and that the cable is attached to the door mechanism.


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[Project Vixen]

Have you verified the Hot / Cold control is moving properly Left/Right and that the cable is attached to the door mechanism.

The temperature knob appears to be moving normally. I don't recall where to find the other end of the cable though. Where can it be checked? i.e. where is the other end?)

The other selector (mode switch) works fine and the blown air directs to the appropriate outputs as it should when the switch is moved.

I didn't check the hose temperatures, actually -- didn't think about that. I just checked the pressures.

[DMC5180]

Unfortunately it in a tough location under the housing. I would think you should be able to see it if you pull the carpet back and look under the housing in the vicinity of the drain connection.


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[Project Vixen]

Having a look at the parts diagrams, it looks like it attaches to the temperature "valve & trunnion", which by the shortness of the cable must be within a foot or so of the selector, so I'm guessing it is behind the HVAC controls, near the firewall, somewhere on top of the frame hump.

[Edit] Thanks, Dennis. I will see what I can find there.

[Jonathan]

For all of our cars, the A/C system cycles the compressor on and off based on load, correct? Meaning, if there is more heat to be removed (producing cold), it works harder. Outside temperature, RPM of the engine, whether the car is moving or stationary. They all factor in.

If your A/C system is indeed working normally, and those pressures are where they should be, that cold must be going somewhere, right? If there is no heat being removed and no cold being produced, you would see it in the system not working hard at all. If the system is working hard(-ish), then you should be able to feel around either the cabin or outside and upfront by the passenger wheel well and locate where your cold is going.

If it is a problem with one of those air directing flaps not working, and the cold is still being created, it has to be going somewhere and you can probably find out by feeling around for it.

[Project Vixen]

Found the end of the cable and the lever it's attached to, visible by pulling the carpet back a bit on the hump in the passenger footwell. The movement there looks just fine, so it must be something else... any suggestions?

[DMC5180]

Found the end of the cable and the lever it's attached to, visible by pulling the carpet back a bit on the hump in the passenger footwell. The movement there looks just fine, so it must be something else... any suggestions?

Are the fans cycling? Also put your hand on the accumulator while running A/C. It should be very cold to the touch. If it isn't, I'd say your just low on Refrigerant.

[Bitsyncmaster]

Your high side pressure is about normal so that suggests you have enough R12 but it does not make sense your not getting cold air.

Now if it was cycling at a short on time and long off time than that would suggest a plugged orifice.

[DMC5180]

I would expect a plugged orifice to cause abnormally High side pressure. Unless the system is just very low on refrigerant.


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[David T]

One way to check what is going on is to remove and measure the amount of refrigerant. For the pressures to look correct and no cooling several things would have to be bad at the same time. Verify you are moving enough air over both coils. It is possible to have the correct high side pressure with low refrigerant if the condenser coil is not dissipating heat because of insufficient airflow due to bad fans or dirt blocking airflow. On the low side the evap coil could be freezing over.

[DMC-Ron]

One scenario that would explain the AC working but no cooling is the heating is stuck on and working simultaneously. Combination of evaporator cooling plus heating coil heat gives you warm air. You should be able to eliminate the heat by putting the mode switch to MAX. But that assumes your heat valve works. Observe and verify that the heating valve is closing when MAX is selected. Even if it looks like it's working, you could clamp the heater hose near the valve to close it off for testing.

57726

If you find the heating coil is the culprit, you'll need to look into the heat flap to see why the cooling from your evaporator is going thru the heating coil. Normally this is regulated by your temperature control knob.
Ron

[Bitsyncmaster]

I would expect a plugged orifice to cause abnormally High side pressure. Unless the system is just very low on refrigerant.


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I would expect that also but since the compressor shuts off at 30 PSI on the low side that may not be the case.

[Project Vixen]

At present, I'm still leaning more towards some type of physical blockage in the airbox. First, I would believe it would be highly unusual for the compressor pressures, cycling, etc. to be perfectly in spec if there were problems in that part of the system. Second, my wife said she thought there might be a mouse nest in the box as she saw little puffs of lint keep flying out of the round air portals on the sides of the dash for the door air vents.

Some things I'm thinking I should try:

test the a/c with the car cold. Then there's no interference from the heater core
feel the accumulator/drier to see if it's getting cold.
drop the blower fan and look for signs of blockage
drop the bottom of the evaporator's housing
check the hot water valve, just because it was the last thing replaced.


Thanks to all for the continued suggestions and help!

[Ron]

At present, I'm still leaning more towards some type of physical blockage in the airbox. First, I would believe it would be highly unusual for the compressor pressures, cycling, etc. to be perfectly in spec if there were problems in that part of the system. Second, my wife said she thought there might be a mouse nest in the box as she saw little puffs of lint keep flying out of the round air portals on the sides of the dash for the door air vents.

Some things I'm thinking I should try:

test the a/c with the car cold. Then there's no interference from the heater core
feel the accumulator/drier to see if it's getting cold.
drop the blower fan and look for signs of blockage
drop the bottom of the evaporator's housing
check the hot water valve, just because it was the last thing replaced.!

Sounds like a diverter/duct problem to me too (if accumulator cold).

====

fwiw, a partially restricted orifice would make the high side go high and the low side go low. Blocked, the high side would go extremely high and the low side go negative (vacuum). Bypass LP Switch to test.
If those pressure readings were taken when the clutch cycled out, they are near perfect for that ambient temp.

[Project Vixen]

Ok, finally got a chance to look at a few things. I dropped the fan so I could feel around for any obstructions/mouse nests, but it was all very clean. The accumulator and evaporator both get cool -- but not cold -- when the a/c is on.

Some other things to note:

tested from a cold start so there was no interference from the heater.
the compressor's 'on' cycle is only 2-3 seconds, with around 60 seconds off
the cooling fans don't come on when I turn the a/c to max. Aren't they supposed to stay on? I am running a Fanzilla so there is a delay before they kick on. I jumpered the fan at the otterstat and they ran fine.


Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts? Thanks again, all, for the assistance!

[hippieman9]

Ok, finally got a chance to look at a few things. I dropped the fan so I could feel around for any obstructions/mouse nests, but it was all very clean. The accumulator and evaporator both get cool -- but not cold -- when the a/c is on.

Some other things to note:

tested from a cold start so there was no interference from the heater.
the compressor's 'on' cycle is only 2-3 seconds, with around 60 seconds off
the cooling fans don't come on when I turn the a/c to max. Aren't they supposed to stay on? I am running a Fanzilla so there is a delay before they kick on. I jumpered the fan at the otterstat and they ran fine.


Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts? Thanks again, all, for the assistance!

I believe the Fans should kick on on max. Set the switch to Max, and turn the ignition switch to run, without starting the car and see if the fans kick on. Also check and make sure the recirc door is opening on max and the Hot water valve is closed. I recently learned that they only operate on the Max setting.

[DMC5180]

Ok, finally got a chance to look at a few things. I dropped the fan so I could feel around for any obstructions/mouse nests, but it was all very clean. The accumulator and evaporator both get cool -- but not cold -- when the a/c is on.

Some other things to note:

tested from a cold start so there was no interference from the heater.
the compressor's 'on' cycle is only 2-3 seconds, with around 60 seconds off
the cooling fans don't come on when I turn the a/c to max. Aren't they supposed to stay on? I am running a Fanzilla so there is a delay before they kick on. I jumpered the fan at the otterstat and they ran fine.


Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts? Thanks again, all, for the assistance!

That cycle time seems way off. Usually the clutch would stay ON almost continuously at idle in hot conditions.
Can you post a video of your manifold gauges hooked up and A/C running?

[DMC-81]

Ok, finally got a chance to look at a few things. I dropped the fan so I could feel around for any obstructions/mouse nests, but it was all very clean. The accumulator and evaporator both get cool -- but not cold -- when the a/c is on.

Some other things to note:

tested from a cold start so there was no interference from the heater.
the compressor's 'on' cycle is only 2-3 seconds, with around 60 seconds off
the cooling fans don't come on when I turn the a/c to max. Aren't they supposed to stay on? I am running a Fanzilla so there is a delay before they kick on. I jumpered the fan at the otterstat and they ran fine.


Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts? Thanks again, all, for the assistance!

Hmm. I agree that a 2-3 second on cycle time and minimal cooling typically points to a low refrigerant condition. With a full charge of R-12, my compressor seems to stay on all the time in these summer temperatures. Ignoring any impact of the Fanzilla unit, yes, as mentioned, as soon as you click the Max mode on, the fans come on immediately and stay on.

I agree to check the pressure switches. I originally had similar problems with minimal cooling and after lots of troubleshooting my problem turned out to be an oil logged evaporator. Apparently this problem has degrees of severity, and mine was completely blocked so I had no pressure building.

Off topic, I just want to thank you for the great information on rebuilding the door lock solenoids. I was able to do the same thanks to your information. :thankyou:

[Ron]

Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts? Sorry, I was suggesting bypassing the LP switch to check out the orifice tube for blockage, RE Dave and Dennis' discussion....The pressures you reported point elsewhere, imo.

You didn't say if the accumulator is getting cold now?

I agree that the cycle times are odd. And posting a vid, including where the pressures are when the clutch cycles OFF, would tell a lot.

The fans should also be on anytime the compressor clutch is engaged.

[Project Vixen]

Off topic, I just want to thank you for the great information on rebuilding the door lock solenoids. I was able to do the same thanks to your information. :thankyou:

Thanks! Wow, that was a long time ago. I just ran across that spool of wire a few days ago. Still have like 5 pounds of it...

[Project Vixen]

I'll try to get a video. Not sure I have enough hands to hold everything, but I'll see what I can do. I might be able to hang the manifold on a ladder.

The pressures I mentioned (30L/185H@85F) were the pressures right when the compressor turned off.

[Project Vixen]

Just in case anyone needs a low pressure switch...

The OEM part number for the switch is:
Part Number:100744, LOW PRESSURE SWITCH

From Houston @ $9.99 it's a good price.

Other -possible- cross referenced part numbers I could find were:
SANTECH STE MT0207
UAC SW 1122C
Global 1711250
AC DELCO 15-5861 and 15-2151
Four Seasons 35751
NAPA TEM 207434

[eight8toy]

I just finished my AC overhaul last week and while I was adding the first can of r134 the compressor cycled like every 2-3 seconds like you're describing. After that and adding the second can it was continuous. I would guess you are low on refrigerant.

[DMC-Ron]

The fans should also be on anytime the compressor clutch is engaged.

A Ron notes, even with the mode selected to max, the fans will only be on when the compressor clutch is engaged (unless you have Dave's fan fail relay which has a delay off feature) . If you are low on refrigerant, the compressor will quickly pull the suction below the low pressure switch setting, turning off both fans and compressor clutch. When the refrigerant is very low, it will take a while for the pressure to build back up, reset the pressure switch, and turn the fans (and compressor) back on. The lower the charge, the longer fans and compressor will be off, to the point where the charge is so low (or completely discharged) and the compressor and fans will not start at all.

I think eight8toy's guess is right.

Ron

[Bitsyncmaster]

Sounds like your orifice is plugged. That would cause the long off cycle short on cycle.

Also your fans won't come on if the low pressure switch keeps the compressor off. The plugged orifice is also causing that problem.

[DMC-81]

Sounds like your orifice is plugged. That would cause the long off cycle short on cycle.

Also your fans won't come on if the low pressure switch keeps the compressor off. The plugged orifice is also causing that problem.

Yes, and is a high pressure of 185 completely normal, or perhaps another symptom of a blockage? I recorded 150 ish at 81 degrees ambient. The instructions I followed said that was normal for a full charge.

http://dmctalk.org/attachment.php?attachmentid=57746&d=1533922815


Thanks! Wow, that was a long time ago. I just ran across that spool of wire a few days ago. Still have like 5 pounds of it...

Cool. PM me if you want to sell some of it. I still have a spare set of solenoids that I need to rewind.

[Project Vixen]

I'll bypass the 'jump the low pressure switch' step and just replace it. (According to http://projectvixen.com/week33.htm I purchased the Four Seasons 35751 switch at Pep Boys on 06 June 2000). I'm leaning to believe the low pressure switch is the problem. Here's my reasoning:

When the A/C kicks on, it runs to exactly the same pressure each time and shuts off (30L/185H). Now what in the system would cause that to happen?

A blocked orifice tube would cause the pressure to rise -- but that blocked orifice has no way to shut off the electrical power to the compressor clutch. Pressures should begin to rise and keep rising until either the low pressure switch kicks in (due to the backup of refrigerant on the high side with little being returned on the low side, or until the high pressure switch turns it off (or the pressure release valve blows).

From the gauges I can see that the pressures at rest and when the compressor is running are just fine. That also helps rule out a blocked orifice tube, as the pressures should be too high on the high side and/or too low on the low side. It also rules out low refrigerant, as the rest and running pressures could not be correct with low refrigerant levels (verified by manifold gauge set).

The clutch is cycling short. The only things that can cause that electric clutch to cut in and out are electrical. There isn't a great deal in that circuit... there is no relay for the compressor clutch (although IMHO there should be one). If it was the mode switch, it would be unlikely that it would simultaneously fail on both normal and max a/c, and fail in such a way that the compressor would run for 2-3 seconds and then cut out, very predictably and repeatedly. The clutch itself could also be overheating and cutting out, though it's not behaving that way. If that were the case, since it's purely a coil with no electronics inside it would be unlikely that it would 'click' off so quickly. One would expect the resistance to rise and the clutch start to slip before it disengaged... and if it had that much resistance it's highly likely it would blow the fuse.

The only thing left that I can see are the high and low pressure switches. The low pressure switch is easily visible behind the right wheel attached to the accumulator/drier. The high pressure switch is on the '4-way' diamond-shaped hub on the high pressure line nearby. But how can we tell if one of these is at fault? Jumpering each would be the hands-on way, but I'm also looking at it as a classroom exercise. I'll focus on the low pressure switch as from research here it seems the low pressure switch on a D is much more likely to fail than the high pressure switch, but the logic for testing is the same for each switch.

The low pressure switch is designed to shut off the compressor clutch when the system low side drops below ~25 PSI. We know the system is not low on refrigerant (or at least not very low) by verification with the manifold gauge set (both running and at rest).

So what ways can the low pressure switch fail? I can think of three ways:

Lax failure -- Failure to cut power to clutch under the low pressure trigger point (low system refrigerant level)
Complete/open failure -- the switch is 'dead' and never allows power to the clutch.
Eager failure -- the switch cuts off power to the clutch at a trigger point much higher than desired.


(BTW> If it's the high pressure switch at fault, the troubleshooting is similar but the descriptions of #1 and #3 above would be reversed.)

I'm thinking the low pressure switch is in failure mode #3. This switch is designed to cut out at around 25 PSI. We know the clutch is clicking off at the same point repeatedly & predictably -- 30 PSI. If the low pressure switch 'believes' that 30 PSI is its desired cutoff point, it will cut power to the clutch until the low side pressure again reaches a high enough pressure to allow the switch to click the compressor back on.

So that's where my mind's at now.

The principle of 'Occam's Razor' states that the simplest solution to a problem is usually the correct one. NAPA is holding a #207434 for me, so I will grab it after work and hopefully get a chance to try it out tonight. I will keep y'all posted on the results.

[DMC5180]

Here is is some classroom info [emoji6]

Low pressure switch (aka duty cycle switch) cuts out (OFF) at 30 Psi (R12) setting. It cuts in(ON) at 45 psi approximately. This is the only switch cycling the clutch under normal conditions.

location: Low side of System (after orifice tube and evaporator outlet.

High pressure cut-off switch (aka high pressure (SAFETY) switch. This switch is along for the ride and rarely would activate. It has a pressure setting in the 350 psi or higher range and is only there in the event of a blocked Orifice tube and the low pressure switch fails too cut off power to the compressor clutch. It is purely a safety protection switch and had nothing to do with normal clutch duty cycling. Location: High Side just before Orifice tube.

Your long Off period between short ON period. The refrigerant is slow to pass thru the Orifice tube. It is the expanding liquid to gas that raises the low side pressure to the 45 psi too cycle the low pressure switch ON again. The lack of refrigerant on the low side of the Orifice causes rapid pressure reduction to 30 psi (clutch off)



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[Ron]

As noted before, your pressures at cut-off are near perfect. Your OP said the cycling seemed fine, later it was "perfect". If you didn't change anything and it changed to long off - short on, you should find out why first. (Forget about air flow/duct work/etc for now.) If it has been long off - short on all along, I would suspect the orifice (restricted, not blocked) and since the high side is not going high and the cut-out pressure is correct, I would suspect the charge is low as well.
(If the cycling is OK when dead cold (has set overnight) then goes long off - short on, there is probably moisture in the system, freezing in the orifice...)

If the charge is low, the low side will not be replenished as fast as it needs to be. The pressure will drop causing the switch to turn the compressor off. It would cycle more than normal, short - short/medium. Add a blockage, short-long, high high side, negative low side (if you bypass the switch to force below 30 for a test). Add a restriction instead, short-long, high side pressure would depend on how low the charge was, same for low side. It may not go low/negative before the switch turns off, as your does not...

I agree it is time to look at the orifice...meanwhile recharging will eliminate a lot of guesswork...


You didn't say if the accumulator is getting cold now?


EDIT:
WSManual N:09:01
57895

[Bitsyncmaster]

As others have stated the blocked orifice would make the low pressure side pull down very quickly and take a long time to come back when the compressor switches off.

When I tried the VOV orifice my tubing had a bend in it and it just about shut the thing off. Result was compressor ran for a few seconds and took more than 60 seconds to come back on.

[Project Vixen]

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?

[Bitsyncmaster]

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?

The low pressure setting is to prevent freezing the evaporator. Since your not cooling the evaporator much, just jumper the connector and test the result.

When I top off a system, I set the fan on speed 4 and the selector to NORM AC. Stick a temperature in the center vent and read the temp after running a few minuets. Then add a little refrigerant and see what the temperature does. Also monitor the high pressure side. Usually when your get to much refrigerant in the temperature will stop dropping and even start to rise. The high pressure side will keep going up as your add more refrigerant.

[Ron]

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. :deviltail:}

[Project Vixen]

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

Dunno. It's what all the cool kids with Youtube accounts are doing these days... :hysterical:

I'll try adding some R12 and see what happens.

[DMC-81]

~snip~

EDIT:
WSManual N:09:01
57895

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


Yes, and is a high pressure of 185 completely normal, or perhaps another symptom of a blockage? I recorded 150 ish at 81 degrees ambient. The instructions I followed said that was normal for a full charge.

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?


I'll try adding some R12 and see what happens. I think that's a sound plan.

[DMC5180]

Dunno. It's what all the cool kids with Youtube accounts are doing these days... :hysterical:

I'll try adding some R12 and see what happens.

All the Cool kids are converting to 134a, which does require a lower pressure adjustment down to 25-27 typically.


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[David T]

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

[Ron]

Dunno. It's what all the cool kids with Youtube accounts are doing these days... :hysterical:
:biggrin:


I'll try adding some R12 and see what happens.
Beer says it goes short-on, ~long-off (Low side-30, high side-something higher than 185) @ 85F. Or, you get a big grin on your face...

[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...:biggrin:

[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?1244-Refrigerant-Temp-Pressure-Chart&p=13935&viewfull=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. :deviltail:}

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!