A/C Manifold Gauge Readings (1 Viewer)

[uscmas412]

Background info, A/C blows cool. In the morning on a cloud covered day, it will eventually cool the cabin. On a heat soaked, 95F afternoon in stop and go traffic, it will never keep up.

Yesterday I removed the blower motor and visually inspected the evaporator and flushed it with some degreaser and the water hose; it was surprising clean with very little debris.

Changing the temperature lever definitely adjusts the temperature from cool to hot and the water control valve seems to be fully closed.

Looking for someone with experience to tell me what I'm reading on the A/C manifold gauges.

Ambient temp 90F with 60% humidity.

Static pressures with the engine off were similar on the low and high side:

I followed the FSM steps shown below and converted the normal readings via Google:

The last picture is after I let the A/C run for 5 minutes on max, recirculate on, with RPMs at 1500.

LOW = 20 PSI, HIGH = 235 PSI.

When I look up these numbers on other A/C gauge reading sites, it seems they're both low, but according to the FSM I'm just barely out of spec in each direction. I think with the 60% humidity I should be seeing higher numbers from both, indicated a low charge? Any other thoughts?

 

[flintknapper]

Your 'static' pressures look fine assuming the temp *under hood, not ambient* was about 90-92°F. That they are roughly equal tells us the system was able to successfully equalize after being shut down. Why is that important? Because it suggests there are no significant restrictions between the high and low sides/components. But we needn't put too much stock in 'static' readings beyond that.

Static pressure readings really only tell us two things.
1. The system will or will not equalize.
2. There is enough pressure present to satisfy the low pressure switch and allow the compressor clutch to engage or there is not.

It will NOT tell us how much 'liquid' refrigerant is in the system, only that 'some' is there. The pressure will be the same if you have 3 ozs *liquid* or 20 ozs. OK to take static readings before proceeding, just recognize the limitations.

Your dynamic pressures are low. Not wildly so...but low. I prefer not to run the engine at 1500 rpm to check pressures. Its good to run the engine a few minutes above idle to let the system settle, then just let it run at normal idle. With the compressor running a normal idle will increase automatically to about 900-1000 rpm and that's good enough. It also replicates what your engine will be doing *in traffic at idle*.

I would first try adding about 1/2 can of refrigerant and watch to see what happens with your pressures. I would expect to see 30-35psi on the low side and about 220-240psi on the high side with something less than a full can added. BUT....you will need to monitor your vent temps as you go. If you don't have a vent temp gauge, get one and use it. Otherwise you'll never be able to determine the point of diminishing returns. *adding more refrigerant no longer reduces vent temps*.

IF adding refrigerant to your system and staying within the windows *for ambient temp* doesn't produce cooler air at the vents or pressures get too high, then we need to look at air flow across/through your condenser or consider the possibility of air or contaminants in the system.

Remember, using gauges on a system does NOT tell us how much liquid refrigerant is in there. Just the pressure windows that we can compare to pressure/temperature charts. It's a best guess and assumes a lot with respect to the health of the rest of the system. ONLY by weighing in a charge can we be sure of the correct starting point. But that doesn't mean can't get you cooler air in most cases.

 

[uscmas412]

Thanks @flintknapper

I'll try adding coolant after I get a dash vent.

If that doesn't work, I think a proper vent, vacuum pull and recharge is in order. FSM shows 29.88 +/- 1.76 oz

 

[flintknapper]

Thanks @flintknapper

I'll try adding coolant after I get a dash vent.

If that doesn't work, I think a proper vent, vacuum pull and recharge is in order. FSM shows 29.88 +/- 1.76 oz

Yes Sir, exactly.

 

[uscmas412]

Gauge shows 48 degrees, this is after driving around for about 5 minutes. However, it was cloudy and somewhat cool this morning.

I added half a can of refrigerant without any improvement.

I made an appointment tomorrow morning to evacuate and recharge the system.

 

[Kernal]

Curious how one determines how much oil to put back into a system once it's been evacuated?? Doesn't the oil, or some of it, stay behind in the system? IDK

 

[flintknapper]

Curious how one determines how much oil to put back into a system once it's been evacuated?? Doesn't the oil, or some of it, stay behind in the system? IDK

Yes, its a crap shoot because if the owner (present) hasn't ever had the system completely evacuated or replaced ALL of the component parts then you don't know how it has been serviced in the past.

'Some' technicians are bad about summarily adding oil with any 'top off' of the system. Having a 'little' too much oil won't affect the cooling capability appreciably....but a LOT too much will.

For a U.S. market 80 series *no rear air* you'll want about 7.5-8.0 ozs of PAG46 for the entire system (if starting dry).

IF the system has not been flushed....then it is to be assumed that the components (compressor, condenser, drier and evaporator) have all held 'some' amount of oil. Roughly 4 ozs in the compressor, about 2 in the condenser, an ounce or so in both the drier and evaporator. The FSM specifies the exact amounts to be added to each component when/if replaced...but those are rough figures.

So, yes....depending on what was done and how it was done....it's just a guess as to how much lubricating oil is in the system.

 

[Kernal]

So if just evacuating and recharging the AC system say due to a slow leak, and no major components have been replaced, then no (or very little) extra oil would be needed, correct??

 

[Gascan]

@uscmas412 I see you’re a fellow Floridian... if you were closer to the Tampa Bay Area, I could vac down your system and then we could recharge it (I have a vacuum pump and manifold gauge). Truth be told I’m suffering the same issue this year. I’ve replaced the TXV (expansion valve) and my Denso compressor & filter drier are only a few years old. I suspect it’s the lack of airflow, what with the old condenser and a high mileage fan clutch but have not put the time in to properly diagnose.

Truth be told, I think the best way to tune your A/C isn’t with by measuring the vent temp. but by checking your superheat & subcooling; those numbers wont lie.

Respectfully, I disagree about the significance of compressor oil measuring when replacing components. In my personal/professional experience with small/medium size heat-pumps, measuring the oil amount (after replacing the compressor) shows no significant difference in a very precise lab environment.

 

[flintknapper]

So if just evacuating and recharging the AC system say due to a slow leak, and no major components have been replaced, then no (or very little) extra oil would be needed, correct??

Exactly right. The refrigerant will carry a very small amount of oil out with it....but not much.

 

[flintknapper]

@uscmas412

Truth be told, I think the best way to tune your A/C isn’t with by measuring the vent temp. but by checking your superheat & subcooling; those numbers wont lie.

Yes, works well for residential applications where the tech is equipped to take such measurements. But few automotive techs/shops have the equipment or knowledgeable techs. So vent temps (what we are after anyway) gets the final say.

Respectfully, I disagree about the significance of compressor oil measuring when replacing components. In my personal/professional experience with small/medium size heat-pumps, measuring the oil amount (after replacing the compressor) shows no significant difference in a very precise lab environment.

Regardless, the point is to have enough lubricating oil to not cause premature wear on the compressor, nor so much that it coats the inner linings of the condenser and evaporator (reducing heat transfer).

 

[uscmas412]

Okay, back measuring gauges again. When my engine was rebuilt I had the shop put on a new Denso compressor. AC vent temps have been in the low 40s until the past week, when ambient temps have been 93 with a crap ton of humidity. 30 minute drive yesterday and vent temps never dropped below 60 degrees.

I think I’m reading high now? Definitely according to the FSM of 22-36 low side and 198-228 high side. Also have bubbles in sight glass, which FSM says is a low charge indicator.

86F 77% humidity, engine at op temp

 

[flintknapper]

Okay, back measuring gauges again. When my engine was rebuilt I had the shop put on a new Denso compressor. AC vent temps have been in the low 40s until the past week, when ambient temps have been 93 with a crap ton of humidity. 30 minute drive yesterday and vent temps never dropped below 60 degrees.

I think I’m reading high now? Definitely according to the FSM of 22-36 low side and 198-228 high side. Also have bubbles in sight glass, which FSM says is a low charge indicator.

86F 77% humidity, engine at op temp


View attachment 3656270

Shop did the compressor installation and recharge? Was the compressor the ONLY thing replaced?

Will you be doing the work to troubleshoot this?

 

[uscmas412]

Shop did the compressor installation and recharge? Was the compressor the ONLY thing replaced?

Will you be doing the work to troubleshoot this?

Yes, shop installed compressor and completed recharge. Invoice only states qty 3 for refrigerant. No other parts noted.

I’d prefer to DIY this so I can be sure it’s done correctly and know how to fix in the future.

 

[uscmas412]

I’ll also add as a data point, AC performs well when not heat soaked and not in direct sunlight.

77 degrees OAT and overcast, she just put out 44 degrees at the vents.

Is -30 degrees ambient the limitation of a healthy system?

 

[flintknapper]

It might be helpful to know the reason for the compressor replacement? Hopefully the original didn't seize.

But lets move forward with a couple of things we can try that can give us some direction.

IF your fan clutch is old or otherwise not working well.....you may have an airflow problem. After starting the vehicle and letting get up to operating temperature (A/C on high the whole time) take note of vent temp, you could also take it for a short drive. With engine still running (at idle) use a garden to 'mist' the condenser and radiator and look to see if vent temps drop appreciably after a few minutes.

IF vent temps drop to a good level 45-ish (outside temp above 85°F) then lets take a look at your fan clutch to see if it is moving enough air. We will assume your condenser and radiator fins are both relatively clean.

IF misting the condenser does not result in a significant drop in vent temps, then hook your gauges up and let's add about a 1/2 lb of R134a and watch to see if pressures rise or fall.

Ignore the Sight Glass in the high side line and any bubbles present (or not). R134a is bad about foaming at times and frankly if you are LOW or HIGH it will produce bubbles. The sight glass is there for when you are way out in BFE with no manifold gauges to check the system.

Let us know what you find out.

 

[mudgudgeon]

Helpful thread!

Hopefully the original didn't seize.

What does this change?

I think my compressor may have siezed. Thought it was just the front bearing, but I relaced that a few years ago.

I expect I'll be fitting a new compressor and drier minimum

 

[uscmas412]

It might be helpful to know the reason for the compressor replacement? Hopefully the original didn't seize.

It was just preventative maintenance, worked fine when removed but was an aftermarket piece that was a decade old.

I’ve got one of the Landtank blue fan clutches installed, roughly 5 years and 15k miles on it.

I’ll try out the steps you listed, thanks.

 

[flintknapper]

What does this change?

I think my compressor may have siezed.

I expect I'll be fitting a new compressor and drier minimum

^^^
If 'internally' seized...can mean it sent small metal particles through parts of the system. Some can be flushed (hoses and lines) others would do better to be replaced (condenser and TXV).

 

[flintknapper]

It was just preventative maintenance, worked fine when removed but was an aftermarket piece that was a decade old.

I’ve got one of the Landtank blue fan clutches installed, roughly 5 years and 15k miles on it.

I’ll try out the steps you listed, thanks.

Good to know. Makes me wonder if the shop that replaced the compressor left the full 8 ozs of oil it comes with IN the compressor. If so....then you have too much oil in your A/C system and it won't/can't perform to capacity.

When replacing ONLY the compressor....then the oil from the old compressor should be drained out and measured (will be about 3-4 ozs max). The new compressor drained....and the proper amount added back in (not to exceed 4 ozs). The other 4 ozs (system TOTAL is 8 ozs) is already in the other components (drier, condenser, evaporator).

I would still 'mist' the condenser and see what happens there. We are trying to determine if you have an air flow problem or a system charge issue.