Strange AC manifold gauge behavior

[ggff002]

Hi,
First time tried to use AC manifold gauge, to check the low and high pressure in my car (R134A).
Used Mastercool 89661.
With the valves on the manifold completely closed, I connected the low and high pressure hoses to the car. Car engine was off. Yellow hose not connected to the manifold at all.

Slowly opening the manual hose valves, first one low and then on the high side, the gauges moved from zero.
Started the car, set AC on max cool. Kept the RPMs around 2000.
The low pressure did not change at all. The high pressure increased a little bit.
Shut off the engine.
Closed the hose valves and disconnected the hoses from the car.
Disconnected the low pressure hose from the manifold. The low pressure arrow stayed where it was, without moving at all.
Opened the low pressure valve on the manifold - the low pressure dropped to zero.

Same situation on the high pressure gauge - with the hose disconnected, the arrow dropped to zero only after opening the high pressure manifold valve.

I expected that, with the manifold valves closed, the gauges should show the pressure in the hoses. What I saw is that while they did show pressure, they showed only maximum pressure.
Is this normal? Defective unit?

 

[65ranchero]

Why are you hooking up the gauges in the 1st place?
With the gauges on and opening the valve was there any hissing from the yellow port?
From what you describe it sounds like the system is empty ( as in large leak)
Just to read the pressure there is no need to open the valves,

 

[charbar]

No way the gauge will only show maximum pressure. With the valves on the hose ends open and valves on the manifold closed then the gauges will be reading exactly what the system pressure is at the service ports at any given time.

If you saw zero change in your gauge readings from engine off compared to engine on then you have an issue.

Here's a half a$$ed idea of what is going to happen......

With the engine off, hose valves open, and manifold valves closed you will see equal pressure on both gauges. I'll just throw out 85PSI as a rough number, although that number is going to depend on outside temperature, system charge, etc.

Engine running, hose valves open, manifold valves closed. You should see a drop in the low side and increase on the high side. Lets say 35psi on the low side and 225 psi on the high side. Again, Im just throwing out rough numbers here.

When you close the hose end valves the gauges will maintain whatever pressure it was at when you closed it. Sounds like you opened the manifold valves and released freon from the gauge set so obviously it will go to zero.
When unhooking gauges from the vehicle I like to keep vehicle running, close high side hose valve, open high side manifold valve WITH THE YELLOW HOSE ON AND NOTHING HOOKED TO IT, then slowly open the low side manifold valve. Once reading on low side gauge stabilizes then close low side hose valve. That way you don't have 250psi just sitting in your high side hose and gauge.

We really need some actual gauge readings here.
If pressure was basically the same on the high and low side while the engine was running (assuming you had the hose valves open and closed when you though you did....generally clockwise on your valve will open the valve to the cars system, counter clockwise will close the gauge set off from the car's system) then you most likely have a low freon charge causing the compressor to not even turn on, a very week compressor, or an expansion valve sticking wide open.

 

[American Locomotive]

Are there schrader cores installed in the hose connections on the manifold? That's really the only thing that could be causing that behavior.

 

[ggff002]

Thank you for all the responses!

I wanted to check pressure on low and high side, to see if it is within spec. The car AC works, but the temperature is somewhere around 55 degrees (measured in the center vent, after 10 minutes with the engine at ~2000 rpms).

If you look up that Mastercool kit, you will see that the hose ends that connect to the car have manual valves. These are the valved I opened when connecting to the car. The valves on the manifold body were completely off. No hissing whatsoever.
Lower pressure was lower than the higher.

The hose ends that connect to the manifold - I did not see schrader valves. Only rubber seals.

If the gauges should show the pressure in the hose (when the manifold valves are closed), both increasing and decreasing pressure - then this was not the case with the unit I used. I think the unit was probably defective. I am going to get a different one and try again.

 

[charbar]

If you have another set of gauges handy then go ahead and try them. Keep in mind that there are a lot of factors that depend on whether or not you will see your pressures moving up and down compared to maintaining a pretty steady reading.

55* seems plenty high at the center vent but also depends greatly on outside temperature and humidity levels, whether or not the recirc door is working, etc.

Try again with a new gauge set and report back what the actual pressures are......can usually get a pretty good idea of what is going on by pressures but proper freon charge should be one of the first things checked. It's not always easy to determine from pressures alone and something that is always overlooked by home-gamers and some shops because they don't have the ability to weigh the amount of freon that was in the system and/or charge with the proper amount.

 

[Zewnten]

Maybe I missed it but are you opening the valves on the manifold?

 

[ggff002]

55 degrees when outside was 90 or so and very high humidity.

With the hoses connected to the car - no, I did not open the valves on the manifold.

 

[Wrench97]

What year, make, model, engine car are you working on?
What are the actual pressure reading on both gauges, car off, then car on A/C on hi at idle.

 

[no704]

If you don’t open the blue/ red on the manifold how are the gauges working at all? Unless I drastically misunderstand how these work.

 

[Wrench97]

If you don’t open the blue/ red on the manifold how are the gauges working at all? Unless I drastically misunderstand how these work.

The gauges are open the the hoses, the valves connect the high or low side to the center port to either remove the refrigerant or when connected to a bottle install the refrigerant.
Opening both will connect the 2 sides together something you almost never want to do in automotive A/C work.

 

[no704]

The gauges are open the the hoses, the valves connect the high or low side to the center port to either remove the refrigerant or when connected to a bottle install the refrigerant.
Opening both will connect the 2 sides together something you almost never want to do in automotive A/C work.

Guess I was badly mistaken. Thanks for the lesson!

 

[ggff002]

I tried with different manifold gauge - this time it seemed to work properly - pressure was changing.

The car is Honda Pilot 2007. Original AC system. It was re-charged 6-7 years ago, in a shop.
At around 95 ambient temperature, low side was around 40 and high was below 250. I added, maybe half of 12 oz R134A can. High increased to around 300. But low did not change much. Maybe expansion valve issue.
Unfortunately, I will not have access to the car for the next several months, and will not be able to do anything about it.

 

[DGersic]

If I understood the OP correctly, he’s not opening the valves on the gauge manifold. He is opening the valves on the snap on connectors that go on the high & low ports.



The red and blue tops on the hose connector are valves that open the system to the hose.

 

[ggff002]

If I understood the OP correctly, he’s not opening the valves on the gauge manifold. He is opening the valves on the snap on connectors that go on the high & low ports.

Yes, this is correct.

 

[Wrench97]

I tried with different manifold gauge - this time it seemed to work properly - pressure was changing.

The car is Honda Pilot 2007. Original AC system. It was re-charged 6-7 years ago, in a shop.
At around 95 ambient temperature, low side was around 40 and high was below 250. I added, maybe half of 12 oz R134A can. High increased to around 300. But low did not change much. Maybe expansion valve issue.
Unfortunately, I will not have access to the car for the next several months, and will not be able to do anything about it.

The high side should be below 250 and the low in the high 30's -low 40's you now have it overcharged assuming both radiator fans were running on high.

 

[ggff002]

The high side should be below 250 and the low in the high 30's -low 40's you now have it overcharged assuming both radiator fans were running on high.

According to the R134A chart, at ~95 ambient temperature, high should be 275-300

Automotive Refrigerant Pressure Charts - A/C Pro®

Our R-134a and R-1234yf air conditioning system pressure charts are a helpful tool for interpreting pressure readings when recharging with A/C Pro®.

www.acprocold.com

 

[Wrench97]

According to the R134A chart, at ~95 ambient temperature, high should be 275-300

Automotive Refrigerant Pressure Charts - A/C Pro®

Our R-134a and R-1234yf air conditioning system pressure charts are a helpful tool for interpreting pressure readings when recharging with A/C Pro®.

www.acprocold.com

I do it everyday if I see anything over 250 there is a issue.