AC issue

[Bushwick]

That's kinda pricey for Harbor Freight. Is it advertised as doing anything else? Does it come with the correct fitting for high side port?

 

 

AC took forever to get cold today, and even then it was luke cold. If anymore charge goes in, it'll shut off. Gotta figure out where that high side pressure is at. Have a feeling it's the reason for the shutting off. Suppose a HF gauge will last longer than the car, and can be used for future stuff, so will grab one if a rental can't be found

 

Any idea what the part number for the HF gauge is? Don't feel confident they'd know what it even was off hand.

[Bushwick]

Alright, Auto Zone DOES indeed rent a manifold gauge set. It's a $130 deposit.

 

 

I had a hell of a time getting the gauges to even work. Despite being quality gauges, apparently if you over-tightened the brass hoses to the gauges, gauges wouldn't read anything. Once gauges were actually reading, they didn't seem to fluctuate much if at all. Dunno if that's normal for the high side or not?

 

Anyways, with the gauge knobs turned all the way right, low was around 32 psi and high was around 175 psi. Blipping the throttle for a few seconds saw low side gradually dip, but high seemed static. I ended up adding one more smaller can which brought those values. Surprisingly, adding the can at intervals, the compressor stayed on. Had to keep unhooking the low gauge to connect my feeder gauge with can. Could probably add a little more, but will wait. Only difference with today vs. before was it was hotter out today, but not by much. Probably upper 70's.

 

Had car idling for around an hour later on in the day and compressor stayed on fairly constant. While driving, seems fairly constant AC cold, but could tell a few times where it seemed to get a tad warmer then back to cold.

 

Anyways, not sure what to make of it. Gauges really weren't that helpful. I noticed if the gauge knobs weren't all the right, especially the low side, like if the low side was turned a bit to the left (all the way left was supposedly off) the gauge would go crazy high. Turning it all the way left gauge would peg out at max, turning it a little bit, gauge would increment. High side didn't seem to matter. Was trying to follow the written directions on the carrying case, but they were vague. They just said to turn the knobs all the way clock-wise. I had the middle connector feeding back into itself in a loop. This seemed to allow the sight glass the gauge to fill up if I backed off both high and low knobs then went back all the right. Didn't see a way to connect a fresh can up to it, as wasn't sure if it'd leak out through the one end, hence I "U'd" it back into itself. Maybe this messed the readings up? Either way, once I backed the hose fittings off just a tad, both gauges were seemingly working. The low side was reading 30 psi before the small can was added, then 32 after.

 

Anyone familiar with manifold gauges that can make heads or tails of this information? I was expecting straight-forward gauge readings that'd react instantly with r134 being added, but it didn't seem to be the case. Was also surprised the high-side reading was rather low. 

[Fairtax4me]

High side was low because the system was undercharged.

 

The valves should always be closed when checking pressure. The only reason to open the valves is to add refrigerant through the yellow hose. Which you need a can tap to attach the can to the hose.

 

When the hose fittings are connected to the service ports the guage shows the pressure at the port. The valve opens the port between the charge hose (yellow) and the blue or red hose depending on which side you open. The only time the red valve is open is during evacuation and vacuuming the system. The blue valve is open during evacuation, and is used for adding refrigerant, because you can't add refrigerant to the high pressure (red) side of an operating system.

The valves do not block pressure from reaching the guages.

By opening both valves with the system running, you allow high side pressure to pass through the manifold and back to the low side of the system. This equalizes the pressure of the system, because the refrigerant is just making a loop from the compressor, through the manifold and hoses, and right back to the compressor. This is bad, because as it enters the compressor the refrigerant is supposed to be in its gas state (vapor). As it flows through the condenser it condenses into a liquid, and then is turned back into vapor in the evaporator core. If the service valves on the manifold are both open with the compressor running, the liquid refrigerant can be drawn into the compressor and cause it to seize or explode. (Remember, Liquid can't be compressed)

Never ever open the high side valve with the system running.

And, unless you have a can tap to add refrigerant through the yellow hose, there is no reason to open the low side valve.

 

With the system off. Guages connected, you will see the system static pressure, which will be the same on both the low and high side. The static pressure will vary depending on outside air temp. The hotter it is, the higher the pressure. On a 60° day, you may only see 70-80 psi static pressure. On a 100° day, you may see 90-110 psi, depending on the system.

 

Operating system pressure will reflect charge level more accurately than static pressure, and if you know what to look for, it will also show you if there is air or moisture in the system.

 

The low pressure side will be mostly constant. This is due to the way the expansion valve works. The expansion valve has a temperature bulb that causes the valve to open or close slightly depending on the temperature of the evaporator core, and a regulating valve that changes depending on the pressure difference between high and low. The expansion valve is the change point from the high side to the low side. The expansion valve will try to maintain an even pressure on the low pressure side. Which should be around 30-35 psi on this particular vehicle. This won't change significantly with temperature change. It may be 5 psi lower on a 60° day than on a 90° day. The pressure may drop slightly as engine speed increases, and may rise slightly as engine speed decreases. On a very low charge, the low side pressure will be lower, and can even show vacuum ( negative pressure) if the system is severly undercharged.

 

The high pressure side tells you the other part of the story. The high side will fluctuate anywhere from 30-50 psi from a cool day to a hot day. You may see 180 psi on a cool day, and 240 psi or maybe higher on a hot day.

 

A low pressure reading on the high side (under about 150), with a normal reading on the low side will indicate an undercharged system. As you add refrigerant the high side pressure will increase, and you may not see any change in the low side pressure, unless the system was severely undercharged. Typically normal high side pressure should range from about 200-240 psi, depending on the system and outside temp. If high side pressure is severely high, say 260 psi or above, that can indicate air in the system, since the air can not be condensed into a liquid at that pressure.

 

If high side pressure is high, and the low side pressure is very low or shows vacuum, this usually indicates a blockage in the system, which would typically be at the expansion valve, or it can be due to moisture in the system which has frozen the outlet nozzle for the expansion valve.

[Bushwick]

Wish I'd known more about this before using it. I thought the knobs on the manifold were more or less on/off to the gauges. Even the instructions on the case didn't mention anything about IF the knobs were turned to the "open" position while running. I'm assuming the liquid you are referring to is what ended up in the manifold's sight glass? It definitely got pulled back into the system. Are you saying that'll stay liquid indefinitely? Isn't there oils in the system too? 

 

 

Assuming the small amount that entered the sight glass doesn't pose a serious risk, it sounds like the system still might be a little under charged.

[Fairtax4me]

The refrigerant is in a compressed liquid state for about half the trip around the system. Starting at the compressor as a gas (vapor) the refrigerant is compressed and travels through the discharge hose to the condenser. Compressing a gas creates heat, that heat is what maintains the gas state. As the refrigerant moves through the condenser heat is exchanged with outside air. About a 35mph wind is created by the cooling fans drawing air in through the front grille. As the heat is drawn out, the gas condenses to a liquid. From this point the now cooled (but still hot) liquid travels through the high pressure line from the condenser to the receiver drier, which is used to draw any moisture out of the refrigerant. It then flows out of the drier, and to the expansion valve, still as a liquid. As the liquid passes through the expansion valve, the pressure drop means the refrigerant cools, but not so much that it immediately turns into a gas. If it were to turn or gas instantly, the core temp would drop below 32° quickly, and the core begins to ice over.

Keeping the refrigerant under pressure maintains the refrigerant as a liquid until it has traveled almost completely through the evaporator core. The trip through the core adds heat to the refrigerant, which combined with still being under pressure, ideally keeps the core temp and refrigerant just above freezing as it evaporates back into a vapor. But ideal conditions never exist in a car AC system. Pressure changes, air temp changes, air flow change across the core, compressor speed changes, all cause the temperature and the boiling point of the refrigerant to change, which can still cause the core to drop below freezing.

Anyway, at this point, the refrigerant leaves the evap core changing back to a vapor state. The low pressure line routes the refrigerant back to the compressor, and the heat under the hood helps to ensure that the refrigerant is totally vaporized by the time it gets back to the suction port of the compressor.

 

Oil in the system just works its way around with the refrigerant. A large part of it rests in a non pressurized area of the compressor.